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thesis entitled

THE CHARACTERISTICS AND CLASSIFICATION OF
GENETIC SEQUENCES OF SOILS IN THE COASTAL PLAIN
SANDS OF EASTERN NIGERIA

presented by

Godwill Lekwa

has been accepted towards fulfillment
of the requirements for

Ph , D4 degree in Crop and
Soil Sciences

Major professor

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© Copyright by

GODWI LL LEKWA
l 9 7 9

THE CHARACTERISTICS AND CLASSIFICATION
OF GENETIC SEQUENCES OF SOILS IN THE
COASTAL PLAIN SANDS OF
EASTERN NIGERIA

By

Godwill Lekwa

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Crop and Soil Sciences Department

1979

(3“

ABSTRACT
THE CHARACTERISTICS AND CLASSIFICATION
OF GENETIC SEQUENCES OF SOILS IN THE

COASTAL PLAIN SANDS OF
EASTERN NIGERIA

By
Godwill Lekwa

Fourteen soil profiles located within the Coastal
Plain Sands geologic formation were investigated along a
126 km transect (Umuahia to Port Harcourt) traversing Imo
and River States of Nigeria. Nine of the fourteen pedons
were selected for more intensive studies that included
their clay mineralogy, micromorphology, physical and chem-
ical properties. Their characteristics were established
and classification at the family level of all the pedons
was made using the U.S. Soil Taxonomy. Approximate clas-
sification of the soils was made with the FAQ/UNESCO Soil
Map of the World Legend. Tentative series were proposed
and tentatively correlated with established series used in
Western Nigeria. Litho-, topo-, chrono- and climosequences
of soils were established.

Data indicate soils vary from those with very little
clay or silt in their deepest horizons sampled to soils

relatively rich in silt and clay. Pedons developed on finer

Godwill Lekwa.

materials have relatively more clay than those on coarse
materials.

All the pedons have low cation exchange capacity
and low base saturation.

The profiles are strongly to very strongly acid.
Aluminum saturation percent is lower than 60% in most
horizons.

In all the pedons the organic carbon and nitrogen
contents are low and decrease with depth except in pedon 4
which has a subsoil maximum in the spodic horizon. Regres-
sion analysis shows significant relationships between
organic carbon, nitrogen content and elevation.

Free iron oxides generally increase with depth in
eight of the nine soils studied in more detail. The iron
contents are lower in soils with udic and aquic moisture
regimes than those with an ustic regime. In all the pedons,
except the aquic regime soils, the amounts of free iron
oxides seem to follow the same distribution pattern as the
clay content within the same profiles indicating combined
movement of iron and clay into the subsoil. Dithionite-
extractable Al contents are lower in comparison with those
of iron for pedons with ustic moisture regimes (pedons 1-3
and 7). Extractable A1 contents are higher than Fe in
pedons with udic and aquic moisture regimes, except in
pedon 4 which has low extractable Al throughout and a marked

accumulation of Fe in the Bhirm.

Godwill Lekwa_

The phosphorus distribution shows that organic-P is
the major form of P in these soils. The Fe-P form is rela-
tively higher than Al-P and Ca-P in the ustic regime soils
while Al-P and Ca-P are almost equally distributed in the
aquic regime soils. Phosphorus fixation capacity of the
soils increases with clay content.

The sand and coarse-silt fractions are predominantly
composed of quartz. X-ray diffraction analysis of clay
shows mainly kaolinite. Relatively older soils have more
kaolinite than the younger soils.

Isotic and undulic plasmic fabrics characterize
the micromorphology of most of the soils. Ferri-argillans
are not very conspicuous in the soils.

Most of the ustic soils (pedons 1-3, 7 and 14) are
classified as Typic Paleustults, fine-loamy and pedon 13
as Arenic Paleustult, coarse-loamy. Pedons 5 and 10 are
classified as Typic Paleudults, fine-loamy. Pedon 12 is
Typic Paleudult, coarse-loamy and Pedon 11 is Arenic
Paleudult, coarse-loamy. Pedon 6 is an Oxic Dystropept,
coarse-loamy.

The sandiest poorly-drained soil is classified as
an Aeric Grossarenic Tropaquod. The somewhat poorly- and
poorly-drained soils, pedons 9 and 8, are classified as
Typic Paleaquult, coarse-loamy and Typic Tropaquult, fine-
loamy, respectively. All of the fourteen pedons have
siliceous mineralogy and isohyperthermic temperature

regimes.

Godwill Lekwa

Using the FAQ/UNESCO World Soil Map Legend all the
soils, except pedons 4 and 6, are classified as Dystric
Nitosols. Gleyic Nitosols are proposed for pedons 8 and 9.
Pedons 4 and 6 are classified as a Gleyic Podzol and a

Dystric Cambisol, respectively.

DEDICATION

To my beloved father, George Ukoha Lekwa, who laid
the foundation and contributed immensely for my success
and achievement, but could not live to see the material-

ization of his efforts.

iii

ACKNOWLEDGMENTS

Dr. E. P. Whiteside, my major Professor deserves
special recognition for his helpful suggestions, most of
which were incorporated in this dissertation, for his
availability at all times, reliability and patience. His
active participation and willingness to discuss problems
as they arose made working with him a rewarding experience.
His encouragement, untiring interest and critical reading
of the manuscript are highly appreciated.

The author wishes to thank the other members of
his Graduate Guidance Committee, Dr. D. D. Harpstead,

Dr. M. M. Mortland, Dr. J. C. Schickluna and Dr. H. Stone-
house for their contributions and substantive discussions
throughout the study.

Sincere thanks and appreciation are also extended
to Dr. F. R. Moormann and Dr. A. J. Herbillon of I. I. T. A.
Ibadan, Nigeria for their suggestions and assistance in
setting up the project. The assistance of Mr. G. A.
Abolarin and the Soil Survey Laboratory of Ahmadu Bello
University, Zaria, Nigeria is appreciated.

Thanks to Mr. A. O. Nnodi, Assistant Director,

Federal Department of Agriculture, Nigeria, for many

iv

thought-provoking talks, assistance, field supervision and
discussions. Without his tireless efforts during the data
acquisition period, this study could not have been completed
in its present form.

The author is also indebted to: Mr. Morgan Lekwa
and Mr. Samuel Lekwa for their invaluable contribution both
financially and otherwise to this undertaking; to Dr. G. A.
Ojanuga of Ahmadu Bello University, Zaria, Nigeria, for
his interest and general counseling at the beginning of the
field study; and to the faculty, students and staff members
of the Crop and Soil Sciences Department at Michigan State
University who helped me in so many ways during the course
of this study. A

Finally, I wish to thank my wife, Happiness Lekwa,
for her constant encouragement, understanding and inspira-

tion throughout the entire study.

TABLE OF CONTENTS

LIST OF TABLES
LIST OF FIGURES .
INTRODUCTION
LITERATURE REVIEW .

Genetic Sequences of Tropical Soils

Nature and Significance of Iron in the Soil
Clay Mineralogy . . .

Description of the Study Area .

Land Use . . . . . . .

MATERIALS AND METHODS .

Field Procedures

Physical Methods

Chemical Methods .
Mineralogical Methods
Micromorphology--Thin Sections

RESULTS AND DISCUSSIONS

Soil Formation Factors

Macromorphology .

Physical PrOperties

Chemical Properties .

Mineralogy of the Sand and Coarse Silt
Fractions . . . . . . . . . . .

Clay Mineralogy . . .

Micromorphology--Thin Sections .

Soil Classification and Correlation .

Comparison and Correlation of
Classification Systems

SUMMARY AND CONCLUSIONS .
RECOMMENDATIONS .

vi

Page

viii

153
158
163
175
186
193

204

Page

LIST OF REFERENCES . . . . . . . . . . . . . . . . . 205

APPENDICES
A. PEDON DESCRIPTIONS . . . . . . . . . . . . . 219
B. SOME ANALYTICAL REAGENTS AND PROCEDURES . . . 236

vii

Table

10.

LIST OF TABLES

Data showing variation between methods
of iron extraction . .

Geological formations and groups of
rocks in Eastern Nigeria

Field morphological descriptions of
the soils .

Particle size distributions in percent,
clay fraction ratios and textural
classes

Fifteen bar water percent, total clay
by pipette method or calculated clay
from 15 bar water content x 2.5 and
ratio 15 bar water:clay .

Bulk density, percent water retention at
0.3 and lS-bar tensions, percent avail-
able water, and total available water

Soil reaction in various suspending
media, ApH, organic carbon, total
nitrogen, and C/N ratios

Exchangeable bases, extractable A1,
cation exchange capacity and percent
base saturation . . . .

Correlation coefficients for the rela-
tionships between some soil properties

Percentages of dithionite or oxalate
extractable Fe O , Al 03, and active
iron, active aIuminiufi, clay:
dithionite Fe and dithionite Fe:
coarse silt plus sand ratios

viii

Page

18

38

59

68

77

81

93

103

115

121

Table

11. Amounts of various forms of phosphorus,
organic C/P ratios, and P-fixation
capacities

12. Mineralogical composition of the fine
sand and coarse-silt fractions deter-
mined with a petrographic microscope

13. Proposed tentative series and their
tentative correlations

ix

Page

137

154

181

Figure

10.

11.

12A-E.

13.

14.

LIST OF FIGURES

Page

Location of profile pits in the

Eastern States . . . . . . . . . . . . . 24
Geology of the study area . . . . . . . . . 25
Mean minimum and maximum temperature

(1951-1960) . . . . . . . . . . . . . . . 27
Seasonal march of temperatures at

selected stations . . . . . . . . . . . . 28
Mean annual rainfall, Eastern

Nigeria . . . . . . . . . . . . . . . . . 31
Hydrological provinces in rank order . . . 33
Vegetation--Eastern Nigeria . . . . . . . . 34
Geology of Eastern Nigeria . . . . . . . . 36
Moisture retention percent (1/3 atm.-

15 bars) versus percent clay . . . . . . 86
Available water percent (1/3 atm.-

15 bars) versus percent clay . . . . . . 87
Frequency distribution of exchange-

able Ca, Mg and K . . . . . . . . . . . . 108
Distribution with depth of clay and

dithionite-citrate extractable

Fe and A1 . . . . . . . . . . . . . . . . 124-128
Relationship between clay/

dithionite—Fe ratio and profile

depth . . . . . . . . . . . . . . . . . . 130
Relative distribution of the various

active P forms as percentage of

total active P . . . . . . . . . . . . . 141-147

Figure

15.

16A-B.
17.
18.
19.

X-ray diffraction patterns of clay
fraction (<2u) of selected

pedons
Micromorphology
Micromorphology
Micromorphology

Micromorphology

of pedon
of pedon
of pedon
of pedon

xi

Page

159-162
166-167
168
169
170

INTRODUCTION

The Coastal Plain Sands occupy a significant area
of the Eastern States of Nigeria. In origin they are
fluvio-lacustrine and were laid down in a series of large
shallow lagoons and lakes. With a rather schematic study
of the soils, a generalized soil map of Eastern Nigeria
was published in 1964. In this generalized soil map the
soils of the Coastal Plain Sands are grouped under the
Ferralitic soils (Jungerius, 1964).

Unlike the Western Region of Nigeria, much of the

work that was done in soil survey in the Eastern part of

Nigeria was lost during the Nigerian Civil War (1967-1970).

The Land Resources Division of British Overseas Deve10pment

was engaged by the Federal Government of Nigeria to carry

out soils study of Central Northern Nigeria and some parts

of the Northern region have been studied by the Soil Survey

Staff of Ahmadu Bello University, Zaria. Owing to scarcity

of soils data and trained soil scientists, an update of the

soils map (1964) has not been done. Presently the Federal
government is proposing to collect and collate soils data
with the subsequent plan of drawing up a new soils map of

the country.

One of the immediate tasks facing the few soil
scientists is the diversity in soil classification systems
viz. USDA (Soil Taxonomy), FAQ/UNESCO, French, British,
Dutch, etc. It is envisaged that the FAQ/UNESCO system
may be adopted to a certain degree and that this will be
supplemented by the USDA (Soil Taxonomy) system. For this
to be successful, a lot of soils correlation, character-
ization and classification is needed. This problem is
most acute in the Eastern States of the country. Whiteside
(1963) stressed the need for soil characterization in
Eastern Nigeria. Inadequacy of soils data hindered a com?
prehensive field review/study during the soil correlation
meetings in Eastern Nigeria (January 1973; June 1974;
Lekwa, 1974).

The principal aim of the present study will be to
provide some of the much needed data that are essential
for understanding the genesis and the classification of
soils derived from the Coastal Plain Sands. The scientific
information acquired should not only be applicable to a
considerable part of Eastern Nigeria, but also to other
parts of Nigeria and of the tropics. An added advantage
is that the area provides for research into the so-called
"acid sands” of Nigeria. It has another advantage in that
it is located close to a center of cr0ps research, the
National Cereals Research Institute of Nigeria, Amakama-

Umuahia. Thus, the area is conveniently located for more

extensive related agricultural research. Decisions involv-
ing the proper use and management of soils have a scientific
basis when soil behavior is understood in relation to soil
characteristics, the origin of these characteristics and
the environment where the soils occur.
A broad classification of the soils of the area
has been made as Ferralitic soils. Some soil scientists
have classified them tentatively as Oxisols or Ultisols.
The practical implications of this controversy contributed
to the undertaking of this more complete characterization
of some of the soils of the area. Fourteen profiles repre—
sentative of the area were studied in the field. Nine of
them were then sampled with the following objectives in
mind:
1. To obtain the chemical, physical, mineralogical
and micromorphological characterization of the
soils.
2. To determine criteria needed for their classifi-
cation using the U.S.D.A. Soil Taxonomy.
3. To provide basic information for future soil
mapping and correlation programs in Nigeria.
4. To begin interpretation of the results for land

use planning and cr0p research programs.

LITERATURE REVIEW

Soil genesis and classification advanced when con—
cepts of landforms were applied to explain differences in
soils and relationships between soils. In the developing
countries soils data are often lacking or not collated.
Great demands are being placed upon the soils of tropical
Africa to meet the needs for food and fiber of a rapidly
expanding population. Soil surveys which are fundamental
to land use planning are lacking except for a few places
where surveys of a reconnaissance type are available. Most
of the existing soil surveys were not made with the inten-
tion of classifying the soils according to the USDA system

of soil classification.

Genetic Sequences of Tropical Soils

 

The cocoa soils of Western Nigeria had been mapped
and classified using the catena concept as a mapping unit.
Milne (1935) in his original article on catena noted that
the soil profiles changed along the traverse from the ridge
crest to the stream in accordance with topography and Milne
gt a1. (1935), in discussing a complex of soils in Tanzania
hinted that the red earths may be relict soils, especially
when associated with calcareous black clays. Milne 93 31.

4

recognized how landscape evolution can affect soils when
they stated, "The great plateau lying behind the east-west
facing ranges carry material which was graded down during
a long process of peneplanation, and which therefore may:
--Retain characteristics impressed during its first
weathering.
--Have acquired new ones as its profile developed
under conditions of sluggish drainage and,
--Undergo a fresh adjustment to conditions of rejuve-
nated drainage following the elevation of the
peneplain into plateau."
In Nigeria the constantly recurring succession of soils
downslope from the red hilltop soils, through brown and
yellow slope soils to the gray alluvial sands and clays
will be very obvious to the agriculturist. The essence of
the catena, therefore, is the spread of soil decomposition
products across a topographic sequence, and their temporary
hold up there in a dynamic phase of soils development.
Bushnell (1943, 1945) suggested terminology for
specified groupings of catenas of soils:
—-Chrono-catena for those differing in time of soil
formation.
--Byndel for those differing in parent materials.
--Climo-catena for those differing in climate.
--Thermo-catena for those differing in the tempera-

ture component of climate.

--Hygro-catena for those differing in the moisture
component of climate.
--Floro-catena for those differing in vegetation.

Jenny (1941) considers catenas of soils as differ-
ing in relief or topography, which includes in addition to
degree of slope, shape of slope, convexity and concavity,
length of slope, direction of exposure, and height of
water table.

Aldrich et 31. (1956), Bushnell (1943), Soil Survey
Staff (1951) and Ruhe (1960) have altered the concept of
catena in recent years. Evaluating Milne's definitions of
catena, Ruhe (1960) restated and redefined catenas as, ”the
sequence of soils encountered between the crest of a low
hill and the floor of the adjacent swamp, the profile
changing from point to point of this traverse in accordance
with conditions of drainage and past history of the land
surface . . . .” According to Ruhe (1960), two variants
of catena can be distinguished in the field. In one, the
t0pography was modeled by denudation or other processes,
from a formation originally similar in lithological char-
acter. Soil differences were then brought about by drainage
conditions, differential transport of eroded material,
leaching, translocation, and redeposition of mobile chemical
constituents. In the other variant, the topography was
carved out of two or more superimposed formations which

differ lithologically. Ruhe (1960) emphasized two principal

points in the catena definition, namely, physiographic
history and geomorphic evaluation of the landscape. This
differs from Aldrich gt gt. (1956) definition, "a sequence
of soils from similar parent material and of similar age
in areas of similar climates but whose characteristics
differ due to variation in relief and drainage."

The formulation of the catena concept by Milne
(1935) was the result of a recognition that particular
slope forms were associated with particular soil sequences.
Milne proposed the idea as a mapping unit rather than as a
taxonomic unit (Ruhe, 1960) and the subsequent elevation .
of it to taxonomic status was a recognition of the fact
that the idea implied a real relationship between soil-
forming processes and slope processes (Bushnell, 1943;
Calton, 1954). As a mapping unit it has been constantly
used for the mapping of soils of a number of areas in
Africa (Ruhe, 1960).

Milne (1935) briefly showed that the soils of a
catena are related to the erosion and deposition of sedi-
ment on a landscape in East Africa. There the landscapes
in a fully developed terrain generally have common ele-
ments. Ruhe (1960) defined the slope-profile components
as summit, shoulder, backslope, footlepe and toeslope.

Nye (1954, 1955b) in a study of a toposequence
without rock outcrops in Africa points out that "a study

of only the vertical development of a profile omits half

the picture. The lateral relationships and variations
between corresponding horizons in the catenary successions
of profiles equally deserve study. It is impossible to
understand the formation of the profiles on the lower parts
of the slope unless the formation of the upper slope-
members is understood and the importance of such factors

as lateral drainage and soil creep is appreciated."

Slope-soil relationships may be studied in at
least two ways, either by the study of catenas, in which
changes in soil characteristics from summit to valley
bottom are related to variation in slope; or by examining
planation surfaces and depositional features in relation
to soils, thus relating soil development to erosional
history. Pallister (1956) and Ollier (1959) used the
former approach while the latter approach is exemplified
by Mulcahy (1960).

Moss (1968) states that the assumption that the
soils found on a particular surface will be older than
those on a lower surface cannot be objectively supported.
The surfaces, in terms of their formation, may not be of
uniform age.

Dixey (1955) has pointed out the drawbacks of
characterizing surfaces by the date of their initiation,
and favors other methods, such as using the geological
period of still-stand immediately prior to their disrup-

tion, or the age of the offshore deposits accumulated

during the erosion of the surface. Pedologically, surfaces
take time to develop and thus the soils with which they are
associated may be of widely differing age. Ollier (1959)
postulated that the correlation of the soils with surfaces
in terms of age implies that processes of soil formation
begin with the cutting of an erosion surface.

According to Moss (1968) the general rule seems to
be that many different soils occur on the same surface,
owing to the many different factors affecting soil formation.
He stated that there is little relationship between surface
age and soil age, owing to the confusion which surrounds A
the notation of age with reference to the geomorphic sur-
faces, the non-uniformity of the operation of weathering
and erosion process over one surface, and the fact that
many surfaces in Africa are formed, not on virgin rock,
but on preweathered material.

Daniels gt gt. (1971) divided the geomorphic sur-
faces into two, depositional and erosional. On many land-
scapes, a surface can have depositional and erosional ele-
ments and the two elements are considered as one surface.
They considered time zero of a geomorphic surface as the
time when it was first eXposed to subaerial weathering.
They maintain that on an erosional surface, this would be
when erosion at that point stOpped; on a marine deposi-
tional surface, it would be when the ocean withdrew and

exposed the sediments to subaerial weathering. Thus, the

10

age of the geomorphic land surface and the associated soil
is the same. The authors concluded that results about soil
genesis based on conventional analysis and consideration of
present soil-forming factors usually are improved when geo-
morphic and stratigraphic information is added.

Ollier (1959) pointed out that the connection
between soils and geomorphology is most important in
tropical soils; more so perhaps than in soils of temperate
regions, where parent rock and parent material are often
more nearly synonymous.

DeVillers (1965) considered that in tropical con-
ditions climatic oscillation since the tertiary has been
a prime factor in inducing cyclic erosion-deposition;
thus, it seems more realistic to consider soil formation
in terms of the cyclic rather than the steady-state concept.
In the present study soil formation factors will be broadly
treated to include climate, organisms, t0pography, time
and parent materials (Jenny, 1941).

Rook (1961) classified soil forming processes under
these groups: disintegration of primary minerals, decom-
position and synthesis of organic compounds, solution and
peptization, precipitation and coagulation, ion exchange
dynamics of liquid phase and heat. Simonson (1959) sum—
marized these processes as additions, losses, transloca-

tions, and transformations.

11

Interaction of these processes under tropical con-
ditions gives rise to many different soils. Ferralization
is the common end product where age or drainage conditions
are not limiting factors. Ferralization can occur in place,
or materials can be preweathered and transported, or both;
supporting a two-cycle theory for trOpical soils in many
areas (Ollier, 1959). According to Stepanov gt gt. (1967),
ferralization is associated with a profound decomposition
of primary minerals, accumulation of sesquioxides, and
paucity of other elements.

The type of humus or humic compounds that are
prevalent have a very important role in soil genesis and
evolution. Tricart and Coilleux (1965) state that fulvic
acids are the most prevalent in red tropical soils; pre-
humic and humic acids are destroyed by bacterial action.
The pH, C/N ratios, kind of organic matter originally
present, and microorganisms are factors associated with
the type of humus present in soils.

Thorp and Smith (1949) modified the 1938 Soil
Classification Scheme by grouping the Red Podzolic with the
Yellow Podzolic soils, under the great soil group Red-Yellow
Podzolic soils, and included the Yellow Lateritic soils
within the Reddish-Brown Lateritic great soil group.
Bennema (1963) related the red and yellow soils of the
trepics to latosols and indicated that South America and

Africa represent the greatest concentration of latosols in

12

the world. The principal characteristics of these soils
are:
--Indistinct horizon differentiation
--Absence or scarcity of silicate clay Skins
--Low CEC due to absence or near absence of 2:1
lattice clay minerals
--Red, yellow or brown colors of subsurface horizons
--Absence of well-develOped blocky or prismatic
structure
--Deep sola
--Consistence in moist state; very fiable or friable
--High porosity and high permeability
--Low base status in the whole profile
--Relatively high anion exchange capacity and high
P-fixation power
--Re1atively low amounts of exchangeable aluminium
due to the low effective base exchange capacities
of the clays present
--High resistance to gully erosion due to their
porous structure and deep sola.
Aubert (1964) summarized the characteristics of
Ferralitic soils as follows:
--Low content of primary minerals
-~High content of hydroxides, especially iron,
aluminium, manganese and titanium. Low ratios

of silica/sesquioxide or silica/alumina.

13

--Colloidal materials are characterized in most
weathered horizons by hydrates and hydroxides of
iron, aluminium, and titanium

--Low silt and humus content

--Very small changes in temperature at depth (50 cm)
during the year.

According to Bennema (1963) red and yellow soils
are typical in uplands of subhumid regions of the tropics.
Color is related in many cases to the different parent
materials in which the soils formed. Red soils are associ-
ated with initially higher contents of ferromagnesian
minerals than in red-yellow soils. Reddish soils are, on
the average, lower in silica than the brownish soils and
are related in the tropics to old surfaces.

Nature and Significance of
Iron in the Soil

 

 

In studies of soil classification and genesis it
would seem essential to differentiate between the free
oxides formed as products of weathering and those inherited
from the parent material. The portion of the total iron
in a soil occurring as hydrous oxides, uncombined with
layer silicate structures, and which is reductant-soluble
is designated as free iron. These oxides occur as coatings
on mineral grains, and possibly as intergrowths with the
clay minerals. The iron in these oxides was released from

the silicate lattices upon the weathering of the minerals.

14

Rankama and Sahama (1949) state that most of the iron in
igneous rocks occurs in the lattices of pyroxenes, amphi-
boles, and ferromagnesian micas. Sand size fragments of
these minerals are found in soils, and since they are
relatively unstable (Goldrich, 1938) it can be assumed that
they weather and release the lattice iron during the period
of soil formation.

Halvorson (1931) discussed the relation of carbon
dioxide pressure and pH to the solubility of iron compounds.
He stated that very little iron remained in solution above
pH 6.5. Olson (1947) found iron solubility in soils to be.
related not only to pH, but also to the amount of free
iron oxides present. Thus iron movement could occur more
readily at a given pH in a soil with more free iron oxides.

Rankama and Sahama (1949) state that iron is
capable of forming complexes with organic matter, and that
these complexes are capable of movement as colloidal sols.
Starkey and Halvorson (1927) indicate that iron can remain
in solution as organic compounds at higher pHs than it can
as inorganic compounds, since the organic compounds are
less ionized. Franzmeier (1962) presents a thorough review
of the proposed methods of iron movement, including organic
and inorganic colloidal complexes and ionic solutions.

Numerous workers have devised methods for the
removal of iron oxides from the surfaces of mineral grains

without disrupting their crystal lattices. Most of these

15

procedures were devised with the objectives of cleansing
the clay particles to sharpen the x-ray patterns rather
than that of the quantitative determination of the oxides
removed. Paddick (1948) used a solution of thioglycolic
acid to extract the free iron oxides from calcareous soils.
The thioglycolic acid also formed a color complex with the
iron thus permitting the quantitative determination of the
iron extracted by a colorimeter. Jackson (1956) used
diluted HCl as an extractant, and determined the amount of
iron removed colorimetrically, using orthophenanthroline to
develop the color. Kilmer (1960) used a solution of sodium
hydrosulfite as an extractant and determined the iron
extracted by titration with potassium dichromate. This
procedure is rather complex but is reported to give con-
sistent results.

Free iron is determined usually by the dithionite
method. This does not distinguish between hydrated iron
oxide weathering products and crystalline primary iron
oxides (Deb, 1949; Mitchell, Farmer and McHardy, 1964).

Deb (1949) reviewed several of the extraction and determi-
native procedures including those developed by Tamm, Jung,
Scarseth and Allison, Dion and Truog. He found a great
deal of variability in the amount of iron removed from the
same soil by the various methods. Deb devised his own
extraction procedure using sodium thiosulfite as an extrac-

tant under conditions of low temperature and pH. Mehra and

16

Jackson (1960) used the same extractant but buffered the
system to pH 7.3.

Schwertmann (1964) tested acid ammonium oxalate
as an extractant of iron oxides and found that in darkness
it dissolved only x-ray amorphous oxides. Lundblad (1934)
showed that this reagent was useful in differentiating
certain classes of soils. Extraction with acid ammonium
oxalate has frequently been used to provide an estimate of
the (x-ray) amorphous fraction of ferric oxides in soils
(Schwertzmann, 1964) and to evaluate soil profile genesis
(McKeague and Day, 1966; Blume and Schwertzmann, 1969).

In the tropics the oxides and hydrous oxide of
iron and aluminium in both crystalline and amorphous forms
are among the major components of soils. A small portion
of Fe and Al is also present in soils in the form of organic
complexes. According to Juo gt gt. (1974) approximate
differentiation among these three forms of Fe and A1 in
soils can be made by selective extraction methods.

The profile distribution of the various forms of
Fe and Al has been used as a criterion in interpreting soil
formation processes in the temperate region (McKeague and
Day, 1966; Blume and Schwertzmann, 1969). Working on
some Nigerian soils derived from sandstones, Ashaye (1969)
studied the relationships between clay content and the acid-
oxalate-extractable Fe and Al. He found that the relation-

ships were not significant. However, he concluded that the

17

amount and nature of the various forms of Fe and Al oxides
and organic complexes may greatly influence the physical
and chemical prOperties of the soils.

The presence of amorphous Fe and Al oxides may
modify certain soil prOperties such as anion sorption,
surface charge, specific surface area, swelling, and
aggregate formation (McIntyre, 1956; Sumner, 1963; Sherman
gt gt., 1964; Acra and Weed, 1966; Greenland gt gt., 1968).
Sherman gt gt. (1964), pointed out that drying at elevated
temperature causes the amorphous Fe and Al oxides to dehy-
drate and subsequently shift to a system of greater crystal-
linity. They also reported some evidence showing that the
loss of amorphous materials in tropical soils as a result
of dehydration and crystallization resulted in significant
changes in certain chemical and physical properties of the
soil, such as, the decrease in cation-exchange capacity
and the increase in both bulk density and particle density
of the soil.

Mehra and Jackson (1960) compared four common
extracting procedures. They presented data indicating
the magnitude of the variation between methods as shown
in Table l.

The very existence of so many methods for the
determination of free iron oxides would indicate that none
of them is completely satisfactory. Apparently, each

method removes only a certain group of iron compounds.

18

Table l.--Data showing variation between methods of iron
extraction (Mehra and Jackson, 1960).

 

Percentage of Iron Oxides

 

8011 0T Mineral Mehra and

 

Jackson Truog Deb
Miami B 1.0 2.5 l.
Vermiculite 4.5 6.0 4
Nontronite 0.5 2.9 2.
Glauconite 0.8 0.6 0.
Nontronite (fine) 0.2 7.9 8
Vermiculite (fine) 6.3 10.3 7.

 

Values obtained using any of these methods must, therefore,

be considered relative, rather than absolute.

Clay Mineralogy

 

Clay mineralogy studies constitute an important
phase in the characterization of soils, soil genesis, and
the classification of soils. Directly or indirectly they
reflect clay content and clay composition. Keller (1970)
observed that clay minerals are the result of the reactive
response of geologic materials to the energies character-
izing certain environments over time. Clays found in
soils result either from the inherited clays present in
the parent material or from the synthesis of clay in the

soil profile.

19

Specific processes by which clay minerals are
formed are direct crystallization from solution, replace-
ment, crystallization from a colloidal gel, and weathering
of non-layered silicates. According to Keller (1970),
argillation includes the following factors: temperature,

pressure, pH, eH, concentration of metallic ions, H48104

and/or hydrated SiO2 polymers, concentration of Al+3 to

Al(OH)4 and/or hydrated aluminum hydroxides, polymorphs
of SiO2 and complex soluble compounds of silica and/or
alumina.

Kaolinite has been reported as the major mineral
in the clay fraction of Ultisols. Clay minerals may be
indicative of environmental conditions if they are formed
tg gttg as a response to such environments. In the tropics
kaolinite tends to be produced when H+ concentration is
high relative to metal ions, and when the aluminium con-
centration is high. Keller (1970) relates kaolinite for-
mation to a high Al:Si ratio, high H+ concentration and
low Na+, Ca++’ Mg++ and Fe+++ concentrations. The syn-
thesis of kaolinite is restricted to a pH of about 4.5.
This means a soil and climate where Ca++, Mg++, Na+ and
K+ are removed freely and H+ ions are supplied abundantly
by acids or by dissociation of water.

DeKimpe and Gastuche (1960) concluded that kaolin

appears in acid soils of low base saturation. Kaolinite

is more stable in acid weathering conditions since the

20

2:1 minerals are less resistant to acid attack than the 1:1
species.

According to Mohr and Van Baren (1954) the main
process of alteration in the tropics is desilication or
kaolinization (Jackson, 1965). More intense and protracted
weathering of parent rocks and soils under conditions of
wet and dry seasons has resulted in further enrichment of
sesquioxidic components giving free iron oxides, kaolinite
(DeVillers, 1965) and halloysite (and frequently gibbsite)
in clays of Ultisols (Jackson, 1965).

From well-oxidized (alternately wet and dry clim-
ates) and intensively leached soils of the trOpics, the
element silicon has frequently been severely depleted
enriching the clay in hydrous oxides of Al, Ti, Fe and Mn
during the Quaternary. Jackson (1965) called this process
laterization or intensive desilication. The results of
this process is an accumulation of hydrous oxide clays
and TiOz-rich residues (Jackson gt gt., 1948; Jackson and
Sherman, 1953).

