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HTHS

THE INFLUENCE OF SOIL POROSITY
UPON ROOT BEHAVIOUR

Thesis for the Degree of M. S...
MICHIGAN STATE COLLEGE.

Allen T. Knight
5941

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THE INFLUENCE OF SOIL POFOSITY UPON ROOT BFFAVIOUR
by

Allen T. Knieht

A THESIS
Submitted to the Graduate School of Michigan
State College of Agriculture and Applied
Science in partial fulfilment of the
reouirements for the degree of

WASTE? OF SCIENCE

Densrtment of Horticulture

34/?xy/
WM // 3; : .

Root Gromth in Artificial Soil—Send Cores

Unaereted Cores - . . . . . . . .
fiereted Cores . . . . . . . . . .
DISCUSSION OF FOOT STUDIES
POPOSITY DFTFRWINATIONS
Procedure . . . . . . . . . . . . . .

Expreeeion of Tension . . . . . . . .

Significance of Moisture-tension Curves

Porosity Studies of Undisturhed Cores
Porosity Studies of Artificial Cores.
DISCUSSION OF POPOSITY DFTFFflINfiTIONS . .

STFNRTPF‘I o o o 0 o o o o o o o o o o o o o o

BIBTJIOCTPAPHY o o o o o o o o o o o o o o o

ACKflOVLEDGMFNTS . . . . . . . . . . . . .

O

(1)

INTRODUCTION

Farlier root investigations conducted by the author1 with the
Concord grape in the Niagara Peninsula of Ontario and with the McIntosh
apple in the Okanogan Valley of British Columbia have revealed many
facts for which an adequate explanation has not always been possible.

It has been observed, for instance, that there was poor development of
grape roots, even in the surface horizon, wherever the soil was heavy
and compact, while in other soils of lighter texture there was a luxuri—
ant development of roots at all depths. Also in the case of apple trees
on a heavy silty clay loam soil, roots have been found in abundance at
depths of 6 and 7 feet, while in nearby locations with somewhat differ-
ent soils, most of the roots penetrated only 2% to 5 feet.

It is known that the supply of moisture, nutrients, and oxygen, the
presence of toxic constituents, and physical impenetrability, are the
most common factors limiting root growth, but the relative importance of
each factor has not been definitely known in explaining root behaviour
in specific soils. °
Purpose of the Investigation

The purpose of the investigation was to determine the reason for
the failure of roots to penetrate certain horizons of compact soil, and
some Montnorency cherry trees in the orchard of Michigan State College

growing in a soil characterized by a dense compact subsoil, provided some

Unpublished material

(2)
convenient material with which to work.

Realizing the impossibility of studying all factors relating to this
phenomenon, it was decided to give particular attention to the influence
of soil porosity upon root behaviour. Such an approach to the problem
demanded the introduction of a study of soil aeration, since the influence
of soil porosity on root development is exerted largely through its con-

trol of moisture and air relationships.

RFVIPW OF LITFFATURE

There are in the literature many statements of a definite effect of
aeration in influencing root behaviour. Such conclusions have been reached
from the effect of waterlogging and compaction of the soil on root and top
responses both in the field and greenhouse, from the results of aeration
studies, and from gas and other analyses.

It has been reported by Robbins (14), that tramping and packing of a
heavy orchard soil by cattle resulted in the killing of the fruit trees,
and it is reported that deficient aeration was directly responsible for
the death of the trees. Likewise, Baver and Farnsworth (5) have found
that the failture of sugar beets on some of the heavier soils of Ohio in.
seasons of heavy rainfall is to be attributed to deficient aeration of the
soil, since the losses were most severe where the larger pores had been
largely destroyed through a system of continuous cultivation.

Symptoms of aeration deficiency have been even more pronounced in
waterlogged soils than in those where such a condition has been produced
by compaction, doubtless because aeration beComes even more restricted.
Schuster and Stephenson (17), have shown from studies made in the
Willamette Valley that roots of nut trees do not penetrate to any degree
into waterlogged horizons because of the small amount, or absence, of air
space. There was an absence of roots in soil cores taken from such hori-
zons, and it is concluded that deficient aeration rather than the high
moisture level was the direct cause of root restriction. Boynton and

Reuther (4) and Heinicke (12) report similar observations in New York.

 

1
Continuous cultivation reduces soil porosity largely through its destruc—

tion of organic matter.

(4)

Dean (9) reports that aeration of waterlogged horizons caused the
roots to develop in such soil, even though the moisture content was well
above the optimum for plant growth, and it was also observed that the densest
root growth occurred in the neighborhood of the aerating coils. Tisdale and
Jenkins (21) believe that the rice plant makes better growth and is more
resistant to certain diseases when the soil is allowed to dry out for 2.
to 5 weeks during the growing Season, thus permitting the entrance of oxygen,
and Coville (8) states that even cranberries and blueberries, which endure
submergence for months when inactive, are harmed by waterlogging of the
soil for only 5 to 4 days in summer.

Whether deficient aeration is induced by compaction and high moisture
content, or by waterloagina of the soil alone, it is probable that the
volume of air space becomes the limiting factor to root growth. Air capacity
is usually represented by the volume of non-capillary pore space, i.e. the
voids or spaces not filled with water. It is noteworthy that the volume
of non—capillarv pore soace is a direct function of moisture percentage
at high levels of soil moisture, since the volume of large pores decreases
as the soil becomes filled with water. Eventually at the point of complete
saturation both air capacity and air exchange are reduced to zero. Rchuster
and Stephenson (l?) are of the opinion that soils with less than 5 or 6
per cent air space are not favorable to the penetration of fruit tree roots,
and Baver and Fernsworth (3) believe that non—capillary porosities of 7 to
10 per cent are necessarv to produce large tonnages of sugar beets on
heavy soils of Ohio.