Tamura gt gt. (1955) in Hawaii showed that under
alternating wet and dry climatic cycles, iron oxides are
stabilized and accumulated. They found that in the Low
Humic Ferruginous Latosols the Maui soil has a high
hematite content in the A horizon and gibbsite in the B
whereas in the Kanai soil goethite and hematite are pre-

dominant in the B horizon.

21

According to Frink and Peech (1962) gibbsite is
the thermodynamically stable phase of Al(OH)3 at room
temperature and pressure. Fripiat gt gt. (1954) concluded
that under savanna conditions grasses remove 51, thus
increasing the tendency to concentrate alumina in the soil
and giving rise to a higher gibbsite content when compared
to soils under other vegetation.

Free gibbsite can appear through weathering when
leaching rates are rapid and the supply of silica is
limited. In Hawaii areas that are continually wet or
alternatingly wet and dry for periods of time long enough
to allow removal of silica, the soils contain either
hydrated alumina and Al-Fe gel material, or laterite and
gibbsite if a dry period has been involved. Iron oxides
are probably the most common of the accessory minerals in
clays (Mitchell gt gt., 1964). The name ”limonite" has
often been used for amorphous, hydrated iron oxides with
composition FeZO3 n H20 or with a molecular formula such

as ZFezO -3HZO (Wada and Harward, 1974). The monohydrates

3
of ferric oxide "goethite” and the anhydrous oxide hematite
are the crystallized mineral forms found in soils, but
hydrated ferric oxide gels and anhydrous Fe oxides are also
known to occur in soils (Mitchell gt gt., 1964). Many of

these materials exist as coatings on other particles (Jones

and Uehara, 1973) rather than as discrete units.

22

Segalen (1971) working with metallic oxides and
hydroxides in soils of the warm and humid areas of the
world found that:

--Near the Equator, goethite is the dominant iron
mineral. Iron is always outside the clay mineral
structure which is essentially kaolinite.

--In the tropical zone, goethite and hematite are
common and amorphous materials are ferruginous
or manganic. The former are very often respon—
sible for the red colors of the soils.

--In the subtropical zone, goethite, associated with
amorphous products, appears to be the dominant

iron mineral. The iron content is high.

Description of the Study Area

 

The land-mass of former Eastern Nigeria (now broken
into four states viz. Imo, Anambra, River and Cross River
States) extends from 4°15'N to 7°05'N, and from 5°32'E
to 9°16'E. It thus occupies very much of a humid, tropical
location. For three of its four sides, the physical
boundaries of the area are reasonably well defined. The
400 km shoreline along the Gulf of Guinea, while it com-
prises a confused complex of sandy, barrier beaches,
lagoons, creeks and swamps, nevertheless forms a clear-cut
southern boundary to the Eastern states. On the west, the
Niger River and its distributaries, culminating in the

vast and intricate drainage network of the Niger Delta,

23

serves to delimit the boundary between the Eastern states
and Bendel state.

The Coastal Plains of Eastern Nigeria comprise the
entire area adjacent to the coast line along the southern
edge of the country. Thus they embrace not only substan-
tial portions to the south of former Eastern Nigeria but
also parts of the Mid-Western state (now Bendel State)
including Benin Division. These plains extend from Port
Harcourt northwards for more than 104 km. It is adjoined
by the Oban Hills in the Cross River State while the
Kukuruku hills adjoin it at its western boundary.

The Coastal Plain Sands or Benin Sands is a geo-
logical formation and does not embrace the entire geo-
graphical area defined above, the portion excluded being
the narrow southern tip of the geographical coastal plain
generally referred to as the mangrove swamps of the Niger
Delta, an area south of an imaginary line running approxi-
mately East-West across Port Harcourt. It embraces much
of former Owerri Province, Benin, and the northern areas

of former Rivers and Calabar Provinces (Figures 1 and 2).

Climate.--The Eastern States of Nigeria experience
a high intensity of solar radiation throughout the year
because of their latitudinal location. Daylight hours
are nearly constant from month to month; as far north as
Nsukka, the difference between the shortest and longest

day is only forty-eight minutes. This ensures a steady

24

 

 

LOCATION OF PROFILE PITS IN EASTERN STATES

 

 

OWERRI

IKOTEKPENE

M Mayor roads

0. Profile pats

‘. PORTHARCOURT

 

o - Scale I- 500,000
a no
’II
Iko
. .
I2

 

 

 

Fig. l.--Location of profile pits in the Eastern States.

 

25

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26

input of solar energy although absorption, scattering and
reflection due to the earth's atmosphere may modify the
actual amounts of solar radiation received at ground level.
Air temperatures within the region are predictably
high the year round. Mean annual temperatures are every-
where above 24°C (75°F) although they do not exceed 29°C
(85°F) (Figure 3). Mean minimum temperatures do not fall
below 18°C (65°F) while mean maximum temperatures do not
exceed 32°C (90°F). The hottest months of the year are
February and March, and the coldest month is usually
August (Figure 4). February and March are late dry season
months when insolation is increasing with the apparent
northward migration of the sun, the Harmattan haze has
already disappeared, and clearer skies permit uninterrupted
sunshine. August is in the midst of the rainy season and
the heavy rains of the preceding months have led to a
lowering of atmospheric temperatures. Despite the little
dry season--"August break," a high degree of cloudiness
persists which deflects incoming solar radiation.
Temperatures across the area are high enough to
permit year round growth of domesticated plants and agri-
cultural activities. A seasonal pattern to crOpping is
necessary due to lack of moisture, not a temperature-
induced dormant season. Year round farming is feasible
where water can be supplied by irrigation and the terrain

and soils are favorable.

27

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28

 

 

 

 

 

 

 

 

 

 

[PORT HARCOULRU °c [CALABAR]

35" 35..
32~ 32-
IIII| lul'
""IIIIIII 2~| "IIIIIII
m-. m4
1w 1%
w m

JFMAMJJASOND JFMAMJJASOND
°c ENUGU m [NSUKKA]

l

35- 35-
32 l

III I“ II I
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-L_._.P :..__....
w JFMAMJJASOND m JFMAMJJASOND

Mean daily maximum
temperature
Mean monthly
tennperature _ ..
Mean daily minimum
temperature

Fig. 4.--Seasonal march of temperatures at selected stations.

29

The rainfall pattern of the region indicates a
continuous decrease from the coast inland. There is a
strong influence of a maritime location and altitude and
the configuration of the Eastern Nigerian shore is such
that the region faces almost squarely the onshore moisture-
bearing winds of the Atlantic. The monsoon winds of the
wet season give rise to an annual rainfall in excess of
4064 mm (160") in the Delta while Bonny peninsula receives
over 4250 mm a year. This convectionally-induced rainfall
diminishes rapidly inland. The central areas of the region
have an annual rainfall of 2032-2286 mm (80-90"). However
local variations in annual rainfall due to terrain, vege-
tational cover, etc., do exist.

In the light of the mean annual figures, all of
Eastern Nigeria may be said to experience a tropical wet
climate (Koppen's Af climate), which implies a rainfall
surplus. However, it is the seasonal distribution of rain
which is critical, rather than the overall receipts, for
an excess of rain at one season may be followed by a
deficit at another. This is the pattern which obtains in
Eastern Nigeria. The relationship between incoming rain-
fall and evapotranspiration is the true measure of humidity
or aridity of a place. If rainfall is greater than poten-
tial evapotranspiration the place is humid. If the
reverse is true, the place is arid. In these terms, many

parts of Eastern Nigeria are "arid" for some part of the

30

year, humid for the remaining months (Figure 5). Unfor-
tunately, measurements of evapotranspiration are not yet
available over a wide enough area to permit precise calcu-
lation of the water balance on a regional basis.

Basically there are two seasons, namely, the dry
season--taken as months with rainfall lower than 62.5 mm
(2.5") and this is less than one month at the coast, e.g.,
Bonny and Brass. This increases gradually inland. In
southern inland locations it is two to three months, e.g.,
Ahoada and Ikot Ekpene; in central inland locations it is
three to four months, e.g., Umuahia, Owerri, and Afikpo;
in northern inland areas it is four to five months or over,
e.g., Nsukka and Ogoja.

The number of rainy days--days with more than 0.25
mm of rain, shows a distribution similar to the quantity

of rainfall and the length of the dry season.

Water Resources.--Of the rain which falls over

 

Eastern Nigeria, only a relatively small percentage reaches
the water table (Floyd, 1969). This is as a result of a
combination of factors, namely, runoff, evaporation and
transpiration, geological formations, soil structure and
vegetative cover.

From an estimated total of 159.6 billion liters
of rain falling on Eastern Nigeria every year, only some
9.9 billion liters remain after deductions accruing from

losses via run-off, evapotranspiration, etc. (Mitchell-Thome,

31

 
 
 
 
 
 
 
    

- Above 4064 mm
[IIIIIIII] 3556-4064 mm
3048-3556 mm
m 2540-3048 mm
B 2286-2540 mm
E 2032-2286 mm
1778-2032 mm
Below 1778 mm

 

 

9 . . . . 39
Kilometers
1 Enugu 7 Abakiliki 13 Owerri 19 Yenagoa
2 Nsukka 8 Ogikwi 14 Umuahia 20 Port Harcourt
3 Onitsha 9 Afikpo 15 Ahoada 21 Uyo
4 Awka 1o Ugep 16 Aba 22 Calabar
5 Udi 11 Obubra 17 Arochuku 23 Opobo
6 Ogoja 12 Ikom 18 lkotEkpene 24 Oron

Fig. 5.--Mean annual rainfall, Eastern Nigeria.

32

1963). Of these 9.9 billion liters, only gravity water
attains the zone of saturation, perhaps 5.9 billion liters.
The high rate of natural withdrawal understandably
fluctuates from place to place within the region. Figure 6
shows the areal distribution of hydrological provinces in
rank order. The area of highest potential as regards
recharge (and the lowest rate of natural withdrawal)
extends south-eastwards from Onitsha to Oron and includes
Awka, Orlu, Owerri, Umuahia, Aba, Ikot Ekpene, Uyo and
Eket (Floyd, 1969). It also includes the eastern side of

the Nsukka-Okigwi Plateau.

Vegetation.--The human factor which entails settle-

 

ment and agricultural activities has done a lot to trans-
form the vegetational cover of Eastern Nigeria (Floyd,
1969). Farming invariably involves marked modification,
even destruction, of the natural vegetation over wide areas.
In the Coastal Plain area, e.g., Owerri, Annang, the wide-

spread cultivation of oil palms (Elaeis guineensis) has

 

 

produced a distinctive type of vegetation referred to as
"oil palm bush" which is quite different from the original
rainforest.

The main vegetational zones of Eastern Nigeria are
illustrated in Figure 7 though there are variations in the
actual vegetation types within each zone. The study area
comes under Lowland Rain Forest (tropical rainforest)

greatly modified (oil palm 'bush') and hence mature 'climax'

33

30

A

1

mMmmms

 

   

E Fifth rank

m Fourth rank

k\\\‘ Second rank
I M Third rank

 

Fig. 6.-Hydrological provinces in rank order (after Mitchell-Thome).

34

 

- 0" ' 80

 

Kilometers
IIMANGROVEFORESTAfiK)COWSIAL IZDHNVHDSAMANNA
VEGETATION (WOODLANDSAVANNA MOSAIC)
- Beach Ridges with Fresh Water Vegetation Sizeable 'relict' outliers of Rain
Salt Water Swamp with Mangrove Forest Forest
11 @ FreshWater Swamp and Rain Forest Wooded Grassland
m LOWLAND TROPICAL RAIN FOREST 099“ Grassland
Largely unmodified Y“ Montane Rain Forest and
Partially modified Grassland
Greatly modified (oil palm'Bush')

Fig. 7.--Vegetation--Eastern Nigeria (Floyd, 1969).

35

forests are rare. Over much of the rainforest zone west

of the Cross River few areas of undisturbed timber remain.
Virgin rainforest in other than small, remote and isolated
stretches has virtually disappeared. High secondary growth
forest is more abundant, particularly northwest of Port
Harcourt, but it is neither as lush nor as luxuriant as

the original plant cover. The main trees are the palms

and other trees include mangoes (Magnifera indica), avocado

 

pears, bananas (Mush sapientum), oranges (Citrus spp),

 

paw paws (Carica papaya) and plantains.

Geology.--Basement Complex Formation consisting of
outcrops of metamorphic and igneous rocks (schists,
gneisses, and granites) cover about 15 percent of Eastern
Nigeria, Figure 8. It has been described as Pre-Cambrian
in age while certain of the schists have been considered
Archean in age (among the oldest in the world), dating
back nearly three billion years. Reyment (1965) has sug-
gested that two of the major series of rocks, older and
younger granites may be much younger, dating from the
Lower Palaeozoic and Mid Jurassic ages, respectively.

The Basement Complex is overlain by a series of
much younger Upper Cretaceous and Tertiary sedimentary
strata, both marine and continental in origin. The sedi-
mentary rocks were laid down either in a great embayment
of the sea which reached north and east along the approxi-

mate lines of the present Niger and Benue Valleys, or in

  
 
  
   

1 4 7

10 12
11/ 13 12 101213

36

K

n h
l" l in

,IIN: l' I till

1| limb

n?

ill:

    

    

 

 
   
 
   
  

4?—

» ' 52-— u
:f-fll ‘ i
,3" .lil ll

"l
ll ll
ll} ll 1
r
r

         
 

1"" 1, ll.
‘ ‘ll ‘ ‘ ll
; in}!
l. l’lrlhp,
M" Mr

0 80
Kilometers
u

 

 

Y
Diagrammatic vertical section; vertical scale exaggerated

_‘

ERTIARY

[:53 Alluvium

:3 Delta Formation

Coastal Plains Sands

E Lignite Formation

5 IIIIIIII Bende-Ameki Formation
and Nanka Sands

6 [CE Imo Clay-Shales Formation

14 E22] Basement Complex
(undifferentiated)

‘un-n

 

CRETACEOUS

7 Upper Coal Measures Formation

a Falsebedded Sandstones Formation
9 Lower Coal Measures Formation

to Nkporo Shales Formation

11 Awgu-Ndeaboh Shales Formation
12 g Eze-Aku Shales Formation

13 [LEI] Asu River Group

- Minor Basic and Intermediate
lntrusives and Extruswes

 

Fig. 8.--Geology of Eastern Nigeria (after R. Monkhouse).

37

the lacustrine or terrestrial environments left by the
retreat of the seas southwards into the present Bight of
Benin (Monkhouse, 1966). The sedimentary materials are
largely derived from subaerial denudation and erosion of
the rocks of the Basement Complex, with possibly smaller
contributions from the sedimentary areas bounding the banks
of the Niger beyond the territory of present-day Nigeria.
Figure 8 illustrates the geology of Eastern Nigeria while
Table 2 outlines geological formations and groups of rocks

in Eastern Nigeria.

The Coastal Plain Sands.--The sedimentary rocks of

 

the Coastal Plain Sands are the youngest Tertiary beds that
occur in Eastern Nigeria. They outcrop over a very wide
area, almost 25 percent of the region, and they are probably
the most extensive also. They constitute a broad plain
sloping gradually towards the sea but northwards they rise
to form low hills with elevations of 1800 meters or more
above sea level. They occupy the area between a line
through Umuduru, Orlu and Ihiala and the coast and form

the extensive featureless plains of former Owerri Province
which slope gradually seawards. Their extension to both
the east and west of this area is also known. Further
north they occupy a considerable area around Orlu, Oba,

and Nnobi. Evidence of uplift of these younger beds can
be gleaned at Itu where there is an obvious juvenile river

system actively cutting into the beds. The Imo River cuts

38

 

 

 

 

:mfiunEmo-opm xaowhmq “soEommm xoflasoo acoEommm .z
fill
macho uo>fim om< .2
macho :mem po>fim mmOho moamcm sx<lon .4
mofimnm conmowzlsw3< .x
msooumpouu mofimgm ouomxz .w
acoeapmomm monommoz Hmou ooze; .H
cam snowman monoumwcmm powwonomamm .m
mopsmmoz Hmou Mono: .0
,ll. - :OMHmEpom
moflmcm zmau oEH moamnmrxmau 05H .m
sea «Ego H 05 .o co .
ESE. 32:22 253?»? Moon?” .m
Flmwcmm mewmfid Hmummou mpcmw mammam Hmummou .u
:owumehom muHoQ .m
xpmcpoumso fiH.E:w>:HH< new mufioa Ezfl>aafi<
po>wm mmOHU can homfiz .<
ow< macho newumehom

 

mfismmfiz :poummm aw mxoop we mmsohw paw

.AmomH .esofiav
mcofiumEuom HNUMonooo--.~ oHnmh

39

through the western part of the Coastal Plain Sands and
exposures at certain points along its bank (e.g., near Ife
in Mbaise) give a fair picture of the formation which con-
sists of a soft friable yellowish/white falsebedded pebbly
sands overlying grey gritty clays not unlike those of the
lignite group.

The Coastal Plain group consists of a series of
white and honey-colored clayey sands with bands and beds
of clay, usually of very fine texture. The sands, which
make up by far the greater part of the deposit, possess
several characteristics which are always present where
good exposures are found, and without which it is unsafe
to identify beds as belonging to this group. They are
always falsebedded and this is usually a very prominent
feature, and they always contain rounded grits and pebbles,
sometimes in patches and bands, but more often distributed
heterogenously through them. Their lack of consolidation
is another striking feature, for they are soft enough to
be easily broken out with the hands and lack any cementing
material whatever. Good exposures occur also in Itu
Division of the Cross River State but current-bedding is
nowhere evident.

Their color too is a persistent characteristic
where fresh sections are obtained. In the more northern
areas of the Coastal Plain Sands the beds may be stained

deep red by iron oxides, the direction of staining being

40

top to bottom. The white and honey-colored varieties are
abundant but there are variations to purple and red depend-
ing on the amount of iron staining present. Their mode of
weathering is striking for where the streams are of high
grade they carve enormous gulches in these soft deposits
and they provide many excellent examples of gully erosion.
On top the sands are usually ironstained and become deep
chocolate-red in color. Vertically downward this often
appears to go to a considerable depth} Instances have
frequently been seen, particularly on vertical faces, where
the red stain is a mere skin on the unstained sands and
has not penetrated horizontally more than about 3 cm.

In the Oba neighbourhood there is a very marked similarity
between the soft clayey sandstones of the Lignite group
and those of the Coastal Plain Sands, but the presence of
the persistent characteristics just enumerated will allow
the Coastal Plain Sands to be identified with certainty,
for the loosely compacted sandstones of the Lignite group
are never current-bedded nor do they contain rounded
pebbles.

Many exposures of Coastal Plain Sands can be seen
in the various tributaries of River Ideh in Agulu. The
River Ideh rises on a low watershed on the Bende-Ameki
group, drains through Agulu Lake and flows westward to the
Niger River. The Coastal Plain Sands are exposed in a

series of deep canyon-like gullies which have scarred the

41

hillsides in a remarkable way. They have been cut by the
numerous small tributaries which join the main stream from
the north and south and their heads are now approaching,
~in some places, the summits of the ridges.

The Coastal Plain Sands are fluvio-lacustrine in
origin and must have been laid down under conditions which
were subject to frequent and rapid changes. They are
widely distributed over the southern portions of Eastern
Nigeria and the Bendel State after a marked subsidence
and more transgression in the late Tertiary times under
conditions of rapidly changing currents. Much of what
some geologists describe as "recent accumulations" are
merely surficial materials derived from the Coastal Plain
Sands over which they form a thick mantle. They are red
sands or sandy earths with a striking monotony of features
and lacking any form of stratification and/or sorting.
Color range is from light to reddish brown to red.

The relation between the Coastal Plain Sands and
the older groups is one of unconformable overlap. South-
east of Orlu they overlap the Lignites completely and rest
on beds of the Bende-Ameki group. Similarly beyond Oba
the Lignites are again obscured by the younger group which
at Abagana overlies shales of the Bende-Ameki group, while
in the Enugu area they overlie fresh water beds of

Cretaceous age, so that there is no doubt that the sands

42

in these areas completely buried the old land surface,

from which they are now being actively removed.

Land Use

Some authors have recognized five different systems
of farming within Eastern Nigeria, viz., shifting culti-
vation; bush fallowing; rudimentary sedentary cultivation;
intensive sedentary cultivation ("compound farming") and
intensive sedentary cultivation (terrace farming). There
are no clear-cut distinctions among some of these systems
and a few, e.g., terrace farming are restricted to specific
areas especially in high elevations like Maku near Awgu,
forty-eight kilometers south of Enugu.

Food crops are grown mainly for local consumption
and the major crop is yam (Discorea spp.). Shifting culti-
vation is sometimes called "slash and burn" agriculture.
This refers to a system in which the land is cleared
manually, most of the vegetation is burned and the land
crOpped by hand Operations for one to two years. The
fertility of the land is maintained by leaving it fallow,
say for three to seven years, the period being dictated
largely by the degree of agricultural population pressure.
The system has also been called migratory agriculture which
entails periodic movement of fields or plots of crop land
(land rotation), also the relocation of villages after the
area in the immediate vicinity of the settlement has been

worked over for agricultural purposes (Allan, 1967;

43

Schlippe, 1956). In Nigeria, the homes of the cultivators
are usually not moved when the cultivators shift culti-
vation sites except the few nomadic cattle-rearers of
Northern Nigeria.

In the Coastal Plain Sands area the farming system
is based on rotation of cr0ps, the use of household refuse
on land within the compound, and the application of fer-
tilizers (in very few areas). Mound cultivation, which is
common, is probably a satisfactory erosion control, but
in many places the farm is still left at the mercy of the
heavy rains. Staple grains are rare. Yams (Discorea spp.),

cocoyam (Colocasia antiquorum), cassava (Manihot utilissima),

 

 

 

maize (Zea mays), and plantains are cultivated for food.

Palm produce (Eleais guineensis) and kola nuts (Cola nitida)

 

 

constitute the major export crops. Cultivation of cash
crops is increasingly p0pular. In many places this has
necessitated the abandonment or mitigation of fallow and
shifting cultivations, a situation which tampers with the

old system of community ownership of farm land.

MATERIALS AND METHODS

Field Procedures

 

Field studies are an extremely important part of
research of this type. The existence of a paucity of soil
data was fully recognized during field trips of the Soil
Correlation Committee in the Eastern States of Nigeria in
1973 and 1974 (Lekwa, 1974).

The area along a line from Umuahia to Ka (travers-
ing about 140 km), being representative of the Coastal
Plain Sands of Nigeria, was selected for preliminary field
studies (Figure l) with the aid of topographic maps (scales
l:50,000; 1:100,000; l:250,000; and 1:500,000, owing to
inadequate coverage of one scale map), the soil map of
Eastern Nigeria (scale 1:100,000) and a geological map
(1:100,000) of Eastern Nigeria.

This general area was investigated in the field
using soils and geomorphological criteria for selection of
topo- and lithosequences of soils. Several locations were
explored using roadcuts and random soil auger borings in
selecting the pedon sites. The transition from "red"
soils in the north to "brown” soils in the south of the

area (a climosequence), was also considered. Field trips

44

45

were also made to Benin Division in Bendel State where the
Coastal Plain Sands formation is also located.

Each pedon was described as to its position in the
landscape and the characteristics of the landform, such as
$10pe, elevation and natural drainage. Observations were
also made regarding evidence of active erosion and current
land use or vegetative cover. These are recorded in
Appendix A.

Fresh pits were dug to a depth of 1.8-2.0 m for
examination, description and sampling of soils. An auger
was used for deeper sampling where necessary, most commonly
to a depth of about 2.5 meters. The deeper samples were
taken at arbitrary depth intervals or upon changes in color
or texture. Fourteen pedons were examined and described.
Particle size distributions were analyzed routinely in the
laboratory. Of these, nine were investigated and studied
more extensively in the laboratory and are numbered one
through nine.

Each pedon was described as to its morphology by
horizons according to criteria and terminology discussed
in the Soil Survey Manual (Soil Survey Staff, 1951) and
were subsequently sampled according to procedures outlined
by the Soil Survey Staff (1971).

The samples were taken from the vertical face at
the end of the profile pit. Two full bags (1 kg each) of

soil were collected from each horizon for laboratory work.

46

Oriented clods for thin sections were collected from
selected horizons. Profile descriptions appear in
Appendix A. Location of the profile pits are shown in

Figure 1.

Physical Methods

 

The soil samples were numbered, air-dried, and
weighed prior to crushing with a wooden roller. They were
then sieved through a 2 mm sieve to separate the fine-earth
fraction from the coarse fraction. The material greater
than 2 mm was washed, air dried and weighed to determine

the coarse fraction content.

Particle-size distribution analyses.--Soil samples

 

corresponding to 50 g of oven-dried samples were shaken
overnight using sodium hexametaphosphate as the dispersing
agent. Silt and clay fractions were determined by the
pipette method (Kilmer and Alexander, 1949). The total
sand fraction was separated by wet sieving through a 300-
mesh sieve. The sand was further fractionated by dry
sieving with the apprOpriate sieves. All determinations

were made in duplicate for each soil sample.

Water retention at 0.3 and 15 bars.--Water satu-

 

rated samples were equilibrated at 0.3 and 15 bar pres-
sures. Water retention of 0.3 bar was determined in
pressure-plate pressure cookers. Water retention at 15

bar was determined in a pressure-membrane apparatus.

47

The percent of water retained was calculated on an
oven-dry basis (Richards, 1949). Water holding capacity
of the soil material was calculated as the difference
between the 0.3 bar and the 15 bar retentions. The 15
bar water retention multiplied by 2.5 was used to estimate
the clay content in each horizon as recommended by the

Soil Survey Staff (1975).

Bulk density.--Natural clods of about 25 to 100 cc

 

were covered with a saran-methyl-ethyl ketone (1:4 to 1:8
ratio) solution. After being coated each clod was weighed.
in air and in water to obtain its volume by Archimedes'
principle. The bulk density was corrected for weight and
volume of the plastic coating assuming a density of 1:3

g/cc for the coating (Brasher £3 31., 1966).

Chemical Methods

 

Soil reaction (pH).--Soil pH was determined in

 

water (1:1 ratio), lN-KCl (1:1) and in 0.01M-CaC12 (1:2)
(Jackson, 1958; Schofield and Taylor, 1955).

ApH.--The value of delta pH was determined by sub-
tracting the soil water suspension pH from the soil lN-KCl

suspension pH (Mekaru and Uehara, 1972).

Cation exchange capacity.--The CEC of the soils

 

was determined by the lN-NH4OAC, pH 7.0, method (Soil

Survey Investigations Staff, 1967).

48

Exchangeable bases.--Exchangeable Ca, Mg, K and Na

 

were extracted with lN-NH4OAC, pH 7.0. The leachate was
evaporated to dryness and the residue dissolved in 6 N-HCl.
Calcium and magnesium were determined by titrating with
EDTA (Jackson, 1958). Potassium and Na were determined by

flame photometry.

Citrate-dithionite-bicarbonate extractable Fe and
Al.--Fe and Al were determined by the method described by
Mehra and Jackson (1960). Fe in the extracting solution
was determined colorimetrically using the potassium thio-
cyanate method of Jackson (1958). Al was determined

colorimetrically using the aluminon method of Hsu (1963).

Ammonium oxalate A1 and Fe.-- These were determined

 

by the method outlined by McKeague and Day (1966). The

Al in the extracting solution was determined colorimetric-
ally using the aluminon method (Hsu, 1963). The Fe in the
extracting solution was determined colorimetrically by the

Jackson method (1958).

Extractable Al and exchangeable acidity.--Al was

 

extracted with lN-KCl. Al was determined by the titration
method and colorimetrically as outlined by the Soil Survey

Investigations Staff (1967).

49

Organic carbon and total nitrogen.-—The organic
carbon content was determined by the Walkley-Black method
as described by the Soil Survey Investigations Staff (1967).

Total nitrogen was determined by the Kjeldahl

method (Soil Survey Investigations Staff, 1967).

Available phosphorus.--This was determined by two

 

methods:

a. Bray's No. 1 method (1945) with 0.03 N NH4F +
0.025 N HCl as extractant.

b. North Carolina Method (Nelson 33 31., 1953) with
0.05 N HCl + 0.025 N H2804 as extractant.
Phosphorus in all the extracting solutions was

determined colorimetrically by the ascorbic acid-molybdate

blue method of Murphy and Riley (1962).

Inorganic forms of phosphorus.--The fractionation

 

of inorganic P forms was carried out according to the
procedure of Chang and Jackson (1957) using Peterson and
Corey (1966), and Chang (1962) modifications. Phosphorus
in the extracts was determined colorimetrically by the
blue color method of Murphy and Riley (1962). Appendix C

gives the reagents and procedures in detail.

Phosphorus fixation capacity.--The method of

 

Kurtz g; 31. (1946) was used to measure "phosphorus
fixation capacity." A 2 gm sample of soil was shaken for

24 hours with 50 ml of KHZPO4 solution containing 6 ppm P

50

and adjusted to pH 7.0. After centrifugation the phos-
phorus in the supernatant was determined and the amount

sorbed obtained by difference.

Mineralogical Methods

 

Preparation of clay fractions (<2 pm) for x-ray

diffraction.--A known weight of soil sample was treated

 

with 1N-sodium acetate solution (buffered to pH 5 with
acetic acid), digested and centrifuged, to remove the

carbonates (if any) and soluble salts. Organic matter
was removed with 30% H2 2.

The treated sample was subjected to dithionite-
citrate-bicarbonate treatment (Mehra and Jackson, 1960)
to remove the free iron oxides. The residue was washed
thrice with l N-NaCl and once with methanol to remove the
citrate-dithionite reagent.

The clay fraction was separated from the soil by
dispersion and sedimentation according to procedures
described by Kittrick and Hope (1963). The clay suspension,
collected in large bottles was flocculated by the additions
of solid sodium chloride to give a solution of approxi-
mately 0.3 N salt concentration.

The clay samples obtained were washed to remove
excess salt. For x-ray diffraction analysis, oriented
clay specimens were made by depositing clay suspensions

onto glass slides and evaporation. For preliminary tests

51

(pedons l, 5, 8, 9) two clay suspensions were made and
subjected to the following standard treatments suggested
by Jackson (1956):
i. Mg-saturation and ethylene-glycol solvation;
ii. K-saturation at room temperature, heating at
350°C and 550°C.

X-ray diffractograms were obtained on the plate
specimens using CuKa radiation over the range 2° 2 theta
to 30° 2 theta.

From results of the preliminary tests, selected
clay samples were subjected to only K-saturation.

Samples from which the iron oxides had not been

removed were also analyzed by the x-ray diffraction method.

Mineralogy of the sand and coarse-silt fractions.--

 

After the removal of free iron oxides, organic matter, sand
and clay, a particle size separation by sedimentation was
made at 20 pm, the lower limit of the coarse silt fraction.
For preliminary tests two sand fractions (0.05-0.2;
0.2 mm-2.0) and a coarse silt fraction were taken of a
sample from each profile (upper part of B, as shown by *
beside Table 4) was examined by means of Optical microscopy.
There were only small mineralogical differences percent-
wise among the subfractions of a given sample, and sub-
sequently only the predominant sand size subfraction

(0.05-2 mm) was examined.

52

Approximately 50 mg of the size fraction (sand or
coarse-silt) to be examined was placed on a microscope
slide, and mixed thoroughly with five drops of immersion
oil of index of refraction, n = 1.54. Line counts were
made on all samples with a petrographic microsc0pe. Between
300 and 500 grains were counted for each sample.

Four pedons 1, 4, 8 and 9 were further examined for
heavy minerals (>2.85 sp. gravity). The Optical and physi-
cal mineralogic constants reported by Krumbein and Petti-
john (1938) and Kerr (1959) were used as references to

identify Opaque and non-Opaque minerals.

Micromorphology-Thin Sections
Undisturbed soil samples of selected profiles were
collected from the vertical face of the horizons. These
vertically and horizontally oriented sections were pre-
pared for each selected horizon by Gary Section Service,

Tulsa, Oklahoma.

RESULTS AND DISCUSSIONS

Soil Formation Factors

 

Climate-temperature.--C1imatic data for selected

 

stations within the study area have been used to estimate
the soil temperatures. The mean annual air temperatures
range between 24°C (75°F) and 27°C (80°F) (Figure 4).
There are also pronounced dry and wet seasons. Seasonal
temperatures vary primarily with clouds and rain.

As the mean annual air temperature is greater than
8°C (47°F) with adequate rainfall, the soils have a mean
soil temperature of about 1.1°C (2°F) higher than the air
temperature (Smith 33 gl., 1964). Therefore, it can be
concluded that in all the fourteen pedons the mean annual
soil temperature at 50 cm depth is higher than 22°C (71.6°F).

In the Tropics, differences between summer and
winter soil temperatures are small. The differences between
the mean monthly summer and mean monthly winter temperatures
at 50 cm depth are less than 5°C (Soil Survey Staff, 1975).
Consequently, all the soils in this study are classified

in the isohyperthermic temperature class.

53

S4

Climate-moisture.--For pedons 1-4, 7, l3 and 14,

 

the mean annual rainfall ranges from 2032-2286 mm (80-90
in.) in those areas (Figure 5). The precipitation exceeds
the potential evapotranspiration on an annual basis.
However, the soils remain depleted of stored water during
more than 60 consecutive days (January-March) during the
dry season which starts in December. The rainy season
starts in late May, with the most intense period being
July-August. During this period a considerable surplus of
water is in the soil during more than 180 cumulative days,
enough to maintain all of the control section constantly .
moist during more than 90 consecutive days. Based on
these data, the soil moisture regime would be ustic for
pedons 1-3, 7, l3 and 14. Pedon 4 has an aquic moisture
regime.

Pedons 5, 6, 8, 9, 10, 11 and 12 are in an area of
greater rainfall, 2286-2540 mm (90-100 in.) (Figure 5).
The monthly rainfall: evapotranspiration ratio exceeds
one in at least nine months. The dry season is not as
pronounced as in the other section of the Coastal Plain
Sands areas, suggesting the predominance of an udic soil
moisture regime. Pedons 8 and 9 have aquic moisture

regimes.

Organisms.--Over the entire area, rainfall is high

enough to permit luxuriant native tree growth. Man's

SS

agricultural activities modify effectively organisms as a
soil formation factor in this area.

On the higher elevations of the northern sites,
high population density prevails, with more people involved
in farming. The pOpulation density thins as one goes south
in the study area and there more peOple are involved in
fish farming than in land cultivation. Therefore savanna
and derived savanna with few forests prevail on the higher
topographic positions while on the lower sites there are
more forested areas. However, 'Oil palm bush' are found
throughout the area. The establishment of a stable plant '
community is hampered by man's agricultural activities.

The length of fallow in the brush fallow-crOpping
system, which involves natural biocycling of nutrients, is
affected by population density and increased pressure on
land. Consequently, the fallow portion is relatively

shorter in the northern part of the study area.

Topography.--Eleven of the fourteen pedons (1-3,

 

5-7, and 10-14) are situated on gently sloping upland sur—
faces with deep water tables and reflect their regional
moisture regimes. These higher topographic sites represent
well-drained soils.

On the lower gently sloping surfaces locally, the
effect of groundwater is indicated by the somewhat poor

and poor natural drainage of the soils (pedons 4, 9 and 8).

56

Other subfactors in topography such as direction of
exposure, slope percent, and shape of land, are not included

in the study.

Iimg.--All the pedons are developed in the same
geological formation--Coasta1 Plain Sands. This formation
is of Pliocene age (Floyd, 1969). Soils located on higher
surfaces are presumed to be comparatively older than those
on lower surfaces that may have been cut into the Coastal
Plain Sands later. However, studies of the ages of sur-
faces are not available in this area to date. All the
pedons sampled are deep weathered profiles indicating

relatively old soils.

Parent material and lithosequences.--The fourteen

 

pedons are formed from unconsolidated sediments. The
deepest soil horizons most similar to the initial materials
are the B3.

Light-colored minerals predominate in the parent
materials. The clay mineralogy of the soils tend to be
highly kaolinitic as shown later. The textures of the B3
horizons vary with their clay contents. The textures of

the B horizons range from finer fine-loamy to coarse-

3
loamy and sandy.

Lithosequences of well-drained soils with ustic
moisture regimes developed in coarse-loamy to finest fine-

loamy materials are pedons 13; 14 or 2 or 3; and l or 7.

57

Lithosequences of well-drained soils with udic
moisture regimes developed in coarse-loamy to finest fine-
loamy materials are pedons 6 or 11 or 12; 5; and 10.

Pedons 4, 9 and 8 have aquic moisture regimes.
They formed in sandy, coarse-loamy and fine-loamy materials,
respectively. They constitute a chrono—lithosequence as
pedon 4 is located on a higher, probably, older surface as
well as being formed in sandy material. Pedons 9 and 8
form a lithosequence developed in coarse-loamy and fine-
loamy materials, reSpectively, but as pedon 9 is somewhat
poorly drained and pedon 8 is poorly drained, they may

actually represent a tOpo-lithosequence.

Other genetic sequences.--The following genetic
sequences are also observable:

i. Toposequences in udic moisture areas are: pedons 8
(poorly drained) and 5 (well-drained) in coarser
fine-loamy materials; pedons 9 (somewhat poorly
drained) and 6 or 11 or 12 (all well-drained) are
in coarse-loamy materials.

ii. Chrono-climosequences: Pedons 11 or 12 or 6 and 13
are all well-drained, in coarse-loamy materials
and occur on increasingly higher surfaces (4 to
60 m). Of these pedons only 13 is in an ustic
instead of in udic moisture regime. These appar—

ently represent a chronO-climosequence of soils.

58

Pedons 5 and 2 or 3 or 14 are well—drained and in
coarser fine-loamy materials. Pedon 5 is in an udic
moisture regime on a lower elevation and a presumably
younger surface. Pedon 2 or 3 or 14 is in an ustic mois-
ture regime on a higher elevation and a presumably Older
surface.

Pedons 10 and 7 or 1 are formed in finer fine-
loamy materials and situated in udic and ustic moisture
regimes, respectively. They constitute another chrono-
climosequence. Biosequences of soils are not visualized

in sites studied.

Macromorphology

 

The detailed morphological descriptions of the
studied pedons are presented in Appendix A. Table 3 gives
a summary Of the field morphological descriptions of the
soils.

Within the well-drained profiles a wide range for
the color component hue is observed. Soils located on the
highest physiographic surfaces (perhaps oldest) in the
ustic moisture area (pedons 1-3, 7, l3 and 14) have more
reddish subsoils (7.5YR-SYR-2.5YR) than those located in
the udic moisture regime (lOYR-7.5YR) on the lower physio-
graphic surfaces (pedons 5, 6, 10-12). These represent
chrono-climosequences of soils. Each of these groups of

soils are formed from coarse-loamy to fine-loamy materials.

59

 

 

 

 

 

EmOH xmao Accmm Amazv xnm.m - w\v m>m.m omalnmfi mm
EmOH xmfiu chmm mamwv xnm.m - 0\v m>m.m nmarme pNNm
EmOH xmao zvcmm mpmev .uo.~ - O\e mwm.N melmm uHNm
amofi seemm Acwev .eo.N m e\m m>m.m NN-NH Hm
EmOH zpcmm muwev .hO.H m.O ~\m mwm Nfiro Q<
.H.m.m .E omfirrm coped
smofi sefio seeem LON - w\o m>m owH-on mm
smofi smfiu seemm Ama3-mzv .pum - c\e msm on-mm ONNm
amofi seemm Amaze .20 m O\m msm.e mm-om OHNm
EmOH chmm mhw>EV .po w «\m m>oH omrma Hm
Emofl secem Aem>ev .eu m N\e m>OH mH-o e<
.H.m.m .E omH--N coped
EmOH xmfiu spasm figmev .pO - m\m mwm.m owH-mn mm
EmOH zmau zwcmm mpwev .po w w\¢ mrm murme uNNm
amoH smHO seemm mewev .eo fl O\e m>m me-- OHNm
EmOH Apcmm numsv .po w Q\m m>m NN-¢H Hm
amofi seemm Howey .po we N\m m>m.e OH-o m<
.H.m.m .E omfillfi coped
neocoumwmcou
Oe Aumwoev Eu
OC3HXOH pcmv thpcsom cemwpom
eunuosuum we pOHou canon
aw
.meom ecu mo mcofiumfipomow Hmofiwofiocmhos pHme--.m Oanmh

60

 

 

 

 

smofi secmm co - m\m m>m.n ow~-mefi mm
EwOH chmm yo - c\m m>m.n meH-oHH mum
EmOH chmm ho - V\m m>oH oaa-mm HNm
EmOH xvcmm no - «\v m>oH mm-m~ Hm
smoH spasm ho - m\m awofi mH-o d<
.H.m.m .E om--o cocoa
EMOH xmfiu zpcmm xnm - 0\m m>m.n owaroma mm
EmOH xmfio chmm Amazv .HO m.o O\v m>m omalmw «NNm
EmoH meO spawn amazv pu.~ m.O O\m m>oH wwrmm pamm
EmOH xvcmm nhwev w «\e m>OH mmlmfi Hm
eemm aseofl Ahmev .eo m.o m\m awofl mH-o a<
.H.m.m .E oo--m coped
ecmm Amused e - H\N msm oom-oefl sedan
eeem mfisv mm.o fi.m H\a a>cH oeH-om m~N<
eemm flHev mm.o m.u H\m m>OH om-NH m-<
eemm fidev mm6 m.o H\4 m>OH N~-o a<
.H.m.w .E meH--e :ovom
Amocoumwmcou
Ox mpmwoev SO
ousuxoe pcmv zpmwcsom :ONMHOI
a ouauospum we pofiou comma
%

.eosafioeou--.m canny

61

 

 

 

 

awed seeam Ahmad xnm.e.N - eofloooe .N\a >m.~ owH-aw mmHH
saoH seamm Aposv xpm.s.H m.o eofiopoe .N\o >m.~ aw-m~ moNNmHH
eeofi seemm Aew>ev xam.e.N m.o eofioooe .N\m m>oH w~-oa moHNmHH
Emoa chmm mum>ev xnm m>.~ m.O N\v m>OH oflrw ~<
EmOH Apcmm mHEV mm.o m.m N\N m>OH Nro ~<
.H.m.m .E omllm Cowmm
EmOH Avcmm
-seoH sefio secmm Afi>mev .xnm .- N\o >m.~ owH-o~H mmHH
amofi safiu seemm Aflmev xnm.~-H 3 O m\m m>m.e+fi\o awed ONH-~A momNmHH
amofl _flgmev xmm 2.0 e\m m>m+fi\o m>OH Ne-oe moNNm
swofi xecmm homey xnm H H O\e xsofi+fi\e m»oH oe-m~ wofimm
smofi seemm Ahmev .e u H\o m>OH mm-oH m~<
eeoH se:Mm hemav .HO m.e H\m m»OH+H\N m»m oa-o e<
.H.m.m .E ONllw cofiumm
EmoH xeao spasm xnm - m\m mwm.m oHN-omH Nmm
EmOH xwau xwamm nhmsv .hu . c\m m>m.m omalooa Hmm
EmOH meO xvcmm mpmev .uo fl 0\m m>m oOHrmm HNNm
EmOH xmau xpcmm mhmsv .pu . m\v mwm mmlom uHNm
EmOH chmm Ahmev .»O me «\e mwm.n om-o d<
.H.m.m .e OOH--N cocoa
Amocoumfimcou
O mgmaoav Eu
oesuxOH « pcmv xgmwcsom . :ONMHOE
n Ohnuuspum me HOHOU spawn
%

 

.eosefioeou--.m mfinme

62

 

 

 

 

EmoH smfio xvcmm xnm.o.m - O\m mwm.e omH-ow mm
EmoH xwao Apcmm xnm.O.H m.w «\e m>m.n owlom ummm
amofl aeemm xam.s.H so m\e msm.e om-mH OHNm
eemm Asmofi xnm.u.H - M\m m>OH mfl-N m<
eemm samofi Apm>ev po.m.~ 3.m N\m m>oH ~-o H<
.H.m.m .E wllNH EOQOQ
aeofi seemm ”edgy - o\m-e\m m>oH me-mw on
amofi seemm fiew>ev - m\m m>OH mm-oe Hm
eemm Aseofi eo.m.~ - ~\m m>OH oo-o4 Nm<
eemm eO.H m m\m mwofi oe-o~ Hm<
eemm C.H - N\~ m>OH o~-o a<
.H.m.m .E VllHH Gowmm
smofi smfiu seemm xpm.m.H - eofioooe .e\m msm.e omH-omH mm
seofi sefiu zeeem Amaze xom.H e e\m msm.e omH-om BNNm
amofl smfiu seemm flemsev eu.m.H e m\e m>m.e ow-oe OHNm
eeam aem>sv eo.w.H - M\m m>oH o¢-mH m<
eemm saeofi-eemm Aew>sv 2O.C.H - N\N «>OH mH-o m<
.H.m.e .e om--o~ eoeoa
Amocoumwmcoo
OHDHXQH 0% Huang m khmGCSOm AWWHMEV MW :ONHHOI
n Ohsuossum « H u :u a
«

 

.eosefioeou--.m ofinme

63

.xxuwum uozumz .Oflummfla xaunwflam uozummz .Oanmwpm xho> pmwos
num>e .Epww xho> pmwoeufiw>e .OmOOH umflOEnHE .OHamwpm umw0EnpwE .Epfim umwoenflms
.vum: saunwflam xhpucmw .umom xpvumc .OmOOH Appuap .nfifiom xhwv ppmnunpo

.Ocfiw Apo>um> .cfimum Oamcfimumm .xxOOHn hmstcmnsmuxam .O>fimmmEuE .ocflm

um .nezuoupo .omumoouo .mCOhumum .Oumpopoeum .xmozuH .mm0aop5uushumno

.x>m303 .cuooemum .pmfiswmuhwnfi .Hmawmhwuw .Omsmmwpuw .pmoauuo .uushnmnw

n

m

 

 

 

 

seofl smHu seeem anew xnm.u.m - O\m xsm.e om~-oo~ mm
EmOH xmau
seeem-smoH seemm figmev gamme-w.m m.m «\4 m>m.a ooa-oe HHNm
we
eeem samofi xnm ON-HO.C.H m.u m\m mxofi oe-o~ m<
eemm maev mm.o m.m N\a m>OH oa-o a<
.H.m.m .e woe--efl eoema
ENOH zmau
seemm-2eofi seemm fleev xnm.u.~ - O\m m>m.e omH-HHH mm
amofi secmm figmev xnm.o.m m.@ «\m m>m.e HHH-~m HHNm
eemm sauce mmev xnm.oum m m\m m>CH Hm-mH m<
eemm mHeV mm o 3.0 N\4 m>oH mH-o a<
.H.m.m .e oo--m~ comma
O Amocoumwmcoo mumaosv Eu
ousuxo g cm see :30 . :0 ago
9 a Ohmuuwnum me e m MOHOQ some: N. I
k

1.?!

.emsafipeou--.m oflnme

64

Soils developed in finer fine-loamy materials
located on high surfaces show more reddish subsoils than
those developed in coarser fine-loamy or coarse-loamy
materials with the same moisture regime (pedon l or 7;
pedon 2 or 3 or 14; and pedon 13, respectively). These
are a lithosequence of well-drained soils with ustic mois-
ture regime.

Pedons 5 and 10 have comparatively more reddish
subsoils (lOYR-SYR-7.5YR) than pedon ll, 12 or 6 (lOYR;
7.5YR or 7.5YR). All are well-drained soils with an udic
moisture regime and are a lithosequence. Pedons 5 and 10
developed in fine-loamy materials and pedons ll, 12 and 6
are coarse-loamy.

There are redder subsoils in well-drained soils
with ustic moisture regimes while the udic soils on com-
parable textured materials (except pedon 12) have more
yellowish subsoils. These are climosequences of soils.
The udic soils are also on lower surfaces (4-60 m) than
the ustic soils (60-150 m) and are probably younger. So
these may actually be chronO-climosequences of soils.

The more poorly drained soils (pedons 8 and 9)
are paler, more mottled and more yellowish—brown in sub-
soil colors (lOYR-2.5Y) (Table 3). They are found mostly
in the southern zone of the study area at lower elevations.
Pedon 4, though poorly drained, has a more reddish and

darker (5YR 2/1) subsoil than pedons 8 and 9. It is

65

situated on an area with an ustic moisture regime, at a
higher elevation, and it has a spodic instead of an argillic
subsoil horizon.

Pedons 8 and 5 are located in an udic moisture
regime area with higher annual rainfall and both develOped
from coarser fine-loamy materials. Pedon 5 has a brighter
less gray and more reddish subsoil (lOYR-SYR-7.5YR) than
pedon 8 (10YR-2.5Y). These differences represent a tOpo-
sequence of soils.

However, within the entire study area, well-
drained soils with reddish subsoils can be found where
soils with yellowish brown subsoils predominate and vice-
versa. This is shown by pedon 5 with yellowish red
(5YR 4/6) subsoil from 88-150 cm in the udic area, and by
pedons 13 and 14 with brown to strong brown (7.5YR 4/4-5/6)
subsoils in the ustic area. Since the increase in the
annual rainfall and its intensity towards the south is
gradual (that is from Umuahia to Port Harcourt, Figure 5),
the boundary between the two groups of 'colored' soils is
not sharp.

The degree of structure development shows little
change with depth in well-drained pedons l, 2 and 6.

There is some difference in the consistence and this is
due to more clay in the subsoil and more plasticity and

stickiness, particularly in pedon 2. Pedon 6 is developed

66

in coarser parent material (coarse-loamy) on lower physio-
graphic positions.

In most of the other well-drained pedons (3, 5, 7,
10, 12, 13 and 14) one can observe structure develOpment
increase from A to the lower B horizons in the form of
crumb to subangular blocky structures. The development of
fairly well expressed crumb structure in the upper horizons
is attributed to the presence of grass and forest regrowth
and higher contents of organic matter. There are no
obvious genetic sequence relationships between the well
drained soils with these observed differences in structure.
development.

Among the more poorly drained soils, the structure
development in pedon 8 increases from A to B horizons,
going from massive to medium subangular blocky. Compared
to the other pedons studied, this one exhibits more sticki-
ness when moist and more hardness when dry. As in pedon 8,
pedon 9 has increased structure development from A1 to B,
structureless to subangular blocky, respectively. Worm
holes and humus coated sand grains are observable within
the horizons.

Pedons 4, 8, and 9 are the poorly drained (pedons
4 and 8) and somewhat poorly drained (pedon 9) members of
the soils under study. All of them have eluvial and
illuvial horizons clearly visible as do the associated

well drained soils, except pedon 6.

67

Pedon 4 has homogenous textural classes of sand
from the Ap to the Bhirm horizon. It shows a weak expres-
sion of structure and consistence which is a reflection of
the low amount of clay and high sand content. An iron-
humus pan is encountered below 140 cm in the massive Bhirm
horizon. The consistence at this horizon is very firm and
compact. The base of this iron-humus pan was not reached.
This kind of soil is associated with very sandy parent

materials in poorly drained situations in the study area.

Physical Properties

 

Particle size distribution.--The particle size

 

distribution data for the fourteen pedons are shown in
Table 4.

The upper horizons of these soils have a lower
content of clay (except pedons 4 and 6) and are higher in
sand (except pedons 4, 6 and 9) than the subsoil horizons.

Several processes help to explain these prOperties.

a. Eluviation and i11uviation.--Eluviation is the

 

process whereby clay is mobilized in the upper horizons
and is transported in suspension down to the lower horizons.
This process is reflected in the development of eluvial
upper horizons (A1, A2). Illuviation is the process Of
formation of a zone of accumulation of the clay or other
eluviated material in the lower part of the solum. The

presence of an A2 horizon does not automatically result

(58

 

 

 

 

 

 

 

 

 

 

m o.N o.H o.H o N NH mm ocN-caH ECHHN
w o.H o.H o.o o H NH oo ovH-om NNN<.
m N N.H N. o H mm we cm-NH NN<
m N H.H a. o H NN 4N NH-o a<
e:=<--v comma
Hum No. o.H HN o.NH c.N N e N co ONH-NNH mm
Hum No. o.N NN c.4H ¢.N m N NH mm NHH-mv HNNN
Hum co. N.H HN m.mH H.H N N NH Nm mv-NN HHNme
Hm om. H.H NH o.m e.N m m mH om NN-NH Hm
Hm em. N.H 4H o.N 4.0 a N NH No NH-c a<
mm:<--m :oeua
Hum no. H.H HN N.HH H.H NH N HH mm owH-NOH mm
Hum no. H.H HN m.NH H.H NH H oH mm NcH-mm HNNN
Hm no. H.H mH m.HH H.H oH H NH Hm mm-om HHNN.
Hm co. m.H eH o.m v.N NH H HH om cm-NH Hm
Hm oo. H.H mH a 0 OH H mH mm NH-o a<
sz<--N coeoa
Hum No. o.H em o.HN c.NH N o N HN ow ONH-mN mm
How He. o.H an a.cN H.HH e m o mH m.Nv mN-mv NNNm
How He. o.H NN N.NH N.oH m o mN Ne me-NN HHNN.
Hm mm. v.H NH m.a H.H e H mm me NN-oH Hm
Hm em. H.H mH H.» m.o N H on Hm oH-o a<
Hmz<--H :oeme
N
meo Hench meu Omumou
mmeHO \HaHu eeHa \smHu oeHm .N v N. v N.-N Noe.-No. No.-mo. ma.-mN. mN.-N so soNHoo:
«capusuxou. kuoh. mafia omhmou ocwn— Unhmou mafia mmhmou can—mo .
mOHumm
: .meu as .uHHm EE .vcmm

 

 

Ilufli

.mommmHO Hmpauxou can .moHumH :OHuume meO .ucoouom :H m:OHu:nHuumHv ONHm oHuHuHmm--.v oHnmh

6S)

 

 

 

 

 

 

 

 

 

HN-HON NN. N.N NN N.NH N.N N N NN NN NNH-NNH NNHH
HON NN. H.N NN N.NH N.N NH N NN NN NNH-NN NHNNNHH
H NN. N.H . NN N.NH N.N NH NH NN NN NN-NN NNNNN
HN NN. N.N NH H.NH N.H NH NH NN NN NN-NN NHHNNN
HN NN. N.H NH N.N N.N NH HH NN NN NN-NH NN<
HN NN. N.H N N N NH HH NN HN NH-N N<
NzN--N eoNoN
HON NN. N.H NN N.NH H.HH N N NH NN NHN-NNH NNN
HON NN. N.H NN N.NH N.NH N N NH NN NNH-NNH HNN
HON NN. N.H NN N.NH NH N N NH NN NNH-NN ONNN
HON HN. N.H NN N.NH N.N N N HN NN NN-NN OHNN.
HN NN. N.H NH N N N N NH HN NN-N N<
N22--N eonN
HN NN. N.H NH NH N N H NH NN NNH-NNH NN
HN NN. N.H NH N N N N NN Ne NNH-NHH NNN¢
HN NN. N.H NH N.N N.N H H NH NN NHH-NN HNN
HN NN. N.H NH N N N H NH NN NN-NH HN
HN NN. N.H NH N N N N NH NN NH-N N<
NN<--N NNNNN
HON NN. N.H NN N.NH N.N N H NN Ne NNH-NNH NN
HON NN. N.H NN NH N N N NN NN NNH-NN HNNN
HON NN. N.H HN NH N N N NH HN NN-NN NHNN.
HN NN. N.H NH N.N N.N N H NN NN NN-NH HN
NH NN. N.H HH N N N N NN NN NH-N N<
N:N<--N :oNNN
N
NNHO Hench ANHO ONHNOU
NNNHO NNNHO «eHN \NNHO NcHN .N v N. v N.-N NNN.-NN. NN.-NN. NN.-NN. NN.-N Eu :oNHco
«caausuxOH HNHOH och Omumou ocwm oNHmOU ocwm omhmou gamma . :
NOHumm
: .zmHu EE .uHNm Es .vcmm

 

.NoaeHucoO--.N oHNNN

70

 

 

 

 

 

 

 

 

 

 

 

HUN NN o v N No oNHrcN Nu
HUN NN N m N am owrom uNNN
HN NH oH N HH NN em-NH HHNm
NH N N N NN NN NH-N N<
NH N N v NH Ho N-N H<
NHme--NH covom
HN NH v N NN NN NNH-NN ONN
HN NH N N «N NN NN-NN HN
NH N N o NN No oc-ov NN<
N N N H «N No cv-NN HN<
N N N N NN NN NN-N N<
HHxH--HH :ovom
HUN HN N o He NN NNH-NNH NN
HUN NN v H Ne oN cNH-cN uNNm
HUN NN N H vv NN calov uHNm
N N N N we we ov-NH m<
NH-N N N H NN NN NH-N n<
oHom--cH :ovom
HN NN. N.N NH N.NH H.N o H NH NN NNH-NN NNHH
HN NN. N.N NH N.NH N.N N N NH No NN-NN NONNNHH
HN NN. N.N NH N.NH N.H ' e NN NN NN-NH NuHNNHHc
HN NN. N.H N H.N N.N N NH NN Ne NH-N N<
Hm NN. H.H N N.N N.N N NH cN mm N-N H<
th--N covom
N
NNHO HNHOP NaHu ONHNOU
NNNHU \NNHO ocHN \NNHO och .N v N. v N.-N NNN.-NN. NN.-NN. NN.-NN. NN.-N so :ONHHO
ccHuusuxOH HNOON ocHN omuuou ocHN omuaou o=HN omuaou Hugo: . =
NOHONN
a .NNHO ea .HHHN as .NNNN
.voacHucou--.¢ OHHNN

71

.NcoHuuuuu ABE NN.-NN.V wean och can «HHN ONHNOO mo NNOHNuocHE HON vocHanxo oHnaamc

.INOHIH macaw NENOHuNH chamuN HENOH NNHO AccumuHUN HENOH chamuHmcc

 

 

 

 

 

 

 

 

 

HON NN H H HH NN NNH-NNH NN
HON-HN NN N H OH HN NNH-Nv OHNN
NH N N N NH NN NN-NH N<
N N N N NH NN NH-N N<
.HNN--NH eoNNN
HON-HN NN N N NH NN NNH-HHH NN
HN NH N N NH NN HHH-HN OHNN
NH NH H N N NN HN-NH N<
N N N N NH NN NH-N N<
NHzxo--NH eoNoN
N
AuHu Hench NNHO omunou
NNNHO \NNHO oeHN NNNHO «NHN .N v N. v N.-N NNN.-NN. NN.-NN. NN.-NN. NN.-N so eoNHNo:
acHNuaaxOF HNHOh ocHu omumou och omuuou och ONHNOU canon .
NoHuaz
N .NNHO as .HHHN as .NeaN

 

.voacHucou--.v oHHNh

72

in the development of an argillic horizon. This can be
seen in pedon 4 which, though it has an Azg horizon, has
no argillic horizon. Removal of other materials like
hydrous oxides of Fe, Al, or humus can lead to an ochric
epipedon especially in a sandy material like pedon 4.

This is further shown by the development of a spodic hori-
zon (a humus-iron pan at below 140 cm) in that pedon
instead of a clay accumulation.

Eluviation processes cannot explain the lack of
differences in the sand contents between the A and B
horizons of pedon 9. However, in pedon 9 differences in
clay content of the A and B horizons are evident. In
pedon 6 there is little or no field evidence of argillans
in the B horizon, and no laboratory evidence of more sand
in the surface or more clay in the subsoil. However, it
is possible that the mobilized clay once in suspension has
not been deposited in the solum; or that subsequent weather-
ing and solution, or pedoturbation has destroyed any evi-
dence of clay eluviation and clay accumulation.

These seem to be deeply weathered soils and the

clay contents of B horizons seem to be higher than those

3
of the upper part of the profiles. This may be associated
with more clay decomposition deeper in the profile after
the soils had attained a reasonable degree of weathering.

In pedons 8 and 9 with aquic moisture regimes the

eluvial and illuvial processes could be expected to be

73

more important while on slightly higher better drained
soils on convex lepes (including all pedons except 4, 8

and 9), erosion could also be active.

b. Erosion.--The particle size fraction distribu-

 

tions can be altered due to differential removal through

erosion. The clay fraction once suspended in surface run-
off would remain in suspension for a longer period than
would the coarser fractions. This could result in its
differential removal. Moss (1965) and Vine (1949) have
attributed the occurrence of more sandy surface soils in

some parts of Africa, to this process.

c. Faunal activity.--Working in Africa, Nye (1955a)

 

observed that material moved to the surface by ants was
more sandy than the soil below the surface. Ant-hills
and thin depositions of sandy materials on leaves of the
forest floor have been noticed during field study. In
some areas of the Coastal Plain Sands activities of ter-
mites, earthworms and other burrowing animals have also
been observed. No extensive studies were done on these
materials to determine their physical and chemical com-
positions.

It is possible that burrowing creatures may help
move clay, instead of sand, to the surface and termite
activity, for example, may affect more than the plow layer.

Worm-holes and casts were observed down to 125 cm in some

74

pedons during the field study. However, while no examples
can be cited now, it is likely that other faunal activity

may also contribute to these profile differences.

d. Lithosequences.--In pedons 2 or 3, l3, l4, 1

 

or 7 (all well drained and in the ustic moisture regime),
there are greater clay contents in the B horizons and less
sand content in their A horizons as their B3 horizons are
lower in sand and higher in clay. These pedons are litho-
sequences developed in coarse-loamy to fine-loamy materials.
Among the lithosequences of well drained soils with
udic moisture regimes (pedons 11, 12 or 6, 5 and 10), there
are similar profile texture differences with differences
in the textures of the B3 except that there is little dif-
ference in textural differentiation in pedon 6. The reason
for this lack of texture profile development in pedon 6
is not known. Pedons 11 or 12; 5; and 10 developed in
coarse-loamy to finer fine-loamy materials, respectively.
Considering the soils with aquic moisture regimes
and without spodic horizons, pedons 8 (poorly drained) and
9 (somewhat poorly drained) may represent a lithosequence
or tOpO-lithosequence of soils. Both pedons show evidence
of vertical variations in parent materials in their lower
silt contents of the deeper horizons, those with II's
preceding their horizon designations. This is also evident
in no change of sand content with depth in pedon 9.

Pedon 8 is lower in sand throughout and higher in silt.

75

It is also higher in clay content beneath the Ap horizon.
Both pedons show markedly higher fine clayztotal clay
ratios particularly in their B21tg horizons and these
ratios are higher in the deeper horizons than in their
upper horizons, indicating eluvial surfaces and illuvial

subsoils.

e. Tgposequences.--Considering the toposequences

 

pedons 9 and 11 or 12 in coarse-loamy materials or 8 and

5 in fine-loamy materials, there is a more gradual increase
in clay content beginning deeper from the surface in the
better drained soils. The ratio of fine clay:total clay

is greater in the upper subsoil of pedon 8 compared to
pedon 5 indicating more fine clay movement into the upper
subsoil. However, pedon 8 is higher in silt content than

5 and so this should be considered a litho-toposequence of
soils. Evidences of climo-sequences of soils are not shown

by differences in particle size distributions.

f. Ratios, fine clay/coarse clay and fine clay/

 

total clay.--In Table 4 the fine clay and coarse clay per-

 

centages in eight of the pedons show that the downward
increase in clay is accounted for largely by the fine-
clays. Illuviation of fine clays is shown more distinctly
by the fine clayzcoarse clay ratios. The magnitude of
this ratio increases with depth, in all but pedon 6, as

would be expected if most illuviation is in the fine clay

76

fraction. In five of the seven profiles this ratio
reaches a maximum and decreases with further depth. This
maximum is nearest the surface in the more poorly drained
pedons 8 and 9.

Of all the pedons, the fine clayztotal clay ratio
for identification of argillic horizon (Soil Survey Staff,
1975) is well shown by pedons 8 and 9 which have poor and
somewhat poor natural drainage, respectively. They meet
the one-third greater fine clay to total clay ratios that
has been used as an index of pedological development for
argillic horizons. Their maxima fine:total clay ratios

are in their upper Bt horizons.

g. Ratio; % lS-bar water/% clay.--The ratio,

 

percent lS-bar water to percent clay determined with the
Calgon-pipette method has been used as a substitute way
to judge the degree of dispersion attained in particle size
distribution analysis (Soil Survey Staff, 1975). This
ratio does not exceed 0.6 if the clay readily disperses.
The effect of removal of Fe and Al oxides and hydrous
oxides on dispersion was not determined here. Values
between 0.36 and 0.41, a mean of 0.38, are found in the
nine soils studied (Table 5). In many cases the values
increased slightly with depth.

Comerma (1968) and later Benavides (1973) used this
ratio as a criterion to judge the degree of weathering of

some soils of Venezuela and Colombia. Comerma (1968)

77

 

 

 

 

 

 

NN.N N.HN HN N.N NNH-NNH NN
NN.N N.HN NN N.N NNH-NN HNNN
NN.N N.NN HN N.N NN-NN HHNN
NN.N N.NH NH N.N NN-NH HN
NN.N N.NH NH N.N NH-N N<
NN2<--N eoNoN
NN.N N.HN HN N.N _NNH-NNH NN
NN.N N.NN HN N.N NNH-NN ONNN
NN.N N.NH NH N.N NN-NN HHNN
NN.N N.NH NH N.N NN-NH HN
NN.N N.NH NH N.N NH-N e<
sz<--N NNNNN
NN.N N.NN HN N.NH NNH-NN NN
NN.N N.NN NN N.NH NN-NN HNNN
NN.N N.NN NN N.HH NN-NN HHNN
NN.N N.NH NH N.N NN-NH HN
NN.N N.NH NH N.N NH-N N<
HN2<--H NNNNN
N NNHU N.N x Noam: HmnlmH OHHOQHN N Noam: Eu :ONHHOI
N ON: NNN-NH NNHO NNN-NH NHNNN .

l. I-}ll§1l.'ll .101 I..‘. all 1"...

10 1 ‘ 141.11.- 1 Illll ll I14; . 1.! I111|1l I
.1111 1.1? '1 lIil It, 4.111I.ll| Illll Il' Alt."

cm N.N x Hcoucoo Noun: Han NH EOHN NNHO

.NNHOHNOHmz Han NH OHHNH
Ha >2 NNHO HNHOH .H:OOHOQ Noam: was :OOHNHN--.N OHHNH

pOHmHsuHmO HO pozuoe OHHOQ

78

 

 

 

 

 

 

NN.N N.NH NH H.N NNH-NNH NN
NN.N N.NH NH N.N NNH-NHH NNN
NN.N N.NH NH N.N NHH-NN HNN
NN.N N.NH NH N.N NN-NH HN
NN.N NH NH N.N NH-N NN
NNN--N eoNNN
NN.N N.NN NN N.N NNH-NNH NN
NN.N N.NN NN N.N NNH-NN HNNN
NN.N N.HN HN N.N NN-NN HHNN
NN.N N.NH NH N.N NN-NH HN
NN.N N.NH HH N.N NH-N N<
NNNN--N NNNNN
NN.N N.N N N.N NNN-NNH ENHHN
NN.N N.N N.N NH.N NNH-NN NNN<
NN.N N.N N N.N NN-NH NHN<
NN.N N.H N N.N NH-N N<
sz<--N coNNN
N NNHU N.N x Hon3 HNH-NH OHHOQHN N Noam: Eu :ONHHOI
N ON: NNN-NH NNHO NNN-NH HHNNN .

.NNNNHHNNO--.N NHNNH

79

 

 

 

 

 

 

 

NN.N N.NH NH N.N NNH-NN NNHH
NN.N N.NH NH N.N NN-NN NNNNNHH
NN.N N.NH NH N.N NN-NH NNHNNHH
NN.N N.N N N.N NH-N N<
NN.N N.N N N.N N-N <
NNN--N :owom
NN.N N.NH N.NH N.N NNH-NNH NNHH
NN.N N.NN N.NN N.NH NNH-NN NNNNNHH
NN.N N.NN N.NN N.N NN-NN NNNNN
NN.N N.NH N.NH N.N NN-NN NNHNN
NN.N N.NH N.NH N.N NN-NH N<
NN.N N.N N.N N.N NH-N a<
sz--N :ONNN
NN.N N.NN NN N.NH NHN-NNH NNN
Hv.o N.NN NN N.HH NNH-NNH HNN
HN.N N.NN NN N.HH NNH-NN NNNN
NN.N N.HN NN N.N NN-NN NHNN
NN.N NH NH N.N NN-N n<
N22--N cowom
N meo m.~ x pmumz pmn-mH ouuonflm N News: Eu :oNNpo:
N on NNN-NH NNHU NNN-NH gamma .

'\,'llll'n‘ Ill

.Nmnchcou--.N NHNNN

80

proposed values below 0.3 for highly weathered soils; values
values between 0.3 and 0.4 characterize soils having an
intermediate degree of weathering, and values above 0.4

are typical of recent alluvial soils. Most of the soils

of the study area can be characterized as having an inter-

mediate degree of weathering using this ratio.

h. Available water.--Available water is estimated

 

as the difference between the water available at field
capacity (0.3 bar tension) and the water content at the
permanent wilting point (15 bar tension) (Peters, 1965).

Studies conducted on Hawaiian oxisols have shown
that water retention in the tension range between 0 and 0.3
bar is primarily influenced by the size, shape and arrange-
ment of the soil aggregates, whereas the size, shape and
arrangement of the soil particles play the dominant role
at higher tensions (Sharma and Uehara, 1968a; E1 Swaify
33 al,, 1970).

Table 6 presents data on the moisture retention
properties of the nine pedons. According to the data the
moisture retention capacity at both 0.3 and 15 bars is
closely related to the properties of the clay fraction.
They commonly increase with depth. The moisture retained
at 0.3 bar tension if measured in undisturbed soil samples
is considered to be somewhat indicative of the field mois-
ture holding capacity. However, in some soils, especially

sandy soils, this measurement may underestimate this

81

 

 

 

 

 

N.HN N.N N.N N.NH NN.H NNH-NNH NN
N.N N.N N.NH NN.H NNH-NN NNNN
N.N N.N N.NH NN.H NN-NN NHNN
N.N N.N N.NH NN.H NN-NH HN
N.N N.N H.NH NN.H NH-N g<

NN2<--N cocoa
N.NH N.N N.N N.NH NN.H NNH-NNH NN
N.N N.N N.NH HN.H NNH-NN NNNN
N.N N.N N.HH NN.H NN-NN NHNN
N.N N.N N.HH NN.H NN-NH HN
N.N N.N N.N NN.H NH-N a<

sz<--N cocoa
H.NN N.N N.NH N.NN NN.H NNH-NN NN
N.NH N.NH N.NN NN.H NN-NN NNNN
N.N N.HH N.NH NN.H NN-NN NHNN
N.N N.N N.NH NN.H NN-NH HN
H.N N.N N.NH NN.H NH-N N<

HN:<--H :owom

w m
Eu owa on NEo\w o .IIWWWII Eu
oHNNHMMWMzHNNON NNN-NH - N.N ON: oN: Nonema apnea coNHNo:
. «yoga: oHNNHHN>< NNN-NH NNN N.N NHNN

.Houmz oHanmw>m Hmuou cam .poum: oHanHm>m ucoopom
.mcoflmcou pan-mH cam m.o um :oflucouoh Noam: pcoonon .zuwmcow xasm--.o oanmh

82

 

 

 

 

 

N.NH H.N H.N N.NH HN.H NNH-NNH NN
N.N N.N N.HH NN.H NNH-NHH NNN
N.N N.N N.NH NN.H NHH-NN HNN
N.N N.N N.NH NN.H NN-NH HN
N.N N.N N.NH NN.H NH-N a<
NN<--N cocoa
N.NN H.N N.N N.NH NN.H NNH-NNH NN
N.N N.N N.NH NN.H NNH-NN NNNN
N.N N.N N.NH NN.H NN-NN NHNN
N.N N.N N.HH NN.H NN-NH HN
N.N N.N N.N NN.H NH-N a<
NDN<--N cocoa
HN.H NN.N NN.N NN.H NN.H NON-NNH ENHNN
HH.N NH.N NN.N NN.H NNH-NN NNN<
NN.N N.N NN.H HN.H NN-NH NHN<
NH.N N.N NN.N NN.H NH-N a<
sz<--N comma
Eu oma ow NEo\m N oo\m Eu
NHNNHMMwmzHNooe “NN-NH - N.N oN: oN: Noncma apnea coNHNo:
. «Nmomz NHNNHHN>< NNN-NH NNN N.N NHsm

 

.NoscNoaoo--.N NHNNN

83

.cofimcou Nan NH mange Nan m.o

n pcoouoa youmz ofinmafim><x

 

 

 

 

 

 

H.NN N.N N.N N.NH NN.H NNH-NN NNHH
N.N N.N N.NH NN.H NN-NN NNNNNHH
N.N N.N N.NH HN.H NN-NH NNHNNHH
N.N N.N N.NH NN.H NH-N N<
N.N H.N N.HH NN.H N-N H<

NNN--N cocmm
H.NN N.N N.N N.NH NN.H NNH-NNH NNHH
N.N N.NH N.NH NN.H NNH-NN NNNNNHH
N.NH N.N N.NN NN.H NN-NN NNNNN
N.NH N.N N.NH NN.H NN-NN NNHNN
N.NH N.N N.NN NN.H NN-NH N<
N.NH N.N N.NH NN.H NH-N a<

sz--N :oumm
H.NN N.NH N.NH N.NN NN.H NHN-NNH NNN
N.N N.HH N.HN NN.H NNH-NNH HNN
N.N N.HH N.HN NN.H NNH-NN NNNN
N.N N.N N.NH NN.H NN-NN NHNN
N.N N.N N.HH NN.H NN-N g<

N25--N :ONNN

N oo\m
Eu owH ow NEo\m Eu
oHNNHMMwmzHNNON NNN-NH - N.N ON: oN: NNHNcoo cwaoa :oNHNo:
. «Room: NHNNHHN>< NNN-NH Nan N.N NHsm

 

.woscfiucou--.o oHan

84

capacity. The 0.3 bar may or may not be representative of
field capacity in the various soil textures. Fine tex-
tured soils will drain to much higher tensions and coarse-
textured soils usually reach equilibrium at much lower
tensions under field conditions. Franzmeier gt al. (1960)
stated that water contents at field capacity are better
related to those of 0.06 atmospheres than to those at 0.3
atmosphere. Thus, since the 15 bar moisture retention
capacity is considered to be comparable to permanent wilt-
ing percentage the 0.3 bar values shown may lead to an
underestimation of the plant available water holding
capacity of the soils.

Sharma and Uehara (1968a) demonstrated that heavy-
textured kaolin-oxide soils have moisture release curves,
which in some respects resemble those of sandy soils.

Water in large soil pores moves rapidly under gravitational
forces, and field capacity is attained at low tensions

(0.1 to 0.15 bars) because at these low values the hydraulic
conductivity is also very low, much like that of sandy
soils. The water in the fine intra-aggregate pores is for
the most part, immobile and can be extracted only by apply-
ing tensions higher than 100 bars (Sharma and Uehara,

1968b; Uehara and Keng, 1973).

From the data (Table 6) the soils with more clay
seem to have more available water than the sandy soils

when the 0.3 bar and lS-bars are used in the computation.

85

Under certain conditions these moisture constants may be
in considerable error. For example, in sandy soils the
field capacity may be represented by tensions as low as

50 cm of water. Also the field capacity is strongly
influenced by stratification (Bartelli and Peters, 1959).
It has been stated that in many instances the lS-bar per-
centage does not give a true representation of the wilting
point of soils in place (Richards and Weaver, 1944). The
wilting point is controlled by such factors as root rami-
fication, evaporative demand and soil texture.

Clay content is the principle factor affecting the»
relative magnitudes of water retained at 0.3 and at lS-bar,
Figure 9. The water at 0.3-bar minus the water at lS-bar
tensions plotted against percent clay (Figure 10) show an
almost linear relationship, as does the lS-atmosphere mois-
ture percent which increases gradually with clay content,
with some obvious deviations of the values for some
samples--upper horizons of pedons 5 and 8. A high degree
of correlation, r = 0.70 is found between available mois-
ture and clay content of the nine soils. This is not in
agreement with results found by some workers in the

temperate areas.

86

 

 

 

 

 

 

 

 

 

C
. 8. Q
g; 20- ‘ ' 0 0
'IO 0..
>
. ' 8 Q“
‘5 o. .
C% 0> .JL. 0
I IO- 0 ’f
o\° o
1;:
00 lb 2'0 30
0/0 Clay
5 20-
O
E!
‘6 3
O
.-'
0 IO- .3
.\ - “'58
. m-
. C Q
Oiliv . u
0 lb 20 30
°/o Clay

 

 

 

Fig. 9.--Moisture retention percent (1/3 atm.-lS bars)
versus percent clay.

87

 

 

 

 

 

l5
0
o
o
o o
E?
g .
T ' . o 0
£2 '0‘ o o
3 0
.§ 0 o o
o o
3: . .0
.2 o .g
.n o
.2 .0
a O. O O
>
‘4 o
E C C
0 oo
0 H
3 5 o o
a’ o
o
o
o
0
Chi. T T I '
0 IO 20 3O 40
Percent Clay

 

 

 

Fig. 10.--Available water percent (1/3 atm.-lS bars)
versus percent clay.

88
Regression coefficients and partial regressions of avail-
able water percent on_percent clay and organic carbon

Regression
coefficients of 'r' 't'

 

 

% clay organic % clay O.C. % clay O.C.

 

carbon
Available
water 1.76 -0.071 .70 -0.28 22** -35.5°
percent

 

**High1y significant, at 1% level
°Nonsignificant at 20% level
The prediction equation for the above significant relation-

ship, where X is available water and Y % clay, is:
Y = 1.76X + 5.3

In the temperate region silty soils have the most
readily available moisture and the available moisture
capacity decreases with clay content (Franzmeier 3: al,,
1960; Jamison and Kroth, 1958). Franzmeier gt El' (1960)
have stated that readily available water capacity (R.A.W.C.)
of a soil is a better estimate of the water available for
plant growth than the usual measures of available water.
R.A.W.C. is defined as the difference between the water
content of the soil at field capacity and the lower turgor
pressure percentage (Franzmeier gt al., 1960). Franzmeier

t l. (1960) found an insignificant negative correlation

coefficient when they compared readily available water

89

capacity with percent clay in some soils of the temperate
region.

Clay particles by themselves form pores so small,
suctions holding much of the water are greater than a
plant can overcome in obtaining moisture. Sand particles
form pores too large for the surface tension to overcome
gravitational forces and, therefore, available water-
holding capacity is low unless smaller clay or silt
particles fill the large pores. Pores between silt
particles are in size ranges that hold water in the
tension range available to plants.

Regression analysis as presented indicated an
insignificant negative correlation coefficient (r = -0.28)
when available water percent is compared with organic
matter content. It is still uncertain as to what extent
and under what conditions organic matter and other aggre-
gating agents will improve water storage. Jamison and
Kroth (1958) found for soils of the South-Eastern United
States, that except for sandy soils, organic matter
increases did not increase the capacity of a soil to store
available moisture. Feustal and Byers (1936) found that
there is little to be gained by adding peat or muck to a
clay soil in equal proportions by volume. They found
that retention of water by organic soils at the wilting

point is high.

90

The bulk density values (Table 6) range from 1.50-
1.67 g/cc. It has been pointed out that the clod method
used here usually gives higher bulk density values than
other methods (Tisdale, 1951; Blake, 1965). Most of the
values obtained seem to be close to those expected from
soils with the textures--sandy loam to sandy clay loam.

The total available stored water for the nine
soils, except pedon 4, range from 18.0 g/cm2 to 180 cm to
30.1 g/cm2 to 180 cm (Table 6). Pedons 1 or 7 have more
total available water than pedon 2 or 3 to 180 cm. This
lithosequence of soils (pedons 2 or 3 and l or 7) are
developed in coarser fine-loamy and finer fine-loamy
materials, respectively, in an ustic moisture regime.

Pedon 5 developed in fine-loamy materials has
more total available water than pedon 6 developed in
coarse-loamy materials. They are a lithosequence of soils
with udic moisture regimes.

In the more poorly drained soils, pedon 8 can hold
more available water than pedon 9 and much more than
pedon 4. Pedon 8 has the highest silt content and is
developed in fine-loamy materials while pedon 9 is coarse-
loamy. These represent a lithosequence of soils with
aquic moisture regimes. The poorly drained members of a
tOpo-lithosequence have shallower water-tables at least
seasonally and consequently more available water (if all

other factors are constant and roots can penetrate the

91

saturated layers) when the water-table is present. Pedon 4
is sandy throughout with much less clay content. The low
total available water of pedon 4 is associated also with
the iron-pan develOpment at 140 cm. While that horizon
has the highest available water in the profile, it may not
actually be available to plants, if their roots cannot
penetrate it.

Pedons 5 and 8 developed in coarser fine-loamy
materials. Pedon 8 with aquic moisture regime and located
on lower physiographic surfaces has more total available
water to 180 cm than pedons 5 (well-drained) with udic
moisture regime on higher physiographic surfaces. The
higher available water can be associated with organic
matter and high silt contents. These represent a topo-
sequence or litho-toposequence of soils.

Pedons 6 (well-drained) and 9 (somewhat poorly
drained) developed in coarse-loamy materials. Pedon 9
with aquic moisture regime has more available water to
180 cm than pedon 6 and this is associated with higher
organic matter content. These represent a toposequence
of soils.

Pedons 2 or 3 developed in coarser fine-loamy
materials in an ustic moisture regime show no difference
in total available water to 180 cm from pedon 5 developed

in similar materials in an udic moisture regime. They are

92

a well-drained climosequence or chrono-climosequence of

soils.

Chemical Properties

 

Soil reaction (pH and ApH).--The results of pH

 

values determined in water (1:1 ratio), KCl (1:1), and
CaCl2 (1:2 ratio) are given in Table 7.

The data obtained in a 1:1 soil-water suspension
show that nearly all of the soils, except Pedons 2 and 6,
are very strongly acid (pH 4.5-5.0) and they are strongly
acid (pH 5.1-5.5). Pedon 4 which is the sandiest of all
the pedons is extremely acid (pH 2.6-4.4). It shows a
decrease in pH value with depth, except for the Bhirm
horizon (pH 3.0) which has high organic matter content.

In the other pedons there is little change with depth, the
pH values vary only by a few tenths (0.2 to 0.6) pH through-
out the entire depth of the pedons.

The pH measurements made in KCl and CaClz sus-
pensions show depth distribution patterns similar to those
obtained in water suspensions although they are lower in
absolute values (-0.3 to -l.4, ApH). The range in pH
differences with depth are similar in KCl and CaCl2 to
those in H20, except for pedon 4, which show a smaller
difference in water (1.8 versus 2.4—2.6 pH units). All
the soils show a negative charge according to the negative

values of ApH (pH in KCl minus pH in H20) (Mekaru and

93

 

 

 

 

 

N.N NN.N NN.N N.N- N.N N.N N.N NNH-NNH NN
N.N NN.N NN.N N.N- H.N N.N N.N NNH-NN HNNN
N.N NH.N NN.N N.N- N.N N.N N.N NN-NN NHNN
N.N HH.N NN.H N.N- N.N N.N NHN NN-NH HN
N.N NH.N NH.H N.N- N.N N.N N.N NH-N a<
NNz<--N :ONNN
N.N NN.N NH.N N.N- N.N N.N N.N NNH-NNH NN
N.N NN.N NN.N N.N- N.N N.N H.N NNH-NN HNNN
N.N NN.N NN.N N.H- N.N N.N N.N NN-NN HHNN
N.N NH.N NN.N N.N- N.N N.N N.N NN-NH HN
N.N NH.N NN.H N.N- N.N N.N H.N NH-N a<
Nz:<--N :oumm
N.NH NN.N NN.N N.N- N.N N.N N.N NNH-NN NN
N.N NN.N HN.N N.N- N.N N.N N.N NN-NN NNNN
N.N NN.N NN.N N.N- N.N N.N N.N NN-NN HHNN
N.NH HH.N NN.H N.N- N.N N.N N.N NN-NH HN
N.NH NH.N NN.H N.N- N.N N.N N.N NH-N H<
HN2<--H eocmm
N N NHumo Hog oN:
owumm z :onumu Ia< Eu :oNfiho:
z\o HNNoN oHcaNHo :H NHHNN

 

.mofiumu z\u was .comouufi: Hmuou
.connmo oflcmmpo .:m< .mfiwoe mcwwcommsm msofihm> :N newuumop HNom--.u oanme

94

 

 

 

 

 

N.N NN.N HH.N N.H- H.N N.N N.N NNH-NNH NN

N.N NN.N NH.N N.H- N.N N.N N.N NNH-NHH NNN

N.N NN.N HN.N N. - N.N N.N H.N NHH-NN HNN

N.N NN.N NN.N N.N- NH.N N.N N.N NN-NH HN

N.N NH.N NN.N N.H- N.N N.N N.N NH-N a<
NN<--N :ouom

N.N NN.N NH.N N.N- N.N N.N N.N NNH-NNH NN

N.N NN.N NN.N N.N- H.N H.N N.N NNH-NN HNNN

N.N NN.N NN. N.N- H.N N.N N.N NN-NN HHNN

N.N NH.N NN.N N.N- N.N N.N N.N NN-NH HN

N.NH NH.N NN.N N.N- N.N N.N N.N NH-N a<
N:N<--N cocoa

N.HH NH.N NN.H N.H- N.H N.H N.N NNN-NNH EHHNN

N.N NN.N NN.N N.N- N.H N.H N.N NNH-NN NNN<

N.N NN.N HN.N N.N- N.N N.N N.N NN-NH NN<

N.NH NH.N NN.N N.N- H.N N.N N.N NH-N a<
sz<--N cocoa

N N NHUNu HUN oN:
ofiumm z :onumu 2Q< so :oNNHo:
z\u HNNoe oHcNNHo In Hogan

.NmscHoaou--.N NHNNN

 

95

 

 

 

 

 

NN.N HH.N N.N- N.N N.N NNH-NN NNHH
NN.N NN.N N.N- H.N N.N NN-NN NHNNNHH
NH.N NN.N N.N- N.N N.N NN-NH NHHNNHH
NH.N NN.N N.N- N.N H.N NH-N N<
NH.N NN.H N.N- N.N N.N N-N H<
NNm--N cocoa
NN.N NH.N N.N- N.N H.N NNH-NNH NmHH
NN.N NH.N N.N- H.N N.N NNH-NN NHNNNHH
NN.N NN.N N.N- N.N N.N NN-NN NNNNN
NH.N NN.N N.N- N.N N.N NN-NN NNHNN
NH.N NN.N N.N- H.N N.N NN-NH NN<
NH.N NN.H N.N- N.N N.N NH-N H<
sz--N cocoa
NN.N NH.N N.N- N.N N.N NHN-NNH NNN
NN.N NN.N N.N- N.N N.N NNH-NNH HNN
NN.N NN.N N.N- H.N H.N NNH-NN NNNN
NN.N NN.N N.N- H.N N.N NN-NN NHNN
NH.N NN.H N.N- N.N N.N NN-N m<
N22--N comma
N N NHUNU HUN
2 Conth Ind EU :ONMHOI
HNHOH owcmwuo In gamma

 

.NoaaHNaoo--.N NHNNN

96

Uehara, 1972; Van Raij and Peech, 1972). Negative ApH
values are indicative of a net negative charge of the soil
colloids, meaning the soils are therefore cation exchangers
(Uehara and Keng, 1973). This negative charge in turn is a
result of the predominance of layer silicates with a per-
manent charge, over oxides and hydrous oxide systems, or
it could be partially due to organic matter specifically
absorbed on colloids or minerals of constant surficial
potential (Mekaru and Uehara, 1972).

ApH like the pH in H20 shows little difference
with depth, except in pedon 4 which is sandy and has a
spodic subsoil horizon. It shows a marked increase in ApH
from the surface to the subsoil horizon. This increase is
associated with increased carbon content of the Bhirm
horizon.

Differences in pH do not show significant relation-

ships among the pedons in the genetic sequences.

Organic carbon content and nitrogen.--Table 7

 

shows the organic carbon and nitrogen contents. All the
pedons show a decrease in organic carbon and nitrogen con-
tents with depth except pedon 4 which has a spodic horizon.
Pedons 2 and 3 have organic carbon content values
that range between 1.03-1.10% for A horizons. Pedon l or 7
have higher values (1.76-1.26%) than pedon 2 or 3. Among
other things, this is associated with finer parent materials

as pedons 1 or 7 and 2 or 3 are lithosequences of

97

well drained soils developed in finer fine—loamy and coarser
fine-loamy materials, respectively. Pedon 5 (coarser fine-
loamy) has the highest surface organic carbon content
(2.60%) and is located on a lower physiographic surface

with an udic moisture regime. Pedon 6 (coarse-loamy) has
lowest organic carbon (.34%) and nitrogen contents even
though it has an udic moisture regime.

Pedons 4, 8 and 9 have aquic moisture regimes and
are located on lower physiographic surfaces (except pedon
4). They constitute a chrono-lithosequence with pedon 8
(coarser fine-loamy) having slightly higher organic carbon
(1.98%) and nitrogen content than pedon 9 (coarse-loamy
1.80%). Pedon 4 has the highest organic carbon content,
2.89%. Though it has the highest total sand content, this
high organic carbon content can be explained by the culti-
vational history of the soil and its poor natural drainage.
The pedon is located at a vegetable plot nursery.

It is evident that soils located on lower physio-
graphic surfaces with udic or aquic moisture regimes and
develOped in fine-loamy materials have more organic carbon
and nitrogen contents than their counterpart on higher
physiographic surfaces developed in similar parent materials.
Thus some of the differences in relative levels of organic
matter content can be related to differences in parent

materials and clay content.

98

Allison (1973) stresses the importance of soil
texture as a factor in determining the level of organic
matter content. He states that under similar climatic
conditions the quantity of organic matter present may be
2-4 times as great in a fine textured clay soil as in one
that is very sandy. He attributes this in part to greater
aeration and increased oxidation in the very sandy soils
but more importantly to:

a. the formation of organic inorganic complexes.
b. sorption of organic matter on clay particles.
c. the formation of metal-organic compounds such as

Ca, Fe and Al humates.

There is no great variation in texture of the soils
studied except pedon 4. It seems that clay content and the
formation of Fe, Al humates and organic-inorganic complexes
will probably play greater roles in the reasons presented.

Difference in altitude could be another factor that
can account for the different levels in organic carbon con-
tent among well-drained soils. The relief of the area under
study is gently sloping to almost flat with the highest
site $10pe being about 2%. The difference in elevation,
however, would be reflected in the relative air temperature
levels. Trewartha (1954) states that there may be an
increase of 6°C per 100 m decrease in altitude. Ignatieff

and Lemos (1963) showed that at high temperatures above

99

25°C organic matter is destroyed more rapidly especially
in coarse textured soils.

Much of the differences in organic carbon content
may also be related to land use. Cultivation of crops in
most of the area involves burning the vegetation before
tillage of the land. Since much of the area under study is
used for cultivated crops with the subsistence fallowing
system, the lower organic matter content would be attributed
to this factor as well. However in all the pedons which
have low organic matter content the decrease in organic
carbon with depth is gradual. Despite this decrease with
depth all the pedons contain some organic carbon even in
the lowest horizons examined.

The carbon contents of the well-drained soils can
be considered in relation to nitrogen contents, rainfall
and elevations. The regression coefficients show a sig-
nificant positive relationship between these factors and
the carbon contents of the soils. In other words, the
data presented show that soils with high nitrogen contents
have higher carbon contents than those with low nitrogen
contents; soils from areas of high rainfall have higher

carbon contents than those from areas of low rainfall.

100

Regression coefficients, their standard errors and t values
of partial regressions of carbon on nitrogen andielevation

 

 

 

 

 

Regression " H
coefficients Standard Errors t value
Nitrogen Elevation N2 Elev. N2 Elev.
Carbon on 0.056 0.012 0.0059 .0017 9.49** 2.65

0.052 +

Prediction equation of % carbon on nitrogen is §

0.056X where X % carbon

y 6 nitrogen

 

**Highly significant at 5% level.

However, the partial regression coefficient of
carbon on elevation is barely significant. The nitrogen
contents of all the soils are low (Table 7). At high
annual temperatures the nitrogen and organic matter con-
tents seem to increase as precipitation becomes higher.

As one climbs from sea level to high altitudes a slight
increase in soil nitrogen and organic matter becomes
noticeable. The soils of the area are located on gently
sloping to almost flat topography but there is no marked
increase in elevation between them (approximately 150 m).

The nitrogen content of the soils range from 0.03-
0.19% with a mean of 0.09%. The carbon-nitrogen ratio
ranges from 2.2 to 15.2 (mean of 6.6). The results of the
data (Table 7) show that there is a slight increase in C/N
ratio with increasing rainfall. There is no significant

relationship between C/N ratio and elevation. With limited

101

data on ranges of mean annual temperatures of the area, no
relationship is found between the C/N ratio and temperature.

Cultivational history and rainfall appear to be some
of the major factors governing the organic matter and
nitrogen contents of the soils studied.

Temperature may play a minor role. Prescott (1931)
found no direct effect of temperature on the nitrogen con-
tent of Australian soils over a range of 13°-25°C (56°-77°F).
Presumably the relationship between rainfall and organic
matter or nitrogen content of the soils is a relatively
simple one in which increasing rainfall favors increasing
vegetative growth and therefore production of organic
matter without having much effect on its rate of decompo-
sition. The role of the cultivation method involving
"slash and burning" of vegetation can be seen in the area
especially with soils on the higher surfaces (pedons l, 2,
3, and 7) where there are fewer thickets and thus greater
ease of burning the vegetation. Except in pedon sites of
uncultivated areas the increasing total nitrogen content
of the surface soil can be clearly associated with rise
in elevation.

The influence of temperature on the other hand is
more complicated in that increasing temperature, within
limits, favors both the rate of production and decomposition
of organic matter. In the studied area with its alternating

dry and wet seasons the effect of high temperatures on

102

microbiological activity seems to be nullified during part
of the year by the adverse effect of dry conditions. This
would slow down organic matter decomposition in the sur-
face soil without necessarily slowing down vegetative growth
and organic matter production provided moisture is avail-
able in the subsoil. Moreover, a rapid breakdown of plant
residues does not preclude a build up in the soil of stable
humus more resistant to microbial activity. In view of
such modifying influences it appears that too much emphasis
has been placed, in the past, on temperature as a major
factor in organic matter decomposition in the trOpics.

It becomes more evident that within the study area
the problem is not so much one of building up the available
nitrogen in the soil as of conserving that already present.
This could be done by determining the best cropping systems
and management practices to maintain a desirable available
nitrogen balance. Such practices should include adequate
protection against erosion, a prOper selection of crop
rotations (including legumes), conservation of crop residues
and doing away with bush burning which is prevalent in the

area .

Exchangeable bases.--In all nine soils exchangeable

 

bases extracted with neutral 1N-NH4OAC are very low
(Table 8). Most pedons have 1.2 to 0.12 m.e. exchangeable
bases/100 g soil throughout their profiles. However,

pedon 4, a sandy spodosol, has less than 0.06 m.e./100 g

103

 

 

 

 

 

 

 

 

 

N.NN N.N N.NNH N.N NH. HN. NH. NUNNN HN. mung» NUNNN NNN-NNH ENHNN
N.NN H.H N.N NN. HN. NN. NUNNN HN. NUNNN compo NNH-NN NNN<
N.NN N.H H.H NN. NN. HN. «UNNN HN. HN.N conga NN-NH NHN<
N.NN N.H N.NNH N.N NN. NN. NN. HN. NN. HN.N HN.N NH-N N<
sz<--N conN
N.NN N.N N.NH N.N HN. NN. NN. NN. NN. NN.N H.N NNH-NNH NN
N.NN N.N N.NH N.N NN. NN. NH. NN. NN. NN.N H.N NNH-NN NNNN
N.NH N.NH N.NN N.N HN.H HN.H NN. NN. NN. NN.N N.H NN-NN NHNN
N.NN N.N N.N NN. NN. NN. NN. NN. NN.N H.N NN-NH HN
N.NN N.N N.NN N.N NN. NN. NN. NN. NN. NN.N H.N NH-N N<
NN2<--N NNNNN
N.NN H.N H.N N.H NN. NH. NH. NN. NN. HN.N NN.N NNH-NNH NN
N.NN N.N N.NH N.N NN. NH. NH. NN. NN. HN.N NN.N NNH-NN NNNN
N.NN H.N N.NN N.N NN. NN. NH. NN. NN. NN.N NN.N NN-NN NHNN
N.NN N.N N.N NN. NN. NN. NN. NN. NN.N NH.N NN-NH HN
N.NN H.N N.NN N.N HN. NN. NN. NN. NN. NN.N NN.N NH-N N<
Nz=<--N cowoN
N.NN N.N N.N N.N NH. NH. NN. NN. NN. NN.N HN.N NNH-NN NN
N.HN H.N N.HH N.N NN. NN. NH. NN. NN. NN.N NN.N NN-NN NNNN
N.NN N.N N.NH N.N NN. NN. NH. NN. NN. NN.N NN.N NN-NN NHNN
N.NN N.N N.N NN. NN. NN. NN. HH. NN.N NN.N NN-NH HN
N.NN N.N N.NN N.N NN. NN. NN. NN. NH. NN.N NH.N NH-N N<
HNz<--H NoNoN
HNoN N NNH\ms
N N NHx
N M EU
.NNN .NNN NNHu N\omu umo H< + momma momma M+H< +Nz +N ++ z ++No NNNNN NoNHNoz
c~< ommm mo saw we saw

Huxm

momma oanmowcmaoxm

 

.coNumuaumm ommn acouuon pcm Nuwomamo owcmguxo :owumo .H< ofinmwomuuxo .mommn oanmowcmsoxm--.m oHnmh

104

 

 

 

 

 

 

 

 

 

O.Hw N.m O.N~ N.N HN.H. Hm. ON.H HO. NO. NH. OH.O ONH-ONH mmHH
m.mm N.O 0.0H O.m NO.H mm. OO.H mO. NO. NH. OH.O ONH-~N mumNmHH
N.No m.O O.NH N.N OO.H Om. ON. NO. mO. NH. mH.O ~N-ON NHNNm
H.Hm H.N O.N~ H.N HH.H HN. OO. HO. NO. OO. OH.O ON-m~ muHNm
0.0m 0.0 O.N Nm. NN. NN. HO. OO. NH. N0.0 m~-OH m~<
o.mH N.NH m.~o O.m OO. on. NH. NO. NO. ON. OH.O OH-O n<
w:m--w :owom
0.0n m.HH 0.0H H.m Om. mm. mH. NO. NH. NO. OH.O OH~-OmH Nmm
0.0N N.HH O.NH m.m ON. ON. OH. NO. mH. OH. HH.O omH-OOH Hmm
N.NN O.N 0.0H O.N NO. mm. on. mO. OH. OO. OH.O OOH-mm uNNm
N.On m.O O.~N N.N mm. mm. mN. mO. NO. OH. NH.O mm-ON uHmm
H.HN H.N H.mm m.m mm. mN. On. NO. HH. NH. OH.O O~-O q<
N22--N cocoa
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o.mm m.o O.m~ m.m Nm. NN. om. NO. NO. OH. O0.0 mNH-OHH -m
~.mm m.o O.~m N.N no. Om. mm. mO. NO. OH. OH.O OHH-mm Hmm
5.0m O.m ~.m OO. om. Nm. mO. NO. NO. O0.0 mm-wH Hm
N.Hm N.N N.Nm N.N mm. NN. Om. mO. OO. OO. OH.O wH-O . m<
Ox<--o cocoa
N.Nm N.N 0.0~ N.N HO. ON. mm. NO. NO. OH. N0.0 OwH-OmH mm
O.Hm m.o 0.0H N.N NO. Hm. mm. NO. OO. OH. N0.0 omH-ww HNNO
O.NN m.o O.N~ H.N mo. mm. Om. OO. OH. OH. N0.0 NN-Nm uHNm
m.n~ O.m~ 0.0 NN.H ~H.H NN. NO. NO. OH. O0.0 mm-mH Hm
O.m~ N.OH 0.0mH m.N HN.H NN.H um. NO. HH. NN. HO.H mH-O AH<
m:w<--m cocoa
N N HHoN N NNH\NE
NpOHOH w so
.umm .uwm meu N\umu umu H< + momma mommm m+H< +mz +x ++ z ++mo gumbo :oNHHo:
«H< ommm mo Esm mo saw uuxm momma oHpmomcmcuxm

 

 

.NNNNHNNON--.N NHNNN

105

 

 

 

 

 

 

 

H< .Huxm + mommm mo Ezm
OOH x u :oHumHSHNN H< N«
H< oHnmuompuxm
0.0m 0.0H O.NH O.N ON.O ON. ON. NO. OH. NO. N0.0 ONH-NN NNHH
N.NN N.OH 0.0N N.N NN.O mm. mm. NO. OH. NO. N0.0 NN-NN NuNNmHH
N.NN N.NH O.NN N.N Nm.O Om. HN. NO. ON. NH. OH.O NN-OH NHHNNHH
H.No O.N N.N NN.O Om. cm. NO. OH. NO. N0.0 OH-N N<
N.NN N.NH N.NN N.N NN.H NN. ON. NO. OH. om. mm.O N-O H<
mkm--m :owmm
. N HHoN N NNHEE
a
OHx
N Eu
.umm .umm meu N\umu omu H< + woman woman M+H< +mz +x ++Nz ++ canon :oNNHo:
«H< mmmm mo Esm mo Esm

Huxm

mommm oHnmomcmcuxm

.NNNNHNNNN--.N NHNNN

106

throughout. Differences associated with genetic sequences
are not very evident.

Among the more poorly drained soils (pedons 4, 8
and 9) pedons 8 and 9 have more exchangeable bases than
pedon 4. Pedons 8 and 9 developed in fine-loamy and coarse-
loamy materials, respectively. It is probable that, among
other things, a more suitable environment for microbial
activities is provided by pedon sites 8 and 9 than pedon

site 4.

Base saturation.--All the soils have low base

 

saturations, ranging from 0.3 to 28%. The percent base
saturation is highly dependent on the method used for its
determination due to the considerable portion of the pH-
dependent charges (pH—dependent CBC) present on soil
colloids (Kamprath, 1970; Kamprath and Foy, 1971). As
mentioned earlier the magnitudes of these percent base
saturations may be associated with microbial activities
and the forest ecosystem of the individual pedon sites.

The very low contents of the exchangeable bases
in these soils indicate a deficiency in weatherable min-
erals. Continuing leaching losses may also account for
their low content. Some conclusions can be made from the
exchangeable bases as follows:

1. Associated with high sand contents (47 to 98%)
and sandy to fine-loamy materials in these soils,

especially pedon 4, all the pedons have low K, Mg

107

and Ca contents. Thus they have low content, if
any, of weatherable minerals. These data appear
to represent the minimal values that are reached

by advanced weathering in such soils.

 

2. In a few cases the upper horizons (pedons 5, 8 and
9) have a slightly higher content of exchangeable
bases. This is attributed to recycling of nutri-
ents by vegetation and manuring.

3. A comparison of the average values (X) and ranges
in Figure 11 with a few soils in other parts of the
country (Ford, 1968) shows similar values as indi-
cated below.

Site H Exch. K Exch. Ca ExCh. Mg Tentative
p me/100 g me/100 g me/100 g Classif.
Abak 4.5 0.09 0.25 0.18 Typic
Paleustult
Calabar 4.6 0.04 0.12 0.12 Typic
Paleudult
Benin 5.2 0.05 0.44 0.25 Oxic
Paleudult
NIFOR 5.4 0.08 1.88 0.52 Oxic
Paleudult
Umudike 5.0 0.06 0.90 0.15 Ustoxic
Paleustult
x (ranges) .07(o.-o.2) 0.17(0.-1.01) 0.09(0.-0.6)

The Calabar, Benin and NIFOR sites are within the Coastal
Plain Sands, but located outside the study area. All
these soils are used for oil palm production.

108

.x Nam N2..mu oHnmowcmgoxo mo :oHuanHHme Nocoscmpm--.HH .NHN

000.}... 52320.". 000.88 828502 009qu E228
cm. 2. 2. mo. 3. 0 mm. N.N. ON. 9. ~_. No. No. 0 mm. ow. Q. ~.. mo. v0. 00
_ f H

r

 

 

 

 

 

 

 

 

 

 

 

 

 

 

o.

 

 

 

 

 

 

 

 

 

ON

 

 

 

 

 

No.0 u u. 8.0 n M :6 u m.

«d. o HNN....N. m.e-o "3:3. 5.70 88m o
IIIIIJ m

 

 

 

 

 

 

o¢

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

seldwos 4o iuaomd

109

Significance to land use and management.--It has

 

been suggested that the absolute value of exchangeable Mg
may not be an effective way to evaluate the availability of
this nutrient for crOp growth. A level of 4% of Mg satu-
ration has been used for Mg soil test interpretation on
Ultisols (Adams, 1962; Henderson, 1970). Only the surface
horizons of pedons 5, 8 and 9 exceed this value.

According to Juo and Ballaux (1977) leaching
losses of applied Ca in highly weathered soils in the low-
land tropics may not be as great as generally believed.

A substantial portion of applied Ca would be adsorbed by

Fe and Al oxides and hydrous oxides. Any Ca ions leached
downward may be retained in the lower horizons which are

also normally depleted in calcium.

Low levels of available Ca and Mg and the high
level of exchangeable Al saturation are important limiting
factors in food production on Ultisols and Oxisols in the
humid tropics (Juo and Ballaux, 1977). The low levels of
Ca and Mg under the high rainfall prevalent on the lower
Coastal Plain Sands area must have been affected to some
extent by the prolonged cropping of the area.

Excellent yields may be given by oil palms (Elaeis

guinnensis) on some of the very acid soils of extremely

 

low exchangeable Ca content, especially if K and Mg supplies
are maintained by manuring. However, with other cr0ps,

these soils may cause special problems, and it may then be

110

more necessary to recognize distinctions between these
soil conditions and those of forested regions of less
intense leaching. In the study area, 'liming' has long
been practiced by local farmers in the form of "slash and
burn" of vegetation. With increasing population density
this practice has approached a level of diminishing return
as a result of shortened fallow periods. To establish
more permanent and productive cr0pping system on these
Coastal Plain Sands, the use of commercial fertilizers
and liming materials are highly encouraged. If a major
portion of applied calcium is leached below the rooting
zone of shallow-rooted crops, planting short-term fallow
species with deep-rooting systems could effectively
recycle calcium back to the surface layer of the soil
providing that the plant residue is returned either as
mulch or by burning. However, if economic lime amendments
are not available this may prove impractical with current
land use pressures.

The low values of exchangeable K still support
cr0p production. This may be due to the fact that even
in soils containing large amounts of total K, only a small
part, usually less than 1% of the K is in exchangeable
form, and much smaller amounts are in soil solution (Doll
and Lucas, 1973). Most of the nonexchangeable K tends to
be released very slowly as the exchangeable K is depleted

from the soil. Consequently, taxonomically speaking, these

111

data are not very helpful as they give only an indication
of a minimum amount of the available K present in these
soils. They are, however, very important from the fer-
tility viewpoint as good and very conclusive correlations
have been obtained between the exchangeable K and crop
responses to fertilizers (Conyers and McLean, 1969; D011
and Lucas, 1973). Content of weatherable K minerals is
also needed in addition.

With the prevalent high rainfall of the area
(2000-2500 mm annually) accumulation of nutrients such as
K is not likely to be appreciable. Lutrick (1958) con-
firmed the downward movement of added KCl in profiles of
two Ultisols. Despite the effect of leaching, his data
showed a considerable accumulation of K in the surface
soil with or without K fertilization. It has been a
common experience for some previously uncultivated soils
of the Coastal Plain Sands to show little or no crop
response to K fertilization especially after a long fallow
period. In oil palm growing areas, this lack of response
could continue for several years before yields decline.

These observations suggest that the soils either
contain an adequate supply of K (the determined low
exchangeable values notwithstanding) to satisfy crop needs
for a considerable period of time or possess properties
through which the soils retain K against leaching losses.

This might seem an anomaly considering the low values shown

112

by data in Table 8. Another explanation may be the fact
that the root system of the crops extend to various depths
of soil profile for nutrients and this may account for

the lack of crop response.to K fertilization in these soils
during the first few years of cultivation. This would be

particularly true for annual crops such as corn (Zea mays)

and soybean (Glycine max L) which have a relatively low K

 

requirement.

Exchangeable Al and Al saturation.--Exchangeab1e

 

Al represents aluminium extracted with unbuffered lN-KCl.
In calculating Al saturation the sum of exchangeable bases
and Al is used as an approximation of the cation exchange
capacity (ECEC) at field pH. According to Kamprath and
Foy (1971), the A1 saturation of the ECEC of the soil
greatly affects the Al concentration of the soil solution.
Aluminium is so tightly held by soil colloids that only
when the A1 saturation is about 60 percent of the ECEC
does a substantial amount of Al become present in the soil
solution (Nye g; 31., 1961; Evans and Kamprath, 1970).
Table 8 shows data which indicate that exchangeable
Al and Al saturation increases with depth in pedons 5 and
8, attaining Al saturation values of 57 and 83% in a lower
horizon. In these pedons A1 saturation and % base satu-
ration show an inverse relationship. Pedon 4 has the

highest Al saturation ranging from 87.5% in the Ap horizon

113

to 97.8% in A22g and 92.3% in the Bhirm horizon. The
sandy nature of this pedon and its extreme acidity in
addition to its low content of exchangeable bases account
for the high A1 saturation values.

For soils on the higher surfaces pedons (l, 2, 3
and 7) considering their relatively higher clay contents
and their low base saturation percentages, the exchangeable
Al values are low. These values are 0.25 to 0.34 m.e./

100 g in the surface horizons and they decrease with depth
to as little as 0.05 in the lower parts of the sola. Al
saturation values are high, however, up to 55.7% due to
the low CEC of these soils.

In acid soils exchangeable Al is generally recog-
nized as the predominant cation as determined by the
exchange process with a neutral salt solution, like KCl.
This determination is based on the assumption that aluminium
ions are counter ions balancing excess permanent negative
charge on the surface of the solid phase. According to
Amedee and Peech (1975) this method of determining exchange-
able Al by extraction is not satisfactory.

In view of these results it is interpreted that in
the poorly to somewhat poorly drained soils (pedons 8 and
9 respectively) on the lower surfaces, most of the Al is
in exchangeable form. On the other hand, for soils located
on higher surfaces, where ApH values are low and negative,

most of the Al is in nonexchangeable form. However, the

114

level of A1 saturation in most of the soil samples is not
high enough (less than 60% Al saturation), at plow layer
at least, to prevent or seriously affect the growth of
most crops in the study area. A possible potential limi-
tation, if the Al saturation is too high, may arise from
the effect of the fertilizer salt concentration on the
concentration of aluminium in the soil solution. It can
be expected, for example, that heavy K fertilization will
increase the A1 concentration in the soil solution of these
soils with low exchangeable K (MacLeod and Jackson, 1967).

Present theories concerning acidic Al in soil
suggest that concentrations of the specific forms of Al
depend on soil pH and the types and amounts of clay and
organic matter present (Pionke and Corey, 1967). Using
simple correlation it is found that pH is most highly
correlated with exchangeable A1 (Table 9). The negative
correlation coefficient is affected by the salt concen-
tration used in the pH measurement, being greater for pH
(KCl) than for pH (H20). This phenomenon is probably due
to the effect of the hydrogen ions released during the
hydrolysis reaction associated with the aluminium displaced
by lN-KCl, the extractant for Al.

At constant pH and a given concentration of Al+3
in solution, an increase in organic matter would increase
the nonexchangeable acidic A1 (that is Al extracted at pH

4.8 lN-NH OAC minus KCl-exchangeable Al) most likely

4

115

Table 9.-—Correlation coefficients for the relationships
between some soil properties.

 

 

Soil Properties All soils, n = 45
pH (H20) vs. Exch. Al -0.59l**
pH (KCl) vs. Exch. Al -0.725**
pH (H20) vs. % Organic matter -0.154*
pH (H20) vs. % Clay n.s.
ApH vs. Exch. Al -0.332*

 

*Significant at 5% level.
**Significant at 1% level.

n.s. Not significant (significant at 20% level).

at the expense of the exchangeable Al (Pionke and Corey,

1967).

Cation exchange capacity (CEC).--The cation

 

exchange capacity determined by lN-NH4OAC (pH 7) is
shown in Table 8. The summation of exchangeable bases
plus aluminium is also shown. It is realized that the
NH4OAC (pH 7) method includes both the permanent and pH-
dependent components. In all the soils the CEC by lN-NH4OAC
(pH 7) is much greater than the CEC by sum of bases plus
aluminium.

In comparison with soils of the temperate regions,
the CEC of these soils, including pedon 4, a Spodosol

developed in sand, is very low. In the well-drained pedons

116

differences associated with genetic sequences are not evi-
dent.

Within the soils with aquic moisture regimes,
pedon 8 (fine-loamy) has more CEC than pedons 4 and 9,
sandy and coarse-loamy, respectively. These soils con-
stitute a lithosequence of soils.

In all the soils, except pedons 4 and 8, the CEC
values decrease with depth and this seems to be correlated
with a decrease in organic matter content. Pedon 8 shows
only small variations of CEC with depth. The small organic
carbon content in the Ap, decreasing with depth, is appar-'
ently offset by the increasing clay contents with depth in
this Tropaquult. It is apparent from the usual depth
distribution of CEC values that the organic matter fraction
would be one of the major sources of CEC especially in the
upper horizons and in the Bhirm of pedon 4. A regression
analysis was made of the data relating CEC to organic
carbon and clay content on all of the nine profiles as a

group, on the surface horizons (A A2) plus Bhirm, and

1:
subsequently on the other subsoils and on the subsoils
on the lower and higher surfaces.

The regression coefficients show a significant
positive relationship between CEC of the surface horizons
plus Bhirm and organic carbon or percent clay. It is also

shown that the clay of the subsoils on lower surfaces and

CEC are significantly correlated. All subsoils (B2 or B3

117

horizons), except Bhirm, have low CBC/percent clay values

(Table 8).

This is to be expected since kaolinite, as

will be shown later, is the dominant mineral in the clay

fraction.

These subsoil values are higher for soils on

lower physiographic surfaces than for those on higher

physiographic surfaces.

Regression coefficients, "F" and "r" of CEC on percent clay

 

andgpercent organic carbon

 

 

 

 

 

Regression
coefficients "F" "r"
% clay O.C. % clay O.C. % clay O.C.»
CEC (all horizons .46 .17 .44° 3.45° .09 .27
of 9 soils)
n = 45
CEC (A , A , .38 .26 126** 12.41** .95 .71
Bhirm horizons)
n = 14
CEC (all other -.04 .39 1.94° -98.9° .25 .57
subsoils except
Bhirm) n = 31
CEC (all other -.05 - l3 l.28° l.93° -0.29 -0.36
subsoils on
higher surfaces)
n = 16
CEC (all other -0.22 .29 80.69** .23° .52 .41
subsoils on
lower surfaces)
n = 15
°Not significant, even at 20% level.

**Significant at 5%.

O.C.

r:

= organic carbon

correlation coefficient.

= variance ratio

118

The prediction equations for the above significant rela-
tionships are as follows: (Y is the % organic carbon,

X is the CEC, Y = CBC and KC = % clay)

a. Y .26X + .35 for all horizons A A2, Bhirm.

1’

b. Y = .15 + .38Xc for all horizons A1, A2, Bhirm.

8.62 = .22Xc for all other subsoils on lower
surfaces.

0
r<
II

The CEC values given by the sum of exchangeable
bases and exchangeable A1 are appreciably lower than those

represented by lN-NH4OAC pH7. As mentioned previously the

CEC obtained by the NH4OACpH7 method includes both the
permanent and the pH-dependent components, hence would
not be indicative of the magnitude of this property at

field reaction (pH) level of the soils.

According to the data obtained the CEC values of
the subsoils on the higher surfaces are very low consider-
ing that they are higher in clay content (pedon 1 has a
maximum of 34% clay) and contain more organic matter than
most other soils. According to Sumner (1963) this marked
reduction in CEC of the soils on the higher surfaces can
be attributed to one or both of the following:

1. Reduction in negative charge or even generating

a positive charge by the mutual neutralization of

the negative charge on the clay by the positive

charge in the iron oxides. This effect is

119

reflected in the low CBC/100 gms of clay found in
the lower part of the sola of the soils on the
higher surfaces (8.-25. m.e. versus l6.-32 m.e.).
2. The negative charge sites on the clay may be
blocked by iron oxide covering. According to
Sumner (1963) this blocking effect increases with
increasing degree of crystallinity of the iron
oxides. The appreciably higher Fe2 3-d minus
FeZOS-O in the higher well-drained soils (.90-3.0%)
indicate more crystalline iron oxides than are

found in the well-drained soils on the lower surfaces

(045-090%)o

Extractable iron and aluminium oxides.--Iron and
aluminium oxides and hydrous oxides are among the major
components of soils in the tropics. It has been shown
that these oxides exist in the soil in the forms of amor-
phous and crystalline inorganic oxides. Selective extrac-
tive methods have been used to differentiate these forms
of Fe and Al. The acid ammonium-oxalate method (Schwert-
mann, 1964) extracts mainly the amorphous forms of inor-
ganic Fe and A1 while the free oxides as well as the major
portion of organic complexes can be determined by the
method of Mehra and Jackson (1960) using a dithionite-
citrate-bicarbonate system. These procedures have been
successful in differentiating forms of Fe and Al in some

temperate soils (McKeague et 21., 1971).

120

Table 10 shows the contents of these two forms of
extractable Fe and Al in the nine pedons studied inten-
sively. The dithionite-citrate-bicarbonate Fe is referred
to as the free iron. However, it cannot be assumed that
all of the Fe not retained in silicate mineral lattices
is extractable by this procedure (Moore, 1973). The
dithionite-extractable Fe content generally increases
with depth in eight of the nine soils studied (all but
pedon 9), but shows a maximum in the upper B horizon of the
more poorly drained soils (pedons 4, 8 and 9). The well
drained soils on the higher physiographic surfaces (pedons-
l, 2, 3 and 7) have Fe contents ranging from 0.76 to 3.22%
in their horizons. In all these well-drained pedons the
AlZOs-d is less than the FeZOS-d, and the amount of free
Fe oxides seem to follow the same distribution pattern as
the clay content within the same profiles as shown in
Figure 12A, B and C. This clearly indicates the combined
movement of Fe and clay into the subsoil.

The values of the FeZOS-d contents for the soils
on lower physiographic surfaces (pedons 5, 6, 8 and 9)
are considerably lower, within the range of 0.09 to 1.12
in the 0-30 cm and 1.12 to 0.10 in the subsurface horizons.
In all of these the A1203-d exceeds the FeZOS-d.

In the more poorly drained soils (pedons 4, 8 and

9), the extractable Al parallels the clay contents as

shown in Figures 12 D and E. Apparently Fe may be lost

121

O.NH

 

 

 

 

 

 

 

 

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122

 

 

 

 

 

 

 

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123

 

 

 

 

 

 

 

 

 

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125

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127

 

 

 

 

 

Clay Content, °/o
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Fig. 12D.--Continued.

128

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129

during profile development under these moisture or drainage
conditions. The differences may be related to their aquic
moisture regimes.

As shown in Figure 13, the clay—dithionite Fe
ratios in the well drained pedons 1, 2 or 3 and S or 7
usually increase as a function of depth, but is higher in
the last two on the lower surfaces. By contrast, these
ratios are much more variable with depth in the more
poorly drained pedons (4, 8 and 9). Also, the clay:
dithionite-Fe ratios (Table 10), may decrease or increase
with depth instead of following the clay content distribu-'
tion as in the aforementioned pedons. A marked accumu-
lation of FeZOS'd is evident deep in pedon 4, Figure 12D.
This is paralleled by a high organic content as mentioned
earlier. In pedon 6, there is no particular trend in the
clayzdithionite-Fe ratio with depth. This soil is an
Oxic Dystropept, as discussed later, with no argillic
horizon. This increase and decrease pattern in this pedon
suggest that either the small Fe content difference is
partially independent of any clay illuviation or it is
due to differences in parent material.

Examination of the Fe203-dithionite/coarse silt +
sand values in Table 10 shows almost constant pattern and
high values with depth in all the first three soils on
higher land surfaces. These values are lower and more

variable with depth in the more poorly drained soils

Fig.

130

 

 

 

 

 

 

 

 

o
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HHHHH Pedon | ------- Pedon 5
Pedon 2 -—---— Pedon'?
—--— Pedon 3

 

13.--Relationship between clay/dithionite-Fe ratio
and profile depth.

 

131

(pedons 4, 8 and 9). This also confirms the movement of
iron oxides in the more poorly drained soils since the
coarse-silt and sand are weathered to a fairly uniform
suite of resistant minerals in all the horizons. This
fact is indicated in the mineralogy of the coarse-silt and
sand fractions studied with a petrographic microscope as
discussed later. The FeZOS-dithionite/coarse-silt + sand
ratios are also lower on the well drained soils on the
lower land surfaces (pedons 5, 6 and 7) but not so variable
with depth as in the more poorly drained soils.

In the comparison between iron oxide and clay
movements one or two means of movement can be implied.
The iron oxides may have moved by physical translocation.
Reduction, solution, migration with organic matter and
reprecipitation may also occur as shown by pedon 4. The
different amounts of iron oxides in the different horizons
in pedons l, 2, and 3 seem to show a trend typical of
illuviation by suspension with finer particles increasing
into the lower horizons. Another pathway by which iron
could have been transported down the soils is by absorp-
tion on, and subsequent movement with, clay particles.
According to Parks (1965) when hydrous iron oxides are
freshly precipitated they have iso-electric points above
pH7 and will be positively charged in soils with lower
pH values. At an early stage of weathering, they may have

been absorbed on to negatively charged clay surfaces and

132

moved downwards with the clays. Although this mechanism
seems highly speculative and there is no present evidence
either for or against it, it does in part explain the
similarity between the clay and iron oxides distributions
with depth and the close range of the clay-dithionite Fe
values in the old well drained soils. However, even if it
prevailed at an early stage of soil formation, it is no
longer operative as the iron oxides are presently highly
crystalline with very low iso-electric points.‘

The amounts of dithionite or oxalate extracted A1
are lower in comparison with those of iron for pedons l, 2,
3, 4 and 7, but the distribution of these forms of Al
according to depth is similar to that of Fe except in
pedon 4 which shows no strong accumulation of Al deep in
the profile. For the soils on the higher surfaces (pedons
l, 2, 3 and 4) the amounts of Al extracted by the two
methods are less than on the lower surface soils while
for Fe the reverse is true. This increase in extractable
Al contents as one goes from high elevations to sea level
is in agreement with increasing rainfall in the area
studied. However, the more poorly drained pedons 8 and 9
show much larger amounts of extractable Al and these paral-
lel the profile clay contents, Figure 12E.

The relative abundance of Fe and Al in the amorphous
forms is much less when compared with the results obtained

by McKeague and Day (1966) and McKeague 33 31. (1971) on

133

soils of the temperate region. McKeague and Day (1966)
found amorphous Fe203 values between 0.47-5.56% in their
study. The low content of amorphous Fe and Al (ammonium-
oxalate extractable) fractions in the present study may
be due to the high temperature condition and the alter-
nating wet and dry season prevalent in the Coastal Plain
Sands area. Sherman £3 31. (1964) stated that drying at
elevated temperature causes the amorphous Fe and Al oxides
to dehydrate and subsequently shift to a system of greater
crystallinity.

The distribution of oxalate extractable Fe or Al
to total free Fe or Al within each pedon can be conveniently
expressed as "active Fe or Al ratio" (Blume and Schwertmann,
1969). These ratios are obtained as follows:

Oxalate Fe or A1

Active Fe or Al ratio = x 100
Dithionite Fe or Al

 

Table 10 gives the active Fe and Al ratios for the nine
pedons, as percents of the dithionite forms of each.

Ratios of active Fe in most of the six well-drained pedons
(except pedon 3) decrease with depth while active Al ratios
increase (except pedon S). The comparatively low values

of oxalate-Fe and Al in pedon 4 may account for the erratic
pattern of the active ratios in this profile. The high
oxalate and dithionite extractable Fe in the deepest

horizon of this profile, a spodic horizon, are most striking.

134

The decreases in active Fe with depth indicate that
higher proportions of Fe are present in more crystalline
forms in the lower horizons of the well-drained profiles.
The higher ratios observed in the surface horizons reflect
the large amount of amorphous Fe released by weathering
near the surface (Follet £3 31., 1965) and the retarded
rate of crystallization of Fe oxides and hydroxides in the
presence of organic matter (Schwertmann £3 11., 1968).

Nevertheless, the same relationship does not hold
true in the case of Al where the active Al ratios are cal-
culated in the same way as for Fe. The actual chemical
and physical forms of Al in soil extracted by the dithionite
and oxalate reagents are poorly understood. In view of
this no further interpretative comments can be made at this
point.

The low amounts of extractable and active Fe in
the more poorly drained soils, pedons 8 and 9, can be
explained by the oxidation-reduction regime and related
soil-ground water movement at each site. Soils located
on the higher surfaces have deeper water-tables than those
on the lower surfaces. The former soils have undergone
longer periods of alternating oxidation and reduction than
soils near sea level. This fact can be reflected by the
larger amounts of extractable Fe shown by the results
(Table 10). Soils at the higher elevation and older in

age probably have lost the smallest amount of iron from

135

the solum and may have had the Opportunity to accumulate
iron through biocycling. Since these soils are well
drained and the sola have less reducing conditions, little
or no iron is lost. As stated earlier, there have been
translocation of iron along with clay within the 5013 of
some pedons (l, 2 and 3). The pedons with the more pro-
nounced increase in clay and iron within the solum may have
been changed much less by soil development, in terms of
total iron and clay than soils at the lowest surfaces with
little or no increase in iron content in the B horizon.
Also in the poorly drained sites with permanently gleyed
horizons, precipitation of iron oxides can be envisaged

as being caused by occasional flushes of oxygenated water,
which oxidize ferrous compounds and decrease their solu-
bility. There is also the possibility of microbial oxi-
dation, for example, by Ferrobacillus ferro-oxidans.
Silica and aluminium are unaffected by change in redox
potential and will tend to stay in solution, leaving com-
paratively unadulterated amorphous ferric compounds to
precipitate, as in pedon 4 herein (Deshpande 33 31., 1968).
Alternating wetting and drying seem to be essential for
clay movement in well-drained soils (Thorp 33 31., 1959),
and this cyclic moisture regime would reach its maximum
intensity in the areas with shallow water-tables, such as
in pedons 8 and 9. Juo 33 31. (1974), working on selected

soils of Nigeria, found similar amounts, distribution

136

and co-migration of clay and dithionite-extractable Fe
(Lessivage).

Distribution ofgphosphorus forms.--The amounts of

 

the various forms of phosphorus are given by horizons in
Table 11.

All the forms of phosphorus usually decrease with
depth in these soils. Pedon 4, a poorly drained, very acid
sandy soil with a spodic subsoil horizon, has an increase
of organic-P and Fe-P in the subsoil. Al-P and Ca-P are
very low in this pedon. Ca-P while very low in pedon 3,
increases somewhat with depth.

Similar amounts of available phosphorus are

extracted by the Bray P and North Carolina methods. There

1
is a close correlation between the available phosphorus
determined by these methods (r = .79, significant at 5%
level). The relative merits and demerits of these methods
are not determined in the present study as this would
involve correlation of the chemically determined available
P and applied phosphorus with crop response. The two
extractants are similar in selectivity of solution of
available P in acid soils hence the high degree of corre-
lation. The selection of a suitable routine soil chemical
test for soil available P must be based on the distribution
pattern of the active inorganic P forms present in the

soils according to Uzu 33 31. (1974) and Chang and Juo

(1962).

137

 

 

 

 

 

 

 

333 333 3.3 33. 3.3 3.3 3 3.3 3 333-333 33
33 333 3.3 33. 3.3 3.3 33 3.3 3 333-33 3333
33 333 3.3 33. 3.3 3.3 33 3.3 3 33-33 3333
33 333 3.33 33. 3.3 3.3 33 3.3 3 33-33 33
33 333 33.33 33. 3.3 3.33 33 3.3 3 33-3 33
3323--3 33333
33 333 3.3 3.3 3.3 3.3 3 3.3 3.3 333-333 33
33 333 3.3 3.3 3.3 3.3 33 3.3 3.3 333-33 3333
33 333 3.33 3.3 3.3 3.3 33 3.3 3.3 33-33 3333
33 333 3.33 3.3 3.3 3.33 33 3.3 3.3 33-33 33
33 333 3.33 3.3 3.3 3.33 33 3.33 3.33 33-3 33
3223--3 33333
33 333 3.3 33. 3.3 3 33 3.3 3.3 333.33 33
33 333 3.3 33. 3.3 3 33 3.3 3.3 33-33 3333
33 333 3.3 33. 3.3 3 33 3.3 3.3 33-33 3333
33 333 3.33 33. 3.3 33 33 3.3 3.3 33-33 33
33 333 3.33 33. 3.3 33 333 3.33 3.33 33-3 33
3323-3 33333
3 03333 Ema Eu
.333 .mm% 3 3>3333 3-33 3-3< 3-33 3 .333 .z 33 3333 33333 333333:
.333-3 33333 3 3>333< 3333333 3 3333333><
.moHHHUmmmo

:ofiumXHW-m can .303333 3\u oHcmmpo .mzpocmmozm mo mapom 330333> mo mucsos<-.HH manmh

138

 

 

 

 

 

33 333 3.3 3.3 3.3 3 3.3 3 333-333 33
3.33 333 3.3 3.3 3.3 3 3.3 3 333-333 333
33 333 3.3 3.3 3.3 33 3.3 3 333-33 333
33 33 3.33 3.3 3.3 33 3.3 3 33-33 33
33 33 3.33 3.3 3.33 33 3.33 3 33-3 33
333--3 33333
33 333 3.3 3.3 3.3 33 3.3 3 333-333 33
33 333 3.33 3.3 3.3 33 3.3 3 333-33 3333
33 33 3.33 3.3 3.3 33 3.3 3 33-33 3333
33 33 3.33 3.3 3.33 3H 3.3 .3 33-33 33
33 333 3.33 3.3 3.33 333 3.33 3 33-3 33
333<-3 33333
33 333 3.3 33333 3.3 3.3 33 3.3 3.3 333-333 33333
33 333 3.3 33333 3.3 3.3 33 3.3 3.3 333.33 3333
33 333 3.3 33333 3.3 3.3 33 3.3 3 3 33-33 3333
33 333 3.3 33333 3.3 3.3 33 3 3 33-3 33
3zz<--3 33333
3 03333 Sun
.933 3\u o>3 um . . M3 :033302
333 .3 3-33 3-3< 3-33 3 333 z 33 33
.333-3 33333 3 3>333< 3333333 3 3333333><

 

.333333333-.33 33333

139

.ycmuumppxo vommz.z mNo.o + Hu:.z mo.o ”uogpoz acfifiohmu gupoz
.Hcmpumuuxm Hu:.z mmo.o + quz.z mo.o "Hm xmhm

 

 

 

 

 

 

mu mm a.“ m.m N.N m.H mm Q m.m omH-hw mmHH
om moH m.w o.e e.~ H.N mo m e 5w-wN maNNmHH
mu mu O.NH o.m c.q o.m on m m ”N-OH muHNmHH
on mm H.mH m.m N.m o.q CNH w w OH-~ N<
mm me N.HN N.” m.o o.o omfl NH HH N-O H<
mam--m cocoa
on mmfi o.m m.o O.N H.N ow o.m o.m owH-oNH mmHH
mu o“ m.m m.H O.N O.N oo o.m o.m om~-~h mumNmHH
on em m.h o.~ ~.m H.N on m.m m.m NN-OV muNNm
mo em Q.HH m.m m.v o.m mm O.N o ov-m~ muHNm
mo ow o.mH m.¢ N.m o.m moa m.ea CH mN-oH w~<
om ovfi o.o~ m.m H.o o.v ova O.NH ma OH-O g<
mzm--m cocmm
om omH q.m H.O w.o m.v a m.m m oHN-omH Nmm
mm mhfi m.“ H.O m.o m.o ca m.m m omH-ooH Hmm
om QVH o.m H.O m.o w “N e m oo~-mm HNNm
ow ooH o.a H.O m.o w mg m m mm-o~ ufiNm
on mnfi ¢.NH m.c m.H OH mu m w oN-o a<
52:--5 comma
w caumu Ema Eu
.amu mmu o>flpum m-mu Q-H< m-em m .pmu .2 Ha xmpm canon coNflpo:
.xww-m o HmHOH m o>fluu< uficmMHo m manmawm><

.coscfiucou--.flfi ofinme

140

The active fractions of P can be grouped into Ca-
bonded phosphates (Ca-P), aluminum-bonded phosphates (Al-P)
and iron—bonded phosphates (Fe-P). The relative amounts
of these forms in the active fraction are shown in Figure
14, for all but pedon 4, where Fe-P is the predominant
active form. The relatively inactive fractions, occluded
and reductant-soluble forms, were not determined in this
study. The Fe-P accounts for over half the total extrac-
table inorganic phosphate forms (70-80%) in most of the
well-drained pedons (pedons 1-3, 6 and 7). Even in pedon
5, also well-drained, Fe-P is the most abundant active P
form, but Al-P and Ca-P are relatively more abundant in
this pedon. There is very little P present as Ca-P in
the well-drained soils. This is associated with the low
pH values of these soils.

Pedon 1 has slightly higher Fe-P contents than
pedon 2 or 3. These are a lithosequence of well-drained
soils with ustic moisture regimes. Pedon 1 developed in
finer fine—loamy materials and pedon 2 or 3 in coarser
fine-loamy materials. However, pedon 7 also developed in
finer fine-loamy materials in the ustic area but it has
the lowest Fe-P content of these four pedons. Pedon 5 has
slightly more Fe-P content in the upper part and less
below than pedon 6. They are a lithosequence of well-
drained soils developed in coarser fine-loamy and coarse-

loamy materials, respectively, with udic moisture regimes.

141

 

 

 

 

 

 

Pedon I
C
l.’
\‘ I"
/ .I/
/ ,!’
30'/ II!
. a
. .!
! .‘ !
i : !
. .’ l
60 ' g !
E. I . .I
0 g '| I
. o
4:" I I| !
§ : | .'
ca‘ {1
904 II
g.
I ‘l
! 1!
1'
l
I h
'50 I T I I
O 20 4O 60 80 I00
Percent
—- Fe-P ----- AI-P ---- Co-P ------ Total in ppm

 

 

 

Fig. 14.--Relative distribution of the various active P
forms as percentage of total active P.

142

 

Pedon 2

IOO

 

Percent

------ Total in ppm

----- AI-P —-- Co-P

Fe-P

-" .III"""-'
"I
-‘l'"l" ""l"

 

 

30"

 

 

 

14.--Continued.

 

Fig.

143

 

 

Pedon 3

 

 

 

 

0
'
4.
0
o I
\.\I ..... I 2
. .l.
\.\ [.1I.’ \.\u\o
.I. .I \r“. III|
II, ,.Io \‘I-‘IIIIII. \ovnol
IIII I.’. ‘\\III \.\.\.
""""" “\ lo’v‘ \ \ \
“‘ """""
“\ IIIIIIIIIIIIIIIIII
“\
cllln . . . m . o
w w w m m

Percent

------ Total in ppm

---- Co-P

-.-.... Al-P

Fe-P

 

 

l4.--Continued.

Fig.

144

 

 

Pedon 5

 

 

 

 

 

20

 

30‘

so .530

Percent

----- Total in ppm

—--- Ca-P

Fe-P
----- AI- P

14.--Continued.

 

 

Fig.

145

 

 

Pedon 6

IOO

 

‘o
‘-
|o‘o'-'u OIIIUI-
o|o|o 'o'o'o'.'o u'o|o|o'
'u'n'ul-lo'o'

-'
""'
'
""‘
‘
"
'
""""'."""""I'
"""l'
"
’-"'|l'""l"

Percent

 

 

 

Eu .580

_--—- Co-P ----- Total in ppm

.—.-.- AI-P

Fe-P

 

 

14.--Continued.

Fig.

146

 

 

thxt7

 

 

 

 

 

 

l||||||||||||lul
\ 1w
'0
6
o
14
rm
[to llllllllllllllllllllllllllllllllllllllllllllllllll
lio/ I.|Ilu..llu
. t"..- a llllll ll. ....... Mu 0
w w w m w m
:5 538

Percent

°--"- Total in ppm

------ Al-P ---— Ca-P

Fe-P

 

 

l4.--Continued.

Fig.

.woscwucou--.vfi .wfim

 

147

 

 

 

 

 

 

 

 

 

 

 

 

200ch €09.00.—
8 . o
o ow o . . on.
V 00. Eng \ .
c. .28 ...... .. "
a-8 l... \ u
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N. _ u .7: II .\ u
_ . I ~ . ION—
.. _ m ON. \ n
l _ n .. _
.
M n m .\ n
._ u n
. — p a
l. ._ m .0. u .0. a
o O -
.. _ m a n m
l n u m m m
. . . O
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l _ u w _. .8
.. . n .8 ..
. -
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.— ~ H N
. n
/. .._ ... .8 .. .8
. . . ~
/!\.\.~ ~—. .-
\\\/ u x
l..\%. . \\. ~
.I+r \ o o
m .5qu m .8qu

 

 

148

There are apparently no clear cut litho- or climosequence
relationships to amounts of active P forms in these well-
drained soils. However, the relative amount of Fe-P in
the active P is greater in the well-drained soils of ustic
areas (pedons 1-3 and 7).

For pedons 8 and 9 with aquic moisture regimes and
Bt subsoil horizons the three forms of active P are more
evenly divided. But, as in the well drained soils, there
are little textural (lithosequence) associated differences
in the active P forms. However, Fe-P is the least abundant
of the three mineral forms, Al-P being the most abundant
form in the poorly drained soil (pedon 8) and Ca-P the
most abundant form in the somewhat poorly drained soil
(pedon 9).

When these soils are compared to the well drained
soils in the udic moisture regimes at similar elevations,
the difference in distributions of active P forms are
apparently toposequence relationships. It will be inter-
esting to see if these toposequence relationships also
hold in the ustic moisture regimes at higher elevations.

The low level of Al-P and Ca-P in the well-drained
soils is attributed to the strong weathering of the soils
under well-drained conditions. The low Al-P content sug-
gests that the capacity of these soils to supply plant
available P from this inorganic pool is, therefore, limited

(Chang and Juo, 1962; Juo and Ellis, 1968). In acid soils,

149

such as these under study, Al-P has been found to be more
available than Fe-P for upland crops (Hanley, 1962; Payne
and Hanna, 1965). However, Juo and Ellis (1968) using
synthetic materials found the colloidal forms of Al-P

and Fe-P to have almost the same degree of availability

and they were much more available than the more crystalline
forms of strengite, FePO4-HZO and variscite, AlPO4-2HZO.

Since most of these soils under study are highly
weathered, the relatively high content of Al-P and Ca-P
in the more poorly drained, non-spodic soils (pedons 8 and
9), may be due to poor drainage conditions rather than
degree of weathering (Juo and Ellis, 1968; Westin and
deBrito, 1969; Uzu gt 31., 1974). These soils are found
in the more forested udic areas of the study area.

The organic phosphorus form (like the inorganic
phosphorus) in all the pedons is relatively high in the
surface horizons and decreases with depth, to a very low
level in the well drained pedons l, 2, 3, 5, 6 and 7, while
the soils with aquic moisture regimes and finer subsoils
(pedons 8 and 9) have relatively higher organic P contents
in their subsoils. The high organic P values are related
to the organic matter content of the various sites. The
relationship between organic matter content (Y) and organic
P (X1) is statistically significant (r = 0.69**, signifi-
cant at the 5% level). Regression analysis shows no sig-

nificant relationship (even at 20% level) between organic

150

P (Y), and soil pH (X2). The prediction equations are as

follows:

0.023 + 0.033 X

$13
..<
ll

1 r 0.69** (Organic P)

0‘
..<
II

4.79 - 0.317 X

2 r -0.23 not significant

The presence of organic P at considerable depths,
particularly in pedon 4, may indicate possible mobilization
of organic P and downward movement through leaching. The
high organic P in the poorly drained profiles with th
horizons (pedons 8 and 9) compared to better drained soils
(pedons 1—3 and 5-7), a toposequence relationship, may also
be attributed to low level of biological activities under
these conditions or biocycling of nutrients. Organic P
may supply a substantial portion of available P during the
process of mineralization (Enwezor and Moore, 1965).

The organic C:P ratios (Table 11) are relatively
low in the upper parts of all but the Spodosol, pedon 4,
in most cases they are below 200 in the Ap horizons.

These values represent a much lower relative phosphorus
content in these soils than in the United States where the
average organic C:P ratio is in the order of 110. Since
the C:P ratios for most of the soils are below 200,
mineralization of organic P is expected to readily occur
(Tisdale and Nelson, 1956), leading to an increase in the
level of available P, provided the P thus released is not

fixed in unavailable form by other soil components.

151

As most of the phosphorus in these soils is in the
organic form, the importance of maintaining organic matter
is evident in order to maintain organic P. Fertilization
and liming can provide a more favorable environment for
the microorganisms responsible for mineralization.

The ability of these soils to fix added orthophos-
phate increases with depth (Table 11). The fixation is
considerably lower in the poorly drained, sandy, pedon 4.
Considering the climosequences of well drained soils,
pedons 2 or 3 and l or 7 with ustic moisture regimes and
pedons S and 6 with udic moisture regimes, slightly higher'
fixation capacities are associated with the soils devel-
oped on higher physiographic surfaces in ustic regimes
than soils located on lower physiographic surfaces in udic
regimes.

In the lithosequence of soils, pedons 9 and 8,
there are no significant differences in their P-fixation
capacities. There is somewhat less P-fixation capacity
in these soils with an aquic moisture regime when compared
with the well-drained soils, a toposequence relationship.

As all the soils are acid (pH values range 2.6-5.2)
and contain considerable amounts of hydrated iron oxides an
ideal condition for phosphorus fixation is provided. There
is a close correlation between phosphorus fixation and
free iron oxides (r = 0.65**, significant at the 5% level).

The prediction equation is Y = 70.9 + 7.8x where Y is

152

P-fixation and X is free iron oxides. Al-oxides seem to
be of less importance because they are present in less
amounts except in the finer poorly drained soils, which
show less P-fixation.

Generally phosphorus fixation in the soils studied
seems to also have a close relationship with clay content.
Regression analysis of P-fixation capacity versus clay
content shows a significant relationship between them
(r = 0.84**, significant at the 5% level). This may be
associated with an indirect effect of the iron and aluminium
oxide contents found in the clay fraction and the finer
materials associated with clay contents. Thus, pedon 4
which is sandy, and has a spodic instead of an argillic
B horizon, retains much less phosphorus than the other

finer soils of similar drainage (pedons 8 and 9).

Practical applications.--Though the traditional

 

farming practices involve "slash and burn," organic P can
contribute an important source of P for crops grown in the
study area. However, excessive land preparation and
intensive bush burning may result in great loss of organic
P from the soils of the Coastal Plain Sands. In the
savanna/derived savanna areas, phosphorus deficiencies are
likely to be high as high population density tends to
result in shorter fallow periods and probable loss of top-

soil through erosion.

153

Much work has not been done on fractionation of
phosphorus forms in Nigerian soils. The present study
coupled with results of other workers necessitates a reas-
sessment of phosphorus supplying power of these soils.

The soil test values or patterns commonly observed in the
temperate soils may be quite different in tropical con-
ditions.

Mineralogy of the Sand and Coarse
Silt Fractions

 

 

Mineralogy of soils is used directly at the family
level, and at some higher levels, as a classification
criterion in Soil Taxonomy (Soil Survey Staff, 1975).
Percentages of minerals present in the coarse silt and
sand fractions are used in classifying those soils that
belong to fragmental, sandy, sandy skeletal, loamy or
loamy skeletal families.

The mineralogical compositions of the sand and
coarse-silt fractions of the soils are shown in Table 12
and are expressed in percentage of grains counted. Quartz
is the predominant mineral in all of these soils ranging
from 96.8-99.8% in the family control section. Many of the
quartz grains are irregularly shaped, clean and without
inclusions. They are of different sizes and are of low
order interference color but bright. They exhibit sharp
extinction within a small angle of rotation. In some

grains the extinctions are sometimes wavy. In all the

154

Table 12.--Mineralogica1 composition of the fine sand and
coarse-silt fractions determined with a petro-
graphic microscope.

 

 

 

 

Light Heavy .
. Depth fractions _fractions % $2311; in
Horizon cm Quartz > 2.85 sp. gr. control
% % section
Pedon l--AMSl
Ap 0-10 98.2 0.9
B1 10-22 98.5 0.7
B21t 22-45 97.0 0.8 97.0
B22t 45-75 98.1 1.2
B3 75-180 98.0 0.9
Pedon 2--AMM2
Ap 0-13 96.4 nd
Bl 13-30 98.9 nd
B21t 30-55 99.6 nd 99.6
B22t 55-108 97.2 nd
B3 108—180 99.5 nd
Pedon 3-—AMF3
Ap 0-12 98.9 nd
Bl 12-22 98.8 nd
B21t 22-45 98.3 nd 98.3
B22t 45—137 97.5 nd
B3 137-180 97.7 nd
Pedon 4--AMM4
Ap 0-12 98.6 0.8
A21g 12-30 97.7 0.9 98 S
AZZg 30-140 98.9 0.7 '
Bhirm 140-200 98.5 0.9
Pedon 5--ASU5
Ap 0-13 93.7 nd
Bl 13-38 96.9 nd
B21t 38-88 97.8 nd
B22t 88-150 98.6 nd
B3 150-180 95.4 nd

 

 

 

 

 

155

Table 12.--Continued.

 

 

 

 

Light Heavy 9 .
. Depth fractions fractions ° $2311; 1n
Horizon cm Quartz > 2.85 sp. gr. control
% % section
Pedon 6--AK6
Ap 0-18 95.7 nd
B1 18-53 97.6 nd
B21 53-110 99.8 nd 99.8
B22 110-143 97.5 nd
B3 143-180 96.7 nd
Pedon 7--UM7
Ap 0-20 92.9 nd
B21t 20-55 96.8 nd
B22t 55-100 99.5 nd 96.8
B31 100-150 98.9 nd
B32 150-210 97.2 nd
Pedon 8--PH8
Ap 0-10 98.5 0.9
A2g 10-23 95.6 0.8
B21tg 23-40 96.9 0.7 96 9
B22tg 40-72 97.3 0.9 '
IIB23tg 72-120 98.1 0.9
IIB3 120-180 96.0 1.2
Pedon 9--ET9
Al 0-2 97.7 0.9
A2 2-10 96.4 0.8
IIB21tg 10-28 98.1 1.1 98.1
IIB22tg 28-87 99.0 0.9
IIB3 87-180 98.2 0.9

 

 

 

 

 

nd

= not determined

156

soils the percent of weatherable minerals is less than 10%.
Feldspars and ferro-magnesian minerals are almost absent
in all the horizons studied.

Results of heavy mineral counts on pedons l, 4, 8
and 9 indicate the presence of zircon, tourmaline, rutile
and staurolite in the non-magnetic suite. Some magnetite
was recognized as black, opaque particles. The heavy
mineral fractions are low (Table 12) and change little in
composition or amount from the A to the B3 horizons in
most of the pedons. It is difficult to establish any real
difference in heavy mineral composition throughout the
four pedons studied. The grain counts of these heavy
minerals do not necessarily prove a uniform parent material
for all the four soils since pedons 4 and 8 are extremes in
sand and silt contents, respectively, of all the nine
pedons. It is also difficult to use these grain counts
as evidence of any systematic variation in parent material
that can be related to the soils as they now occur on the
landscape. Soil drainage class appears to have no influ-
ence on the sand and coarse silt mineralogy for these
soils.

All the soils studied have siliceous mineralogy.
The soils occur along the same gently undulating to flat
landscape. Therefore, this general relationship between
soil mineralogy and occurrence on Coastal Plain Sand land-

scapes should aid soil survey in classification at the

157

family level of sandy and loamy soils for it will permit
delimiting soils that are members of siliceous mineralogy
family on a geographic basis on the Coastal Plain Sands
of Nigeria; irrespective of drainage class. Pedons on
elevations below 20. m were not studied mineralogically.
Geologists consider a sediment mature when it con-
tains only the most stable mineral species, such as
quartz. The percentage of quartz in all the profiles is
high. The absence of feldspar from the soils especially
those on higher older surfaces is expected. Judging from
the percentage of quartz and the low heavy minerals con-
tents (less than 1% in most cases) for the four pedons, it
can be concluded that most of these sediments had little
in the way of feldspars or other weatherable minerals in
the sand and coarse silt fractions when deposited or they
are strongly weathered in situ. Moreover the lack of
weatherable minerals is further substantiated by the pre-
dominance of kaolinite in the clay fraction. Since the
parent materials of these soils are of sedimentary origin
it would appear probable that much of the sediment was
highly weathered material prior to its transport from the
source area. However, it is also possible that the very
young soils not fully investigated (pedons 11 and 12 at
elevations 4 and 8 m respectively) may still not have

attained their maximum degree of weathering.

158

Clay Mineralogy

 

X-ray diffraction was used to study the clay
fraction of the soils. Preliminary studies of selected
horizons of all the soils show the predominance of 1:1
type clays. Magnesium saturation and glycerol solvation
gave no 2:1 clays. Subsequently clays from selected
samples of pedons were only K-saturated and heated to
550°C.

Figure 15 shows the x-ray diffraction data of
selected pedons (l, 4, S, 8 and 9). Kaolinite is identified
by its diffraction peaks in the range 7.13-7.14 A: and
3.54-3.64 A° which dissappeared upon heating the specimens
to 550°C. In general kaolinite is by far the most abundant
and predominant clay mineral in all the pedons.

The relative contents of kaolinite in clay by
pedons, expressed in average percentage values are as
follows: pedon l > pedon 4 > pedon S > pedon 9 > pedon 8.
In most of the pedons the content of kaolinite is highest
in the upper two or three B horizons and then decrease
slightly with depth. More kaolinite is found in soils
located on the higher and relatively older surfaces though
these soils may have different parent materials. Some
quartz was found only in the clay fraction of pedons l
and 5 as shown by the x-ray diffractograms. None of the
other common minerals, like geothite, which might be

present, are shown by the x-ray diffraction. However, the

159

 

.mcowoa pouuofiom mo Anva :ofluumpw meo

 

 

 

0’

mo mcpouuma :0wuumumwflw xmp-x--.m~

  

y 'r';

 

 

._ was.

_ ' .

H .3de

‘ y 99':-

 

 

160

 

 

 

 

.._-— -———— ~- ' F’"-

 

 

e
__ Realms

._ l.-- -._. -._- 4

—. —.—--_. —— —— —-——- -._

 

 

 

 

 

 

 

 

....... w - _._.... -..-..
I I am. -. ’° “

 

 

 

 

Fig. lS.--Continued.

161

 

_ -- .______--_-...__- ._- _. -_- -u

1?

 

_. , _. _-...-.. -- ..__.°, ,—_- .04». _.__ .. . -.._..°_. _.-. -.—__.-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 15.--Continued.

 

162

 

.poSCMHcoo--.mH

 

 

 

 

 

 

 

 

 

 

 

 

.mE

 

 

 

 

163

peak identifying quartz, 3.39 A°, may also be due7to gibb-
site. Formation and accumulation of gibbsite might be
expected under the climatic conditions of the study area.
All the soils of the study area have been shown to
possess characteristics that are conducive to good permea-
bility. They are strongly to very strongly acid and have
low base saturation. These properties reflect conditions
that would not have been favorable for montmorillonite.
The absence of montmorillonite in the relatively younger
pedons may indicate high content of 1:1 type clays in the
parent material. This suggests that kaolinite was inherited
from the parent material and may not be of pedogenic origin
even though prevailing conditions of low pH and low base
saturation are also conducive to the formation of kaolinite
from 2:1 layer silicates or primary minerals such as feld-
spars through processes involving dealumination and desili-

cation.

Micromorphology--Thin Sections

 

Thin sections, both horizontal and vertical, made
from clods taken from selected profiles 1, 3, 5 and 7 were
examined petrographically. Features are described and
classified according to Brewer (1964).

Skeletal grains have the same morphological vari-
ability in all the pedons. In general, they are randomly
distributed, large grains are rounded, and many small ones

are angular. All the grains are mainly quartz. Some iron

164

oxide coatings are present on weathered quartz grains.
Weatherable minerals are almost absent. Higher in the
profiles, the number of red-coated grains decreases com—
pensated by a yellowish-red staining of the plasma. The
density of skeletal grains is higher in the B3 horizon
than in the upper horizons. The predominance of rounded
sand grains are due to transportation by water of the
parent rock material.

Voids in all the soils are of mainly two different
types: simple packing voids and vughs or channels. Vughs
constitute the characteristic voids in the subsurface
horizons, especially irregular orthovughs. Almost all the
vughs and some channels are lined by ferri-argillans and
organo-argillans. Vughs, and channels may owe their
occurrence to packing, adhesion and attraction between
particles and also to differential weathering and leaching.
Channels may be caused by faunal activities, plant root
systems or other biological processes. Worm holes and
termite activities are also observable in the study area.

The influence of organic matter is seen especially
in the A horizons. Plant roots occur in the voids and
cross-sections show the internal structure of the roots.
Presence of organic matter decreases with depth in all the
pedons. Some voids and vughs are filled with organic
matter. In comparison with the temperate region, the small

amounts of humified materials observed may be due to rapid

165

destruction under tropical conditions. Fragments of char-
coal observed in the S-matrix of the top horizons may be
due to the persistent bush burning during land cultivation.
Plasmic fabric in most of the pedons are almost
completely isotropic which are described as isotic, isotic-
undulic or isotic-inundulic. The plasma is generally
reddish-yellow in color. The isotrOpic character of the
S-matrix is related to strong weathering of the soils
(Stoops, 1968). Isotic plasmic fabric is found in the A
horizon of most of the soils. Presence of humic compounds
in the plasma may also contribute to the isotr0pic char-
acter of the plasma. Low development of plasmic structure
in the surficial horizons may also be due to the low
amount of clay in the upper horizons. The isotic nature
of the plasmic fabric is apparently associated with the
type and amount of clay and iron. The soil matrix in all
the soils range from granular to intertextic. This
explains the great porosity and friability of these soils.
The most significant micromorphological features
in the studied soils are the illuviation ferri-argillans
(Figures 16-19). The maximum amount of these cutans are
shown in some areas of the thin section of the argillic
horizons. In most of the profiles these ferri-argillans
are very slight and constitute one of the major components
of the plasma and they appear clear, red or yellowish-red

with weak continuous orientation. They also cover the

166

 

Fig. 16A.--Micromorphology of pedon 1 (plain light).
a. B21t (70X)
b. B22t (70X)

167

 

Fig. l6B.-—Micromorphology of pedon l (crossed nicols).

a. B21t horizon with ferri-argillans showing
strong continuous orientation (70X)

b. B22t horizon (70X)

168

 

Fig. l7.--Micromorphology of pedon 3 (crossed nicols).
a. B21t horizon (70X)
b. B22t horizon (70X)

169

 

Fig. 18.-—Micromorphology of pedon 5 (crossed nicols).
a. B21t horizon (70X)

b. B3 horizon (70X)

170

 

Fig. 19.--Micromorphology of pedon 7 (crossed nicols).
B22t horizon (70X)

171

walls of vughs, channels and weathered surfaces of quartz
grains.

The presence of ferri-argillans is considered to
be evidence of clay illuviation. These ferri-argillans
are slightly more expressed in soils located at higher
physiographic surfaces in the ustic moisture regime area
of the study area. The intensity of illuviation can be
assessed by the surface area occupied by these illuvial
features in the cross-section and by the thickness of the
material. The ferri-argillans are slightly more abundant
in the deepest layers in accord with the clay and iron dis-
tribution in the soils.

In general, the co-migration of clay and iron is
the major property associated with the micromorphological
characteristics of the soils studied. Iron, probably in
the form of hematite, covers all the Components of the
soil matrix. Other pedological features observed include
cross sections of roots, generally most common in the sur-
face horizons. The dynamic nature of the plasma is pri-
marily due to its susceptibility to mobilization being the
colloidal fraction of the soil. Consequently soil forming
processes affect the plasma. Free iron oxides play a sig-
nificant part in the evolution of the plasmic fabric
(Stoops, 1968). As the free iron oxide content increases,
the plasmic fabric is less well developed. This is shown

by the nature of plasmic fabric obtained in these studied

172

soils. However, the plasmic fabric for all the soils range
frOm isotic to undulic or inundulic. Detailed micromor-
phological descriptions of the selected pedons are given

as follows:

Pedon l--AMSl

Apgo-lO cm. Randomly distributed skeleton grains

 

with sharp extinction. Simple packing voids present.
Plasmic fabric: at 35X magnification it is undulic, but

at lOOX magnification it is isotic. Soil matrix: porphyro-
skelic to intertextic. Plasma consists of organic matter.

B21t and B22t: 22-75 cm. There are skeletal grains

 

randomly distributed. The plasma consists of clay and
organic matter. There are diffuse reddish brown ferri-
argillans on some skeleton grains. The vughs and some
channels are also lined by ferri-argillans. Voids: many
irregular orthovughs, few simple channels and metavughs.
Plasmic fabric: inundulic to insepic. Matrix: granular.

B3: 75-180 cm. Few skeleton grains and the ground-

 

mass is porous. The plasma is reddish brown in color and
has a high content of highly dispersed iron oxide. Low
organic matter content. Plasmic fabric: isotic-inundulic.

Matrix: granular-intertextic.

Pedon 3--AMF3

Ap 0-12 cm. Skeleton grains of varying sizes and

 

shapes. Simple packing voids. Plasma filled with organic

173

matter. Plasmic fabric: isotic-undulic. Matrix: granular-
porphyroskelic. Few vesicles filled with organic matter
and some opaque materials.

B1: 12-22 cm. Fewer skeletal grains than the

 

overlying horizon, Ap. Groundmass is also porous. Plasma
has less organic matter and it is reddish-brown in color.
More ferri-argillans and clay than in Ap horizon. Vughs
are irregular orthovughs. Simple packing voids are present.
Plasmic fabric: isotic-inundulic. Matrix: granular to
intertextic.

B21t to B22t: 22-137 cm. Skeleton grains, some

 

rounded and varying sizes. Less organic matter in the
plasma. More reddish-brown ferri-argillans and clay fill
the plasma. Simple packing voids and irregular orthovughs.
Plasmic fabric: insepic-undulic. Matrix: granular to
intertextic.

B3: 137-180 cm. More ferri-argillans than in over-

 

lying horizon. Low organic matter. Same as overlying

horizon in other respects.

Pedon 5--ASU5

 

Ap,0-l3 cm. Few skeleton grains and the groundmass

 

is very porous. The plasma consists of organic matter and
clay. Simple packing voids. Few large irregular orthovughs.
Few simple embedded grain ferri-argillans with weak orien-

tation. Plasmic fabric: inundulic-isotic. Matrix: granular.

174

B1: 13-38 cm. Skeleton grains predominantly quartz

 

are distributed randomly. Simple packing voids. Ortho-
and metavughs are present. Less organic matter than in
Ap. Perri-argillans not as concentrated as in pedons l
and 3. Some skeleton grains are coated with ferri-
argillans and organic matter. Plasmic fabric: isotic/
insepic. Matrix: granular.

B21t and B22t: 38-150 cm. Randomly distributed

 

skeleton grains. Numerous irregular interconnected ortho-
vughs, very few channels and vesicles. Simple packing
voids. Few large irregular orthovughs. Plasma filled
with ferri-argillans with stronger orientation than in B1
horizon. Plasmic fabric: insepic. Matrix: granular.

B3: 150-180 cm. Description similar to BZt, but

 

with slightly more ferri—argillans and less organic matter.

Pedon 7--UM7

Ap 0-20 cm. Skeleton grains are randomly dis-

 

tributed. Simple packing voids filled with organic matter.
Plasma filled with dark Opaque materials and few reddish-
brown ferri-argillans.

B21t and B22t: 20-100 cm. Few large-sized skele-

 

ton grains with many small and medium sized grains dis-
tributed throughout the plasma. Simple packing voids,
irregular orthovughs and metavughs are present. Some part

of the plasma consists of yellowish (clay) color around

175

reddish brown iron oxides. The groundmass is porous.
Black opaque materials are evident. More ferri-argillans
than in Ap horizon. Plasmic fabric: isotic-undulic.
Matrix: granular-intertextic.

B31 and B32: 100-210 cm. Same as B22t except that

 

more ferri-argillans are present than in the overlying
horizon. Less organic matter present.

It is probable that deeper sampling depths, 2m - 3m,
would be preferred for more evidence of oriented clay skins
in these soils and possible decreases in clay contents
with depth. In ancient soils it is common that the most
abundant and thickest clay skins are between depths of 2m
and 3m. At that depth there is only a little biologic
activity, and peds and pores should be stable (Soil Survey
Staff, 1975). This may in part explain the scarcity of

oriented clays in these soils under study.

Soil Classification and Correlation

 

A summary of the characteristics of the fourteen
pedons is used to classify the soils in the U.S. Soil
Taxonomy (Soil Survey Staff, 1975) and with the FAO/

UNESCO Soil Map of the World Legend (1973).

In these relatively old soils, judged by their low
base saturations and thick sola, identification of an
argillic horizon in the field is not easy. Hill and Bennet
(1974) working in the Central Nigeria Project area empha-

sizes the field observation of cutans which is often

176

difficult for young soil surveyors. Van Wambeke (1967)
states that the identification of argillic horizons in
tropical areas is often only a matter of appreciating the

amount, the thickness and the location of cutans.

With USDA Soil Taxonomy.--In terms of the USDA

 

Soil Taxonomy, all the pedons except 4 and 6 have subsur-
face horizons that may qualify as argillic on the basis
of increases of clay content with depth; an increase of
the fine clay:coarse clay ratios with depth, although only
pedon 8 and 9 have one-third more fine clay in the Bt
horizon; and the presence of some weak void ferri-argillans
in the B horizons. In most of the soils the diagnosis of
the argillic horizon is based on the ratio of the clay
content of the illuvial horizon to the eluvial horizon
being greater than 1.2 (Soil Survey Staff, 1975). Oxic
horizons were not observed in these soils in the field
or by laboratory data. Therefore, these soils are excluded
from the order of Oxisols.

According to the base saturation and other diag-
nostic prOperties; all the pedons except pedons 4 and 6
are classified as Ultisols. The clay distribution is such
that the percentage clay does not decrease by as much as
20% of the maximum within a depth of 1.5 m of the soil sur-
face. Therefore, the well drained Ultisols are classified
as Paleustults and Paleudults at the Great Group level.

Pedons l, 2, 3, 7, 13 and 14 are classified as Paleustults.

177

The soils classified as Paleudults (pedons 5, 10,
11 and 12) meet the proposed classification of Kandiudults
(Moorman gt 31., 1977) with the following diagnostic
criteria:

1. CBC (lN-NH4OAC) of less than 24 m.e./100 g in
the upper 50 cm of the argillic horizon.

2. Less than 10% weatherable minerals in the 20-200
micron fraction of the upper 50 cm of the argillic
horizon. No saprolitic material present.

3. No fragipan or "continuous phase" plinthite.

In the study area the soil profiles are usually
well aerated and structures are favorable to root pene-
tration. It is possible that argillic horizons in these
dominantly kaolinitic materials have less agronomic impor-
tance than in soils dominated by 2:1 clays. For practical
reasons, it may be more convenient to lower the taxonomic
weight of the argillic horizon as soon as the material in
which it has formed reaches a certain degree of weathering
(Van Wambeke, 1967).

All the well drained Ultisol profiles except pedons
11 and 13 meet requirements for the typic subgroup. Pedons
11 and 13 qualify as an Arenic Paleudult and an Arenic
Paleustult, respectively. The others are classified as
Typic Paleustults (pedons 1, 2, 3, 7 and 14) and Typic
Paleudults (pedons 5, 10 and 12). Differences in particle-

size classes lead to different family classes, namely

178

fine-loamy and coarse-loamy for these soils. All the Typic
Paleustults are fine-loamy. Pedon 13 an Arenic Paleustult
is coarse-loamy. In the Typic Paleudults pedons 5 and 10
are fine-loamy and pedon 12 is coarse-loamy. Pedon 11 an
Arenic Paleudult is also coarse loamy. Next to pedon 4
this is the sandiest pedon studied.

Considering the relative elevation of the more
poorly drained profiles and other prOperties, these soils
may not have attained the same degree of weathering as
rapidly as the geographically associated well drained pro-
files. Their low base saturation may indicate that these
soils might have undergone previously well-drained cycles,
or their parent materials are derived directly from kao-
linitic sources, or that their physiographic positions
(receiving run-on in addition to rainfall) are associated
with more leaching.

Pedon 4 is sandy throughout the profile, has a
spodic subsoil horizon, and is naturally poorly drained.
It is classified as an Aeric Grossarenic Tropaquod. Its
distribution is not extensive in the study area. Pedons
8 and 9 are poorly drained and somewhat poorly drained,
respectively. Pedon 8 has been shown to have more silt
than pedon 9 and is a Typic Tropaquult while pedon 9 is
Typic Paleaquult. Pedon 8 is fine-loamy while pedon 9 is

coarse-loamy.

179

The family texture classes range from sandy to
coarse-loamy to fine-loamy. Weatherable minerals in the
coarse-silt to fine sand fractions in all the soil samples
constitute less than 10%. This places all the soils in the
siliceous mineralogy class. All soils are in the isohyper-
thermic temperature class as previously stated.

On the basis of the data obtained in this study,
the fourteen pedons are classified as follows at the family
(and series) level of Soil Taxonomy:

Pedons l, 2, 3, 7 and 14: Typic Paleustults, fine-
loamy, siliceous, isohyperthermic (l and 7--Amakama; 2, 3
and l4--Itaja).

Pedons 5 and 10: Typic Paleudults, fine-loamy,
siliceous, isohyperthermic (5--Asa; 10--Bori).

Pedon 12: Typic Paleudult, coarse-loamy, siliceous,
isohyperthermic (Ogoni).

Pedon 4: Aeric Grossarenic Tropaquod, sandy,
siliceous, isohyperthermic (Ihie).

Pedon 6: Oxic Dystropept, coarse-loamy, siliceous,
isohyperthermic (Akwete).

Pedon 8: Typic Tropaquult, fine-loamy, siliceous,
isohyperthermic (Port Harcourt).

Pedon 9: Typic Paleaquult, coarse-loamy, siliceous,
isohyperthermic (Eteo).

Pedon ll: Arenic Paleudult, coarse-loamy, siliceous,

isohyperthermic (Ka).

180

Pedon l3: Arenic Paleustult, coarse-loamy, siliceous,
isohyperthermic (Ngwa).

Table 13 gives the proposed tentative series names
of these soils.

Pedons 4, 8 and 9 have aquic moisture regimes.
These form a chrono-lithosequence. Pedon 4 is located on
a higher site and is very sandy. The parent material tex-
ture of pedon 8 (fine-loamy) is finer than 9 (coarse-loamy)
and the difference in elevation between them is slight,
about 10 m. Pedon 4 is an Aeric Grossarenic TrOpaquod.
Pedons 8 and 9, from sandy clay loam and sandy loam
materials, respectively, are a fine-loamy Typic Tropaquult
and a coarse-loamy Typic Paleaquult, respectively.

Pedons 5, 6, 10, 11 and 12 have udic moisture
regimes. Pedons 5 and 10 on sandy clay loam materials,
and pedons 12 and 11 on sandy loam materials may represent
a chrono-lithosequence as they also occur on successively
lower elevations of 60 m, 30 m, 8 m, and 4 m, respec-
tively. Actually all four pedons are Paleudults but pedons
5, 10 and 12 are Typic Paleudults while pedon 11 at the
lowest elevation is an Arenic Paleudult, coarse-loamy.
Pedons 5 and 10 are fine-loamy and pedon 12 is coarse-
loamy. Pedon 6 is at 50 m elevation and is a coarse-
loamy Oxic DystrOpept without a texture profile differenti-
ation. That the differences in elevation (age) are not

evident in the classification of these soils may be because

181

Table l3.--Proposed tentative series and their tentative

correlations.

 

Tentatively Correlated Series

 

Pedon Tentative Series Moss (1957)
1 Amakama Alagba (normal subseries - b)
2 Itaja Alagba (sandy subseries)
3 Itaja Alagba (sandy subseries)
4 Ihie None
5 Asa Uyo
6 Akwete None
7 Amakama Alagba (normal subseries - b)
8 Port-Harcourt Idesan (silty variation)
9 Eteo None
10 Bori Uyo
ll Ka Ahiara
12 Ogoni Uyo
l3 Ngwa Kulfo
l4 Itaja Alagba (sandy subseries)

 

182

they are all weathered beyond stages where such differ-
ences may have been important.

Pedons 1-3, 7, 13 and 14, all have ustic moisture
regimes and form lithosequences. Of these pedons 2, 3
or 14 are from coarser sandy clay loam materials and pedon
1 or 7 are from finer sandy clay loam materials and on
similar elevations (150-106 m above sea level). Pedon 13
is from sandy loam and on a lower elevation (60 m above
sea level). All but pedon 13 are fine loamy Typic Paleu-
stults. Pedon 13 is a coarse-loamy Arenic Paleustult.

The difference in its parent material is reflected in a
difference in its subgroup and family classification from
the soils on the same elevation or higher elevation in
finer parent materials. Some textural differences in the
finer parent materials are recognized at the series or
subseries level.

The general topography of the study area is nearly
flat for most pedons. Variation in aspect and elevation
influence vegetation by varying conditions for organic
activity--savanna/derived savanna, versus forests on the
higher and lower surfaces, respectively. Soils on the
higher surfaces have deep water table while those on lower
surfaces have more shallow water-tables. More conspicu-
ously, locally the soils with higher water tables have
aquic moisture regimes. Their topo-relationships to better

drained soils with textural B horizons appear as aqui

183

suborder Aquults (pedons 8 and 9). In the case of pedon
4, in very sandy materials and with a spodic B horizon,
the tOporelationship appears as an aquic suborder of
another order, Aquod. This is apparently a tOpo-
lithosequence relationship.

Among the well-drained soils, differences between
the Udults and Ustults, as pointed out elsewhere, are a
reflection of differences in their climatic moisture
regimes (a climosequence) but these are also correlated
with increasing elevations inland from the seashore. The
Ustults also reflect the redder subsoil colors particularly
in the finer materials, while coarser materials are more
yellowish in both Ustults and Udults.

The establishment of a stable forest ecosystem
has been tampered with by the slash and burn system of
agriculture in the study area. Within the dominant "oil-
palm bush" vegetation can be found savanna and derived
savanna giving rise to almost similar biotic communities.
Biocycling of nutrients has been shown to take place.
Consequently the extensive leaching of nutrients is counter-
acted by the biocycling process, leading to a decrease of
bases with depth. Clear-cut biosequences of soils may
not be easily prOposed as this is dependent on the litho
and toposequences and land utilization as mentioned earlier.

The prevalent slash and burn system of agricul-

ture affects the establishment of a climax forest or

184

unmodified rain forest. A heterogenous collection of

trees and shrubs of all shape and sizes in various stages
of growth or decay is found. Most of the pedons on the
higher physiographic surfaces and within the ustic mois-
ture regime have either scant forest/oil-palm bush or
derived savanna. The impact of organisms such as fauna

on soil develOpment is dependent on other factors like soil

reaction and climatic conditions for biological activities.

With FAD/UNESCO Soil Map of the World Legend.--

 

Using the FAD/UNESCO soil map legend most of the soils
have an argillic B horizon with a clay distribution such
that the percentage of clay does not decrease from its
maximum by as much as 20% within 150 cm of the surface.
Consequently all the pedons except 4 and 6 can be classi-
fied as Nitosols (FAG/UNESCO, 1973). According to the
base saturation of these soils, all the pedons (including
pedons 8 and 9) are Dystric Nitosols.

Pedons 8 and 9 have impeded drainage but they
satisfy the requirements for Nitosols. However, within
the FAD/UNESCO legend, there is no provision for more
poorly drained Nitosols. Therefore, for pedons 8 and 9,
Gleyic Nitosols are prOposed. It is realized that pedon
9 is somewhat poorly drained. Further separation should
be made at lower categories, if the legend provides for

that, but the lack of this provision presents a setback

185

towards total applicability of the FAG/UNESCO system down
to the series level.

Pedon 4, on the other hand, though poorly drained
is not a Nitosols. It is classified as a Gleyic Podzol
due to the presence of a spodic horizon and hydromorphic
properties within the profile. Pedon 6 because of lack of
a texture B is classified as a Dystric Cambisol.

The topography of the study area is generally
flat to gently undulating. Therefore, the effect of relief
on these soils is limited and the morphology of the soil
profiles is remarkably uniform in most parts of the study
area. The geologic age of the soils is generally about
the same, namely Pliocene/Pleistocene though the soils
located on higher surfaces further inland show character-
istics of relatively older age and a dryer climate than
soils on lower surfaces near the coast. All the soils
belong to a geological formation of similar age--the
Coastal Plain Sands formation.

The existing distribution of forest and savanna
or derived savanna vegetation cannot be said to be stable
due to the subsistence farming system, which among other
things, involves cutting and burning of vegetation in the
course of land preparation. Therefore the effect of vege-
tation on soil formation and soil properties cannot be
attributed exclusively to its present composition, but to

successional sequences.

186

Comparison and Correlation of
Classification Systems

 

 

The Soil Taxonomy is a multi-categoric system that
applies concepts of pedogenic processes and soil charac-
teristics. On the basis of this, categories starting from
orders to series have been established based on the pres-
ence or absence of a variety of combinations of diagnostic
horizons and quantitative values of soil properties. The
FAG/UNESCO legend is a bicategorical scheme which recog-
nizes 26 higher classes subdivided into 104 soil units.
The soil units are differentiated on the basis of quanti—
tative criteria similar to those of U.S. Soil Taxonomy.

In the present study, the U.S. Soil Taxonomy
appears to be by far the most elaborate and most quanti-
tative especially in definitions of taxa in terms of soil
prOperties subject to measurement by defined methods.
However, certain requirements involve costly outlays. For
example, preparation of thin sections for identification
of clay skins, where these are not observed in the field,
in an argillic horizon. Moreover a considerable amount of
morphological, analytical and climatological data are
needed for positive placement of soils in the lowest cate-
gory of the system.

Definitions in the FAO system are less precise.

In many cases it is difficult to place soils down to the
lowest category or accurately within a soil unit. To

correlate the two systems one faces the problem of

187

different criteria employed in their definitions. At the
suborder or Great group level of the U.S. Soil Taxonomy,
most of the soils were broadly classified under ustic and
udic soil moisture regimes. Using the FAO system, this
criterion is not considered. Ultisols can be Acrisols or
Nitosols in the FAO system due to overlap of chosen dif-
ferentiating characteristics.

In correlation of series throughout Nigeria it may
be necessary to classify the soils according to either
the FAO or U.S. Soil Taxonomy system. Using the U.S. Soil
Taxonomy it is possible to classify the soil at all levels
down to the series, provided analytical data are present.
However, the use of the FAO system poses some problems.
A number of soils occurring commonly in Nigeria may not
appropriately fit one of the existing soil units. There
are no criteria for grouping soils into any classes below
the sub-unit level of the FAO system. It is proposed that
new sub-units be defined as necessary to take care of
certain characteristics, e.g., Ferralic Nitosols for
Nitosols having ferralic or ferric properties (e.g., pedons
2 and 3). It would appear as if soils with impeded drain-
age should not be classified as Nitosols. It is proposed
that Gleyic Nitosols be introduced for more poorly drained
Nitosols. Characteristics transitional between the higher
level units should be established, e.g., drainage differ-

ences, presence of petroferric or lithic phases, etc.,

188

should be used to differentiate soils from those typic or
modal within the units. Phases may be used in any cate-
gory in the U.S. Taxonomy for practical purposes.

At the family level it is recommended that the
family differentia for particle size and mineralogy as
defined by the USDA (Soil Survey Staff, 1975) criteria be
used. Mineralogical classes may, in some cases, be deduced
from the nature of the parent material or age of the land
surfaces, with the aid of limited benchmark data, such as
those on the nine pedons in this study.

It is obvious that the two classification systems '
are not compatible in many respects even though they have
similar diagnostic horizon definitions. Their correlation
cannot be achieved with great accuracy. For soil and land
management problems and classification it is recommended
that the U.S. Soil Taxonomy be adopted while exploring the
adaptability of the FAO system. For instance, soils
classified into the same soil families using the Soil
Taxonomy, should have almost the same management require-
ments and similar potentials for crop production. This
would afford a better basis for correlation and establish-
ment of benchmark sites within an ecological zone.

However, as a means of international communication,
the FAO system can be adopted as a basis for higher-
category groupings. The French Classification system used

in some northern parts of Nigeria is based on the evolution

189

of the entire profile and the type of humus; the amount of
"lessivage" is considered for the lower categories. Much
emphasis is placed on climatic zonation as compared with
lithogenic influences. The class boundaries are imprecise
and allow for too much subjective interpretation. For
soil correlation work in Nigeria, an English-speaking
country, the adOption of the French system poses great

difficulties.

Series correlation.--The purposes of the Series

 

category, like the Family, is mainly pragmatic. The
Family and Series differentiae as outlined in U.S. Soil
Taxonomy (Soil Survey Staff, 1975) are therefore recom-
mended. For the present study and future series corre-
lation in Nigeria, the following modifications are pro-
posed, taking into account the difficulty in acquiring
the extensive laboratory data required by the U.S. Soil
Taxonomy:

(1) Depth to bedrock (if any)

 

a. 0 - 30 cm : very shallow
b. 30 - 90 cm : shallow
c. 90 -150 cm : deep
d. > 150 cm : very deep
The 30 cm limit corresponds with the limit for Lithosols

of D'Hoore (1964).

190

(2) Texture of diagnostic horizons present affect
water holding capacity of the soils. Structure, mineralogy
and amount of organic matter should also be considered in
series differentiae and correlation. Attention is centered
on the genetic horizons of major biologic activity below
the depth of normal ploughing.

Nature and degree of expression of one or more
horizons should be considered.

Fine sandzcoarse sand ratio may also be used for
separating soil series. In general the clay content of
the B horizon (if present) can be used.

(3) Drainage

These classes are recognized:

 

USDA FAQ

Poorly drained Very poorly drained
Somewhat poorly drained Poorly drained
Moderately well drained Imperfectly drained
Well drained Well drained

The degree and depth of mottling are considered in poorly-
and somewhat-poorly drained soils. For gleyed conditions
a value of 4 or more and chroma of 2 or less (Munsell
Color Notation) for moist soils are used for separation.

(4) Large differences in color: Color of the B

 

horizon (if present) is used. Three classes can be used

as very approximate guides only:

191

"Red"--Hues of 5YR or redder

"Brown"--Hues of 7.5YR or 10YR (no mottles)

"Gray"--Hues of 10YR (with mottles) or more yellow

Table 13 shows the proposed tentative series and
their proposed correlation for the study area. These
series are prOposed using the above guidelines and field
study. It is strongly emphasized that these proposals are
tentative based solely on data obtained from the study
area. The correlated series are based on soils derived
from sedimentary rocks in Western Nigeria (Moss, 1957) that
are mostly Alfisols. Consequently, approximate corre-
lations have been made in this study pending further studies.

Vine (1956) provisionally defined four "Fascs"
roughly equivalent to Families to cover the "Latosols" of
Nigeria. Using this grouping, the soils of the study area
can be classified as follows:

(1) Benin Fasc. These are deep red porous soils

 

derived from sandy loam to sandy clay loam deposits.
Pedons l, 2, 3, 7, l3 and 14 are included under the Benin
Fasc.

(2) Calabar Fasc. These are derived from similar

 

sandy deposits but are brown as opposed to red. Pedons 5,

6, 10, 11 and 12 are classified under the Calabar Fasc.
The above classification is not recommended for

soil mapping due to variability within them plus the dis-

tinction based mainly on color. Within the Fascs there

192

may be some series that are separated solely on basis of
color. On a broad basis the soils of the study area can
be placed in the Benin and Calabar Fascs but the detailed

subdivision of each into series according to the texture

of the suboils and other characteristics is recommended.
In soil surveys in Western Nigeria the more clayey soils
of the Benin Fasc were placed in Alagba Series and the
more sandy ones in Kulfo Series.

Erosion features were not observed in the study
area. Stones constitute less than 1% of the particle size
distribution of the soils. Consequently the important
phases locally should be soil types within the area. The

texture of the plow layer should be used in this instance.

SUMMARY AND CONCLUSIONS

Fourteen pedons representing soils found along a
126 km transect (Umuahia to Port-Harcourt) traversing Imo
and River States of Nigeria and within the Coastal Plain
Sands geologic formation were investigated in the field
with some supplemental laboratory measurements of particle
size distributions. On the basis of the data obtained 8
nine of the fourteen pedons were selected for more inten-
sive studies that included their clay mineralogy, micro-
morphology, physical and chemical properties. Their
characteristics were established and the classification of
all the fourteen pedons at the family level was made using
the U.S. Soil Taxonomy. Approximate classification of the
soils was also done with the FAD/UNESCO Soil Map of the
World Legend. Tentative series were proposed and these
are tentatively correlated with series used in the Western

Region of the country.

Mineralogy.--Mineralogy of the coarse-silt and

 

fine sand fractions of all the soils indicate the domi-
nance of quartz. Weatherable minerals constitute less

than 10% in these fractions. The heavy minerals in

193

194

selected pedons show the predominance of zircon, stauro-
lite, tourmaline and rutile.

The x-ray diffraction analyses of clay show the
predominance of kaolinite. Thus, the lack of appreciable
weatherable minerals in the coarse silt and sand fractions
is not surprising since kaolinite is the dominant mineral
in the clay fraction. Quantitatively, soils located on
higher land surfaces, and thus relatively older, have more
kaolinite than the presumably younger soils on the lower
surfaces. Clay minerals of the 2:1 type were not found
even in the younger more poorly drained soils. Small
amounts of quartz were found in the clay fraction of pedon 1
an ustic and pedon 5 an udic soil. Since kaolinite is the
predominant clay mineral in these soils, the prevailing
dry and wet moisture conditions, low base saturation and
low pH, preclude the condition conduvice to formation of
montmorillonite. Kaolinite may have been largely inherited
from the parent material instead of as a result of pedo-

genesis.

Micromorphology.--Intense weathering conditions,

 

resistant minerals and the alternately dry and wet seasons
prevalent in the area are reflected in the micromorphological
characteristics of the soils. In most of the soils isotic
and undulic plasmic fabrics characterize the micromorphology,
indicating relatively advanced weathering stages. Insepic

fabric is also shown. These types of plasmic fabric can be

195

due to the masking of the birefringent plasma by the sesqui-
oxides. Perri-argillans are only slightly evident especi-
ally in soils on the higher surfaces with higher amounts of
free iron oxides (pedons 1, 2 and 3). Skeletal grains are
composed mainly of quartz that vary in shape and size.

Some voids are filled with organic matter. Pedological
features include cross-sections of roots, generally most
common in the surface horizons. Voids are predominantly

simple packing voids.

 

Genetic sequences.--Considering the soil formation
factors (the textures of the B3 horizons, the natural I
drainage classes, elevations and other data), genetic
sequences of soils have been recognized. Soils with aquic
moisture regimes (poorly or somewhat poorly drained) con-
stitute a lithosequence of soils formed in sandy (pedon 4),
coarse-loamy (pedon 9) and coarser fine-loamy (pedon 8)
materials, respectively. Lithosequences of well-drained
soils with ustic moisture regimes (at higher elevations
further inland) developed in coarse-loamy, coarser fine-
loamy and finer fine-loamy materials are pedons 13; 2 or
3 or 14; and l or 7; respectively. Pedons 6 or 11 or 12;
5; and 10 represent lithosequences of well-drained soils
with udic moisture regimes developed in coarse-loamy,
coarser fine-loamy and finer fine-loamy materials, respec-
tively. Pedons 9 (somewhat poorly drained) and 11 or 12

or 6 (all well drained); with pedons 8 (poorly drained)

196

and 5 (well drained) represent toposequences of soils
formed in coarse-loamy and coarser fine-loamy materials,
respectively.

Chrono-climosequences of udic and ustic well-
drained soils are: pedons 11 or 12 or 6 and 13 (developed
in coarse loamy materials); pedons 5 and 14 or 3 or 2
(developed in coarser fine-loamy materials); and pedons
10 and 7 or 1 (developed in finer fine-loamy materials).
The pedons are listed in order of their increasing ele-
vations within each parent material group and the ustic
pedons are at the highest elevations (60 m+). Clear-cut

biosequences of soils were not evident.

a. Lithosequences.--Pedons 4, 9 and 8 are a litho-

 

sequence of soils with aquic moisture regimes developed
in sandy, coarse-loamy and coarser fine-loamy materials,
respectively. No textural B horizon is observed in pedon
4, a sandy soil with a spodic B horizon (Bhirm). Pedon 4
is the most acid soil studied, has the lowest P-fixation
capacity and very low available water holding capacity.
There is less clay and silt contents throughout pedon 9
(coarse-loamy) and it has less water holding capacity
(26 cm) than in pedon 8 (coarser fine-loamy, 30 cm), and
both have argillic horizons.

In pedons 13 (coarse-loamy); 14 or 2 or 3 (coarser
fine-loamy); and l or 7 (finer fine-loamy); all well-

drained and with ustic moisture regimes (lithosequences

197

developed in coarse-loamy to finer fine-loamy materials),
there are greater clay contents in the B horizons and less
sand content in their A horizons as their B3 horizons are
lower in sand and higher in clay. The finer textured
pedons also have higher available water holding capacities.
Similar profile texture differences and available water
holding capacities with differences in the textures of the
B3 (except that pedon 6 has no finer subsoil) are observed
among the lithosequences of well-drained soils with udic
moisture regimes (pedons 11 or 12 or 6; 5; and 10 developed
in coarse-loamy to finer fine-loamy materials, respectively).
Some of the reasons for the clay distribution patterns in
these well-drained soils include eluviation with illuvi-
ation, differential removal of the fine particle size
fractions by erosion plus faunal activity. Faunal activity
and pedoturbation may explain the lack of argillic horizon
in pedon 6.

The lithosequences of well-drained soils have more
reddish subsoil colors on the finer materials at higher
elevations, in both ustic and udic moisture regimes
(chrono-lithosequences).

The general pattern of soil structures with depth
for these well-drained soils is mostly fine, porous crumb
to subangular blocky. The subangular blocky B horizons
of most of the soils are weakly developed. The litho-

sequences of soils with ustic moisture regime soils

198

developed in coarse-loamy materials (pedon 13), coarser
fine-loamy materials (pedon 2 or 3 or 14) and finer fine-
loamy materials (pedon l or 7) show an increasingly more
homogenous structure development with depth in the finer
materials. For udic moisture regime soils, pedon 11 or
12 (coarse-loamy), 5 (coarser fine-loamy) and pedon 10
(finer fine-loamy) show increased structural development
from fine crumb throughout to crumb structure in the sur-
face over subangular blocky lower horizons in the finer
materials.

The develOpment of fairly well expressed crumb
structure in the upper horizons is attributed to the
presence of grass and forest regrowth and higher organic

matter contents .

b. Toposequences.--There is a higher proportion

 

of fine-clay in relation to coarse-clay in most of the
subsoils of pedons with argillic horizon (all but pedons

4 and 6). The ratio of fine-clayztotal clay is greater

in the upper subsoil of pedon 8 compared to pedon 5 (topo-
sequence of soils in coarser fine-loamy materials) indi-
cating more fine-clay movement into the upper subsoil.
However, pedon 8 is higher in silt content than pedon 5
and this may be a litho-toposequence of soils. Pedons 8
and 9 show markedly higher fine-clay:total clay ratios
particularly in their B21tg horizons and these ratios are

higher in the lower B horizons than in their Ap or A2

199

horizons, indicating eluvial surfaces and illuvial sub-
soils. This is in spite of the evidently coarser deeper
subsoil materials in the II layers of these profiles.
Illuvial clay skins were observed in the field in pedons
8 and 9.

The more poorly drained soils are also paler, more
mottled and more yellowish in subsoil colors than the well-
drained soils. These are found mostly in the southern
zone of the study area at lower elevations. All of these
differences when compared to better drained soils from
similar parent materials locally, represent toposequences;
of soils.

Increased structural develOpment in the subsoils
is also observed in the more poorly drained members of
these tOposequences of soils (pedons 9 versus 11 or 12 or
6; and 8 versus 5) develOped in coarse-loamy and coarser
fine-loamy materials, respectively.

The more poorly drained members of the toposequences
(pedon 9 and 8) have shallower water-tables, at least sea-
sonally, and consequently may have more available water
within 180 cm than the available water measurements indi-
cate if it is available to the plants. The more poorly
drained soils of the toposequences (pedons 8 and 5; or 9
and 6) have more calculated available water than the well-

drained soils and this can be associated with their

200

greater organic matter and higher silt contents (a litho-
logic difference).

Pedons 8 and 5 (coarser fine-loamy) and 9 or 6
(coarse-loamy) are toposequences of soils with associated
differences in the forms or amounts of active P present.
In the more poorly drained soils Al-P and Ca-P are more
abundant than Fe-P and are almost equally distributed
within the profiles. On the other hand, Fe-P is the most
abundant among the well-drained soils, with Ca-P being the
least abundant.

The more poorly drained soils in these toposequences
also fix less phosphorus. Organic C:P ratios generally
indicate that more mineralization of P may occur with
greater organic matter contents and may provide more suit-

able environments for microbial activities.

c. Chrono-climosequences.--The lithosequence of

 

ustic well-drained soils developed in coarse-loamy to
finer fine-loamy materials (pedons 13 coarse-loamy; 2 or 3
or 14 coarser fine-loamy; and l or 7 finer fine-loamy)
have more reddish subsurface colors with similar textures
of materials than udic well-drained soils (pedons 11 or

12 or 6 in coarse-loamy; S in coarser fine-loamy; and 10
in finer fine-loamy materials). These are climosequences
of soils. But they may be chrono-climosequences as they
occur on higher elevations with the more ustic moisture

regimes in each case. The ustic pedons studied more

201

intensively also contain higher FeZOS-d and less A1203-d
than the udic pedons in this study.

The well-drained ustic pedons (l, 2, 3 and 7) con-
tain larger amounts of dithionite extractable FeZO3 in
most cases (pedons l, 2 and 3) and larger amounts of
FeZO3 than A1203-d in all cases than do the well-drained
udic pedons (5 and 6). The well-drained udic pedons both
contain larger amounts of dithionite extractable A1203
than FezO3 (Table 10). Likewise the active P in the ustic
pedons (Figure 14) is predominantly (> 70%) Fe-P. Pedon
6 in the udic group, which does not have an argillic
horizon, approaches this proportion of active P as Fe-P,
but Al-P is more than one-third of the active P in pedon 5.

In the climosequence of well drained ustic soils
(develOped in coarser and finer fine-loamy materials;
pedons 2 or 3 and l or 7) and the udic well-drained soils
(developed in coarse-loamy to coarser fine-loamy materials;
pedons 6 and 5), the udic soils also have slightly lower
phosphorus fixation capacity than the ustic soils and
higher CBC/100 gms of clay in the subsoil.

Soils with udic moisture regimes also have some-
what higher base saturation than those with ustic moisture
regimes. This can be associated with increased rainfall,
more forests or their somewhat poorer natural drainages.

This may represent a bio-climosequence of soils.

202

Soil c1assification.--On the basis of data obtained

 

in this study, the fourteen pedons are classified as
follows (using the U.S. Soil Taxonomy):

Pedons l, 2, 3, 7 and 14: Typic Paleustults, fine-
loamy, siliceous, isohyperthermic.

Pedons 5 and 10: Typic Paleudults (or Typic
Kandiudults), fine-loamy, siliceous, isohyperthermic.

Pedon 12: Typic Paleudult (or Typic Kandiudult),
coarse-loamy, siliceous, isohyperthermic.

Pedon 4: Aeric Grossarenic TrOpaquod, sandy,
siliceous, isohyperthermic.

Pedon 6: Oxic DystrOpept, coarse-loamy, siliceous,
isohyperthermic.

Pedon 8: Typic Tropaquult, fine-loamy, siliceous,
isohyperthermic.

Pedon 9: Typic Paleaquult, coarse-loamy, siliceous,
isohyperthermic.

Pedon ll: Arenic Paleudult (or Arenic Kandiudult),
coarse-loamy, siliceous, isohyperthermic.

Pedon l3: Arenic Paleustult, coarse-loamy,
siliceous, isohyperthermic.

Using the FAQ/UNESCO soil classification system,
all the soils except pedons 4, 6, 8 and 9 are further
classified as Dystric Nitosols. Gleyic Nitosols have been
proposed for pedons 8 and 9. Pedon 4 is classified as a

Gleyic Podzol and pedon 6 as a Dystric Cambisol.

203

For soil correlation and classification the U.S.
Soil Taxonomy system is prOposed. The FAQ/UNESCO system
is to be correlated at the higher categories for inter-
national communication. Series differentiae of the USDA
system are recommended with slight modifications, where
necessary, to suit local conditions.

Tentative series have been proposed and tentatively
correlated, as near as possible, with series used in the

western part of Nigeria.

RECOMMENDATIONS

The genetic sequences of soils and soil character-
istics based on these pedons need careful testing in the
field and laboratory. Mineralogical studies have not been
made to date of pedons at sites below 20 m elevation.

Most of the sites (11 of the 14) studied are well-
drained. It is thus prOposed that more study be given to
the classification of the more poorly drained soils in the
study area, particularly in the northern part of the area.
This will also facilitate classification of the more poorly
drained soils using the FAQ/UNESCO Legend.

Considering the relatively high content of iron
oxides in these soils, it is proposed that clay contents
(2n and 0.2u) and presence of argillic horizons be tested
after removal of the sesquioxides. As already mentioned
there is need to examine similar pedons deeper, 2-3 m, to
check the relative abundance of argillans in these soils
and their possible decreases in clay content and clay

mineralogy with depth.

204

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APPENDICES

APPENDIX A

PEDON DESCRIPTIONS

APPENDIX A

PEDON DESCRIPTIONS

Pedon l--AMS1

 

Location: About 16 km south of Umuahia on Umuahia
to Aba new road. Site of National Cereal
Research Institute, Nigeria.

Setting: About 150 m above sea level.

Vegetation: Forest regrowth; scattered oil palms,

Elaeis guinnensis.

 

Drainage: Well drained.
Relief: 0.5 - 1 percent slope
Erosion: None

Ap 0-10 cm. Dark brown (7.5 YR 3/2); sandy loam; crumb
structure; friable when moist, nonsticky and non-
plastic; many pores; many fibrous and woody roots;
high organic matter content; intimate humus admix-
ture. Sharp, irregular boundary.

B1 10-22 cm. Reddish brown (5 YR 5/4); sandy loam; crumb
structure; soft when dry, friable when moist,
slightly sticky and slightly plastic; medium

organic matter content; gradual boundary.

219

B21t 22-45 cm.

220

Yellowish red (5 YR 4/6); sandy clay loam;

crumb structure; friable, sticky and plastic;

moderately porous; few fibrous roots; irregular

boundary.

B22t 45-75 cm.

Yellowish red (5 YR 4/8); sandy clay loam;

crumb structure; many pores; faunal activity; low

organic matter content; irregular boundary.

B3 75-180 cm.

Red (2.5 YR 5/6); sandy clay loam; crumb

structure; friable when moist, nonsticky, non-

plastic; few fibrous roots; low organic matter

content .

All colors are moist except where stated otherwise.

Location:

Setting:

Vegetation:

Drainage:
Relief:

Erosion:

Ap 0-13 cm.

Pedon 2--AMM2

 

About 15 km south of Umuahia on Umuahia-
Aba new road. Site of National Cereals
Research Institute Amakama, Nigeria.
About 150 m above sea level.

Forest regrowth and scattered oil palms.

Well drained.

9

o slope.

None

Dark grayish brown (10 YR 4/2); sandy loam;

crumb structure; loose to very friable, nonsticky,

nonplastic; moderate pores; common fibrous roots;

221

medium organic matter; gradual boundary; strongly
acidic.

Bl 13-30 cm. Yellowish brown (10 YR 5/4); sandy loam;
crumb structure; loose when dry, very friable to
slightly plastic and sticky when wet; low organic
matter; gradual boundary.

B21t 30-55 cm. Strong brown (7.5 YR 5/6); sandy loam;
crumb structure; slightly plastic; few fibrous
roots; gradual boundary.

BZZt 55-108 cm. Reddish yellow (5 YR 6/6); sandy clay
loam; moderately developed crumb structure;
slightly plastic and sticky; few roots.

B3 108-180 cm. Reddish yellow (5 YR 6/8); sandy clay
loam; moderately developed crumb structure; very

low organic matter; very few roots.

Pedon 3--AMF3

 

Location: About 16 km south of Umuahia on Umuahia
to Aba new road. Site of National Cereals

Research Institute, Amakama, Nigeria.

 

Setting: About 150 m above sea level.
Vegetation: Shrubs regrowth, Acacia barteri.
Drainage: Well drained.

Relief: 1% slope.

Erosion: None apparent.

Ap 0-12

222

cm. Dark reddish brown (5 YR 3/2); sandy loam;
crumb structure; loose when dry, friable when
moist; nonsticky, nonplastic; medium organic matter
content; abundant pores; very strongly acid; clear,

smooth boundary.

Bl 12-22 cm. Dark red (2.5 YR 3/6); sandy loam; weakly

B21t 22-

B22t 45-

developed crumb structure; friable when moist;
nonsticky, nonplastic; common roots; smooth
boundary.

45 cm. Red (2.5 YR 4/6); sandy clay loam; moder-
ately developed crumb structure; friable, non-
sticky, nonplastic; low organic matter content;
common fibrous roots.

137 cm. Red (2.5 YR 4/6); sandy clay loam; well

developed angular blocky structure; slightly hard.

B3 137-180 cm. Red (2.5 YR 4/8); sandy clay loam; well

developed angular blocky structure; slightly hard;
moderately sticky and slightly plastic; low organic

matter; very few roots.

Pedon 4--AMM4

 

Location: About 16 km south of Umuahia on Umuahia to

Setting:

Aba new road. Site of National Cereals
Research Institute, Amakama, Nigeria.
About 145 m above sea level. About 3 m

from a flowing stream used as a source

223

for dry season irrigation for vegetable

garden nursery.

Vegetation: Mainly shrubs and grasses; bush regrowth.

Drainage: Poorly drained.

Relief: About 1% lepe.

Erosion: No evidence of accelerated erosion.

Ap 0-12 cm. Dark gray (10 YR 4/1); coarse sand; single
grain structure; loose; very porous; some fibrous
roots; strongly acid; clear smooth boundary.

A21g 12-30 cm. Gray (10 YR 5/1); coarse sand; single
grain structure; loose; few fibrous roots with
some thicker roots; clear smooth boundary.

A22g 30-140 cm. Gray (10 YR 7/1); coarse sand; single

grain; loose; a few roots; abrupt, irregular
boundary, varying from 100 to 180 cm below the

surface.

Bhirm 140-200 cm. Black (5 YR 2/1) to dusky red (2.5 YR

2/1); sand; iron-humus pan; very firm, massive and

compact; no roots. Base of iron pan not reached.

Pedon 5--ASU5

 

Location: Asa-Umunka, about 9 km from Aba on Aba-

Setting:

Port Harcourt road.
About 60 m above sea level. Almost flat

terrain.

224

Vegetation: Oil palm, Elaeis goinnensis; cassava,

 

Manihot utilissima; and forest regrowth.

 

Drainage: Well drained.
Relief: l-2% slope.
Erosion: None.

Ap 0-13 cm. Dark brown (10 YR 3/3); loamy sand; crumb
structure; friable when moist; nonsticky, non-
plastic; high organic matter; many fibrous roots;
clear smooth boundary.

Bl 13-38 cm. Dark yellowish brown (10 YR 4/4); sandy loam;
friable, nonsticky and nonplastic; medium organic A
matter; gradual boundary.

B21t 38-88 cm. Yellowish brown (10 YR 5/6); sandy clay
loam; crumb structure, moderately developed;
slightly plastic and slightly sticky; thick
fibrous roots; clear smooth boundary.

B22t 88-150 cm. Yellowish red (5 YR 4/6); sandy clay
loam; crumb structure; slightly plastic and
slightly sticky; low organic matter; few roots;
clear smooth boundary.

B3 150-180 cm. Strong brown (7.5 YR 5/6); sandy clay loam;
subangular blocky structure; low organic matter;

few roots.

225

Pedon 6--AK6

 

Location: About 37 km south of Aba. Two km south
of Rubber Research Institute of Nigeria,
Akwete, Nigeria.

Setting: Almost flat land. About 50 m above sea
level. Groundwater below 1.50 m.

Vegetation: Oil palm, Elaeis gginnensis; cassava

 

Manihot esculenta; shrubs and local vege-

 

tables.
Drainage: Well drained.
Relief: l-l.5% lepe.
Erosion: None

Ap 0-18 cm. Dark brown (10 YR 3/3); sandy loam; crumb
structure; fibrous and woody roots; low organic
matter; strongly acid.

Bl 18-53 cm. Dark yellowish brown (10 YR 4/4); sandy loam;
crumb structure; many woody and common fibrous
roots.

B21 53-110 cm. Yellowish brown (10 YR 5/4); sandy loam;
crumb structure; few roots.

B22 110-143 cm. Strong brown (7.5 YR 5/6); sandy loam;
crumb structure; few roots.

B3 143-180 cm. Strong brown (7.5 YR 5/8); sandy loam;

crumb structure; no roots.

226

Pedon 7--UM7

 

Location: Umuojima-Owerrinta, about 33 km from

Umuahia on Umuahia-Aba old road. South

8 km to Ugba junction.

 

Setting: About 106 m above sea level. Almost flat
topography.

Vegetation: Oil palms, Elaeis guinnensis, Acacio
barteri, shrubs.

Drainage: Well drained.

Relief: 1.5% $10pe.

Erosion: None

Ap 0-20 cm. Dark brown (7.5 YR 4/4); sandy loam; crumb

B21t 20-

B22t 55-

B31 100-

structure; friable when moist, nonsticky and

nonplastic; high organic matter; many fibrous roots;

sharp irregular boundary.
55 cm. Reddish brown (5 YR 4/3); sandy clay loam;
crumb structure; friable when moist, slightly
sticky and slightly plastic; few roots.

100 cm. Yellowish red (5 YR 5/6); sandy clay
loam; crumb structure; friable when moist; moder-
ately porous; low organic matter; irregular
boundary.

150 cm. Red (2.5 YR 5/6); sandy clay loam; crumb

structure; friable; low organic matter; few roots.

227

B32 150-210 cm. Red (2.5 YR 5/8); sandy clay loam; sub-

angular blocky structure; low organic matter; no

 

roots.
Pedon 8--PH8

Location: About 3 km from the Agricultural Farm
Center at Port Harcourt, along Aba-Port
Harcourt Road.

Setting: About 20 m above sea level. Almost flat
tOpography. Liable to high water table
during the rains.

Vegetation: Mainly shrubs.

Drainage: Poorly drained.

Relief: 0.5-1% slope.

Erosion: None.

Ap 0-10 cm. Black (5 YR 2/1); sandy loam; few medium gray
(10 YR S/l) mottles; friable; abrupt smooth boundary.

A2g 10-23 cm. Gray (10 YR 6/1); sandy loam; massive;
friable; many pores and roots; clear faint boundary.

B21tg 23-40 cm. Light gray (10 YR 7/1); sandy loam; with
few brownish yellow mottles (10 YR 6/6); weak, fine
and medium subangular blocky structure; peds have
irregular faces with no coatings; friable; irregu-

lar boundary.

B22tg 40-72 cm.

228

Gray (10 YR 6/1); loam; with many medium

yellowish red (5 YR 5/6) to yellowish brown (10 YR

5/6) mottles; few thin discontinuous clay skins

on some cleavage faces; firm to slightly firm;

many pores and roots; clear wavy boundary.

IIB23tg 72-120 cm. Gray (10 YR 6/1); sandy clay loam;

many fine to coarse strong brown (7.5 YR 5/8)

mottles; weak, medium, irregular angular to sub-

angular blocky structure; firm; clear, wavy boun-

dary.
IIB3 120-180 cm.

Light brownish gray (2.5 Y 6/2); sandy

clay loam-sandy loam; with common pale yellow

mottles; massive.

Location:

Setting:

Vegetation:

Drainage:
Relief:

Erosion:

Pedon 9--ET9

 

Extends directly west of the Port Harcourt-
Bori road, between the villages of Oyu and
Eteo. Approximate geographical position

is 4° 44' N latitude and 7° 11' E longitude.
Almost flat. About 30 m above sea level
Depth to groundwater is greater than 1.30 m.
Secondary shrubs and raffia palms.
Somewhat poorly drained.

0-0.5% slope.

None.

229

Al 0-2 cm. Very dark brown (10 YR 2/2); sandy loam; loose;
structureless; sand grains humus coated; worm
excrements; many roots and worm holes; abrupt
smooth boundary.

A2 2-10 cm. Dark grayish brown (10 YR 4/2); sandy loamy;
weak, very fine, subangular blocky structure;
very friable; sand grains humus coated; many fine
lumps of humus; common roots, and many worm holes;
clear smooth boundary.

IIB21tg 10-28 cm. Grayish brown (10 YR 5/2); sandy loam;
with gray distinct, fine and common mottles;
moderate, medium subangular blocky; very friable;
little organic matter; few roots; clear smooth
boundary.

IIBZZtg 28-87 cm. Light brownish gray (2.5 Y 6/2); sandy
loam; with rust spots and mottles, prominent,
medium to coarse; weak, medium subangular blocky;
moist, friable; little organic matter; few roots;
common worm holes; clear smooth boundary.

IIB3 87-180 cm. Light gray (2.5 Y 7/2); sandy loam; with
mottles, prominent, coarse and many; moderate,
medium subangular blocky; friable; little organic

matter; few roots; common pores and worm holes.

230

Pedon lO--B010

 

Location: Experimental Farm Bori Rivers State,
Nigeria. About 13 km north of Bien Gwara.

Setting: Level to gently sloping topography. About
30 m above sea level.

Vegetation: Under cultivation. Mainly maize, Egg

mays; cassava, Manihot esculenta. Some

 

wild palms, Elaeis guinnensis are also

 

present.
Drainage: Well drained.
Erosion: None.

Ap 0-15 cm. Very dark brown (10 YR 2/2); sand-loamy sand;
weak, fine crumb structure; very friable and porous.

ABl 15-40 cm. Dark brown (10 YR 3/3); sand; weak, fine
crumb structure; very friable and porous; strongly
acid.

B21t 40-80 cm. Dark brown (7.5 YR 4/3); sandy clay loam;
weak, fine crumb structure; very friable and
porous; diffuse boundary.

B22t 80-130 cm. Strong brown (7.5 YR 5/6); sandy clay loam;
weak, subangular blocky structure; slightly plastic;
porous; diffuse boundary.

B3 130-180 cm. Brown (7.5 YR 5/4); sandy clay loam; with
common strong brown mottles; weak, fine blocky

structure .

231

Pedon 11--IK11

 

Location: About 36 km north of Ka-Ogoni in Rivers
State Nigeria. Approximately 4° 35' N
latitude and 7° 22' E longitude.

Setting: Almost flat. About 4 m above sea level.

Vegetation: Regeneration after cultivation. Oil palms,

Elaeis guinnensis and bamboo.

 

 

Drainage: Well drained.

Erosion: None.

Ap 0-20 cm. Very dark brown (10 YR 2/2); sand; weak,
fine crumb structure; very porous; some fibrous
roots.

ABl 20-40 cm. Dark brown (10 YR 3/3); sand; weak, crumb;
few fibrous roots; gradual boundary.

ABZ 40-60 cm. Very dark gray-brown (10 YR 3/2); loamy
sand; weak, fine crumb; fairly porous.

Bl 60-85 cm. Dark brown (10 YR 3/3); sandy loam; very
friable; fairly porous.

BZt 85-185 cm. Yellowish brown (10 YR 5/4-5/6); sandy
loam; friable; fairly porous; roots down to 130

CID.

Pedon 12--KaL12

 

Location: East of the village of Ka-Lori and north

of the village of Sime Luekon,Rivers State,

232

Nigeria. Approximate geographical position

is 4° 48' N latitude and 7° 27' E longitude.

 

Setting: Slightly undulating. About 8 m above sea
level. Depth to groundwater is about 1.8 m.

Vegetation: Dense overgrowth of secondary shrubs of
which Musanga cecroides is one of the
most characteristic.

Drainage: Well drained.

Al 0-2 cm. Very dark grayish brown (10 YR 3/2); loamy sand;

AB 2-18

B21t 18-

B22t 50-

very friable; moderate, fine crumb structure; sand
grains are humus coated; many roots and few pores;'
abrupt wavy boundary.

cm. Dark brown (10 YR 3/3); loamy sand; weak,
coarse subangular blocky structure; sand grains
humus coated; many roots and few pores, few worm
holes.

50 cm. Dark brown (7.5 R 4/3); sandy loam; weak,
medium, subangular blocky structure; little organic
matter; common roots; few pores; clear wavy boun-
dary.

80 cm. Dark brown (7.5 YR 4/4); sandy clay loam;
weak, coarse subangular blocky structure; little
organic matter; common roots; very few worm holes;

gradual smooth boundary.

B3 80-180 cm.

233

Strong brown (7.5 YR 5/6); sandy clay loam;

moderate, coarse subangular blocky structure;

medium organic matter; few roots and very few worm

holes.

Location:

Setting:

Vegetation:

Drainage:

Relief:

Erosion:

Ap 0-15 cm.

Pedon l3--0KN13

 

Okpala-Ngwa on Umuahia-Aba old road.
Approximate geographical position is

5° 8' N latitude and 7° 19' E longitude.
About 60-65 m above sea level. Almost
flat.

Bush regrowth, cassava, and oil palms.
Well drained.

About 1% slope. Depth to groundwater
greater than 1.90 m.

None.

Dark grayish brown (10 YR 4/2); sand; structure-

less, single grains; dry loose; sand grains humus

coated; many roots; strongly acid; clear wavy

boundary.

AB 15-51 cm.

Dark brown (10 YR 3/3); loamy sand; moderate,

coarse, subangular blocky; dry, soft; sand grains

humus coated; common roots; few worm holes; gradual

boundary.

B21t 51-111 cm.

234

Brown (7.5 YR 5/4); sandy loam; moderate,

coarse subangular blocky; dry, slightly hard; few

worm holes; few roots; gradual, smooth boundary.

B3 111-190 cm.

Strong brown (7.5 YR 5/6); sandy loam-

sandy clay loam; moderate, coarse subangular blocky;

dry, hard; few pieces of charcoal; few roots;

common pores; no worm holes.

Location:

Setting:

Vegetation:

Drainage:

Relief:

Erosion:

Ap 0-10 cm.

Pedon l4--EZl4

 

Situated about 24 km from Umuahia at

Eziama. Approximate geographical position
is 5° 24' N latitude and 7° 21' E longitude.
About 105 m above sea level. Gently undu-
lating.

Bush regrowth and oil palms.

Well drained.

1-1.5% slope. Depth to groundwater is
greater than 1.50 m.

None.

Dark grayish brown (10 YR 4/2); sand; struc-

tureless, single grains; dry, loose; sand grains

humus coated; many fine lumps of humus (worm

excrements); many roots; few worm holes; abrupt

smooth boundary.

AB 10-40 cm.

Dark brown (10 YR 3/3); loamy sand; moderate,

coarse, subangular blocky-moderate to weak, fine

235

crumb structure; dry, soft; common roots and pores;
few worm holes, clear smooth boundary.

B21t 40-100 cm. Brown (7.5 YR 4/4); sandy loam-sandy clay
loam; moderate, coarse subangular blocky to medium
subangular blocky; dry, slightly hard; little
organic matter; few roots and common pores; gradual
smooth boundary.

B3 100-190 cm. Strong brown (7.5 YR 5/6); sandy clay loam;
moderate, coarse subangular blocky; dry, hard;

little organic matter; few roots and many pores.

APPENDIX B

SOME ANALYTICAL REAGENTS AND PROCEDURES

APPENDIX B

SOME ANALYTICAL REAGENTS AND PROCEDURES

Fractionation of inorganicyphosphorus forms using
Petersen and Corey (1966) method

 

Reagents

Ammonium chlorideyle - Dissolve 53.3 g NH4C1 in water and

 

dilute to 1 liter.

Ammonium fluoride - 0.5 N, pH 8.2. Add 17.3 ml of concen-

 

trated HF and 33.3 ml of 15 N NH OH to about 900 ml of

4
water. Adjust to pH 8.2 by the addition of 4 N NH4OH
using the pH meter and dilute to 1 liter.

Sodium hydroxide, 0.1 N - Dissolve 4.1 g of NaOH in water

 

and dilute to 1 liter.

Sulfuric acid, 0.5 N - Dilute 14 m1 of concentrated H2504

 

to 1 liter.

Sodium citrate, 0.3 N - Dissolve 88.2 g of tribasic sodium

 

citrate (Na3C6H507-2H20) in 900 m1 of water and dilute
to 1 liter.

Sodium dithionite - Reagent grade NaZSZO4'

 

Saturated sodium chloride solution - Suspend 400 g of

 

NaCl in 1 liter of water. (The solubility of NaCl in water
is 36 g/100 ml of water at 20°C.)
236

237

Activated charcoal (phosphorus free)

 

Primary phosphorus standard (50 ppm) - Dissolve 0.2195 g

 

dried reagent grade KH2P04 in about 400 m1 of water in a
1,000 m1 volumetric flask, add 25 m1 of 7N-HZSO4 and
dilute to volume.

Secondary phosphorus standards (2 and 20 ppm) - Dilute

 

20 ml of the 50 ppm stock solution to 500 ml for the 2 ppm
standard. For the 20 ppm standard, dilute 200 ml to 500
m1. These dilute solutions do not keep well and should be
made fresh each time a standard curve is run. A separate
standard curve should be prepared for each phosphorus
fraction. In preparing standard curves for the individual
fractions, the standards should be made up in the extract-
ing solutions and treated exactly like the samples in the
analytical procedure so that the standards contain the same
concentrations of extractants or added reagents as the
sample solutions. For the present study it is found more
convenient and accurate to prepare standards ranging from
0, .l, .2 to 1.0. The concentration of the phosphorus

in extracts (in ppm) is found by multiplying readings from
standard curve with appropriate dilution factors.

Chloromolybdic boric acid solution - Dissolve 3.5 g of

 

(NH4)6M07024-4H20 in about 150 ml of water by warming to
about 60°C. Filter the solution if cloudy. When the
solution has cooled, slowly add 75 ml of concentrated HCl

with stirring and again let the solution cool. Dissolve

238

30 g of boric acid in 200 ml of water by warming to about
80°C. Allow this solution to cool, add it to the molybdate
solution, dilute the final solution to 1 liter and mix
thoroughly.

Chlorostannous reductant - Dissolve 12 g of stannous

 

chloride, SnCl °2H20 in 50 ml of 6N HCl. Place the solu-

2
tion on a steam hot plate until clear and then dilute to
1 liter with water. When the solution has cooled, it

should be stored under a 10 mm layer of white mineral oil

in a brown bottle with a syphon.

Bray reductant (gmino-naphthol-sulfonic acid reductant) -

 

Grind the following dry materials and mix them thoroughly:
2.5 g l-amino-Z-naphthol-4-sulfonic acid (Eastman 360);
5.0 g sodium sulfite (NaZSO4); 146 g sodium bisulfite
(Meta, NaZSZOS). Dissolve 8 g of the resulting powder
mixture in 50 m1 of warm water. If possible, allow the
solution to stand overnight before using. A fresh portion
of this solution should be made up from the dry powder
every 3 weeks.

Potassiumpermanggnate, 0.25 M - Dissolve 39.5 g of KMnO4

 

in water and dilute to 1 liter.
Isobutyl alcohol

Ethyl alcohol, 95%

Ethyl alcohol, absolute

Molybdate-sulfuric acid reagent - Dissolve 60 g of ammonium

 

molybdate in 800 m1 of water by bringing the water to a

239

boil. Cool the solution, and add 84 m1 of concentrated
H2804. Cool and dilute to 1 liter.

Stannous chloride reductant - Add 3.0 g of stannous chloride

 

to 200 ml of water and follow with an addition of 56 ml of
concentrated HCl. Swirl until the solution is clear (the
heat of dilution of HCl should dissolve the SnClZ-ZHZO if
it is good reagent). Add 400 m1 of water and 130 ml of
concentrated H2804. Cool and dilute to 1 liter. This

solution should be prepared daily.

Procedure

 

Extraction and determination of aluminiumyphogphate.

 

To 1 gm soil is added 50 ml lN-NH4C1 and shaken for 30
min. to remove the easily soluble and loosely bound phos-
phorus and exchangeable calcium. After centrifugation, 50
ml 0.5N NH4F, pH 8.2 is added to the soil after discarding
the supernatant solution. This is shaken for 1 hour and
centrifuged. The supernatant is treated with about 0.5 g
phosphorus-free activated charcoal and placed on the filter
paper. Organic matter is thus removed giving a clear
solution. The soil is saved for extraction of iron phos-
phate.

To about 3 m1 aliquot of the extract is added 3 ml
chloromolybdic boric acid solution and mixed thoroughly.
One dr0p of Chlorostannous reductant is added, mixed and

absorbance of the blue color is measured at 660 mu within

240

15 to 30 minutes. A filter of 650 mu can also be used if
660 mu is unavailable.

Extraction and determination of iron phosphate.

 

Wash the soil sample used for extraction of aluminium
phosphate twice with 25 ml portions of saturated NaCl by
centrifugation and decantation. Add 50 m1 of 0.1N NaOH and
shake for 17 hours. Centrifuge the suspension and decant
the supernatant solution for P analysis.' Add 5 drops of
concentrated H2S04 and swirl the flask so that the organic
matter will flocculate. Addition of concentrated H2804
is continued until the dispersed organic matter flocculates.
Using 3 ml aliquot of the extract the color is develOped
and measured in the same manner as for the aluminium phos-
phate.
Extraction and determination of calcium phosphate.
The remaining soil residue is washed with two successive
centrifuge washings with saturated NaCl (25 ml). Add 50
ml of 0.5 N H2804, shake for 1 hour and centrifuge.

Take a 3 ml-aliquot of the extract and develOp and
measure color in the same manner as for aluminium and iron

phosphate.

0rganio_phosphorus determination (Legg and Black, 1955).

Ignition and extraction.

 

One gm sample of soil was placed in a beaker on a raised
asbestos sheet in a muffle furnace at 240°C for 1 hour.

The ignited sample was transferred to a 100 m1 centrifuge

241

tube, using a brush to remove any particles adhering to
the beaker.

To the ignited soil in the centrifuge tube, and to
a comparable 1 gm sample of non-ignited soil in a similar
tube, was added 10 m1 of concentrated HCl, and heated on a
steam plate for 10 minutes (final solution temperature
about 70°C). The tubes were removed from the steam plate,
and an additional 10 m1 of concentrated HCl added and
allowed to stand at room temperature for 1 hour.

About 50 ml of water was added, mixed with the
acid and suspension centrifuged. The extracts were poured-
into 250 ml volumetric flasks and made to volume with
distilled water.

Determination of inorganic P in extracts.

 

Inorganic phosphorus was determined by Bray's No 1 method
(1945) with 0.03 N NH4F + 0.025 N HCl as extractant.
Phosphorus in the extract was determined colorimetrically
by the blue color method of Murphy and Riley (1962).

Calculation of organic phosphorus content.

 

The difference in content of inorganic phosphorus found in
the extracts of the ignited and non-ignited samples may be
taken directly as an estimate of the content of organic
phosphorus in the soil.

Remarks.

This method of determination comes very handy in cases

where perchloric acid hood chamber is unavailable.

242
Particle size analysis-~pipette method
(Steele and Bradfield, 1934)

Summary. A uniform suspension of soil in water
is prepared and allowed to stand. Samples are removed at
different times from a measured depth (rising gravity on
a tube centrifuge) evaporated to dryness and weighed. The
largest particles in any sample are those with a settling
velocity just equal to the specific value for depth/time.
With all smaller particles, the concentration of the sample
is the same as that of the original suspension. The
relation of particle size and settling velocity is given
by Stokes' Law. By taking samples at different values of
depth/time, a series of summation percentages is obtained.
The difference between two successive summation percentages
gives the measurement of the fraction between those two

limits of size.

Equipment.

 

1. Z-way st0pcock sealed to the upper stem of a
5 cc pipette calibrated for total volume.

2. Sieves--Nos. 300 mesh, and 140 mesh.

3. Centrifuge (International with head carrying
8-100 cc tubes.

4. Evaporating dishes.

5. Large aspirator bottles and weighing dishes.

243

Procedure. Five gm soil from which organic matter

 

has been removed is dispersed with Calgon. After dis-
persion the suspensions are washed through a 300 mesh
sieve keeping the amount of water as small as possible.
The sands remaining on the sieve are washed into an evapo-
rating dish, dried at 105°C, and weighed. The sands are
further fractionated using appropriate sieves.

The suspension of clay and silt is caught in a
weighed 250 cc bottle and made up to volume. The suspen-
sions thus prepared are stirred for 2 min., shaken, time
noted and allowed to stand. Ordinarily in a set of 8
suspensions stirring is done at 5 minute intervals, to
allow time for sampling. Samples of 5 cc are removed at
the time shown below, placed in weighed dishes, evaporated
to dryness at 105°C.

The pipette is slowly filled, stopcock closed and
the suction tube removed. Reversing the st0pcock, the
sample is allowed to drain into the weighing dish and the
excess suspension is washed out with distilled water.

Four successive gravity samples are taken without
again stirring the suspension. After the last gravity
sample is removed, the suspensions are stirred for a few
minutes. Portions of 25 cc are placed in centrifuge tubes
and centrifuged as soon as possible for the required time.
Ordinarily four samples are run at one time, each in

duplicate. The time is taken from the moment the motor

244

Dopth and Time of Sampling Soil Suspensions. Temperature
25°C

 

 

 

Depth of By Gravity By Centrifuge Equiv. Equiv.
Sampling radius diameter
cm hrs mins secs hrs mins secs mu mm
5 0 33 11 2500 .005
5 3 27 40 ' 1000 .002
2 ' s 31 44 500 .001
2 22 6 56 250 .0005
2 O 4 16 125 .00025
2 0 l7 3 63 .000125
2 l 8 1 31 .0000625

is turned on until it is turned off. The speed of the
centrifuge is maintained as nearly as possible at 2200 rpm
using a vibration tachometer.

The temperature is maintained fairly constant by

working in a cool room of constant temperature.

n log

t:
(time of centrifuging) 3.81 N

 

N ”HI FUN

2
r (dl—dz)

where n = viscosity of the medium.

R R2 are distances from the axis of rotation to

1’
the surface of the suspension and to the sampling depth,

respectively. In this case, they are R1 = 22 cm and R2 =

24 cm.

245

2
ll

Revolutions per second

*1
II

Radius of the particle
<11 and d2 = densities of the particle and of the

medium, respectively.