Baver (2) reports that soils with hirh non-capillarv pore space have

usuallv been found to favor a rapid rate of gas exchange, although Buehrer (6)

(5)
has shown that "not all of the void spaces between the particles contribute
to the flow of gases through such assemblages, and that an appreciable
proportion of them either terminate in stagnant estuaries or they are so
small that they offer a much greater resistance to the flow of air than the
larger pores." It is therefore probable that a soil with a considerable
proportion of larger pores, but with little or no continuity between them
may be less favorable to air exchange than a soil with a smaller prOpor-
tion of larger pores, but with minute channels, root and worm holes, cracks
and cleavage planes to act as connecting channels between at least a part
of the pores. Schuster and Stephenson (17) believe that such are the prin-
cipal means of effecting interchange of gases and drainage of Water, and
Buehrer (6) has found these channels of great significance in determining
the rate of flow of air through soil.

There is some disagreement concerning the exact manner in which air
exchange occurs in soil, but regardless of the nature of this phenomenon,
it is generally agreed that continuous free pore spaces are normalLy essential
in effecting this process. Although it is conceded that diffusion occurs
in pores of smaller diameter equally as rapidly as in those of larger diameter,
still under optimum soil moisture conditions, it is the large or non-
capillary pores only that are important in air_movements, for, in most soils,
the small or capillary pores are filled with water.

It is probable, in the case of many soils of low non-capillary pore
space, that rate of exchange may be equally as important, if not more impor-
tant, than air capacity or volume of large pores in determining the suits-
bility of a soil for root growth, since it has been demonstrated by Cannon (7)

that slow artificial streaming of the soil atmosphere permits root development

(6)

to occur in soils of relatively low non—capillary pore space and low

oxygen tension.

(7)

ROOT STUDIES IN THE ORCHARD

Method of Study

 

Three cherry trees were selected for root studies, two of which were
in a decidedly poor state of vigor, while the other represented more
nearly the average of the orchard.

A trench was dug along one side of the tree so that one half of the
root system was exposed for root studies. No attempt was made to record
quantitatively either the extent of spread of the roots or the degree of
branching in each profile, since there was an almost complete absence of

roots wherever compact horizons occurred. A photographic record was

therefore considered more satisfactory.

Results of Orchard Studies

 

Even from casual observation, it was apparent that certain horizons

were definitely unfavorable for the growth of cherry roots, as is illus—

trated in Figs. 1 and 2. There was almost a complete absence of roots

 

 

 

Fig. 1. Response of cherry roots to compact horizons.

(Note how roots have avoided compact soil on
each side of trunk)

(8)

 

 

Fig. 2. Results of unfavorable subsoil for root growth.
(Note how all roots are on left side of trunk)

 

 

 

 

Fig. 5. Red cherry roots in favorable soil.
(Note large trunk and free distribution of
roots throughout profile)

(9)
wherever compact horizons appeared in the profile, while in those areas where
the soil was loose and friable, there was considerable development of roots,
both laterally and vertically as shown in Fig. 5.

Considerable root killing was evident in the surface horizon where-
ever compact subsoils were encountered. Later studies of pore space of the
surface soil in such locations showed only a small volume of non—capillary
pores owing in part to admixture of fine textured materials from the compact
subsoil and also to cultivation. In loose, friable surface soils, such as
those of Fig. 5, the amount of root killing was small. Such trees as those
in Figs. 1 and 2 were therefore dependent upon a root system restricted
largeky to the subsoil. Examination of weather records for the fall of 1939
and early winter of 1940 showed a light snowfall in the fall with a minimum
temperature of -llO F on January 11, and it is reasonable to attribute the
root injury to the occurrence of such extreme cold at a time when the soil
was free from snow. It is emphasized that this injury was of slight extent
wherever the soil was friable and open, and although there are many factors
influencing penetration of frost into soil, it is believed that the absence
of air Spaces in the surface soil has been responsible, at least in part,

for the occurrence of root injury.

(10)

GPEFNHOUSF STUDIES

Procedure

General Remarks

 

By studying the growth of roots of some indicator plant in soils of
varying compactness in the greenhouse, and supporting these observations with
an investigation of root growth in several soil-sand mixtures in which aera-
tion studies were made, it was believed that a more complete determination of
the suitability of soils for root development could be obtained than by limit.
ing the study to what could be observed in the orchard.

Alfalfa was selected as the plant for greenhouse studies, primarily on
account of its fairly rapid growth under short day conditions. As was shown
later in the investigation, however, the outstanding advantage of this plant
for such a study is its high oxygen requirement, in which, according to
Gourley and Hewlett (ll), it may resemble many of our tree fruits. Alfalfa
has the further advantage that it is adaptable to growth under greenhouse con-

ditions.

Removal of Soil_Co 8

“ fl...

 

In order to provide soil conditions for the alfalfa plants in the green-
house approximating those in the cherry orchard, it was necessary to remove
undisturbed cores of soil from the field. This was considered especially
important, since any disturbance of the natural field structure makes radical
changes in soil characteristics, especially in their structural properties.

In removing the Cores from the profile, it was found necessary to sur—

round each core completely with a trench, since a slight strai- was often

(11)
sufficient to split the core along the well—developed cleavage planes.
Cores were approximately cvlindrical in shape, and were 8 to 10 inches in
height and 6 to 8 inches in diameter. Cans of approximateLy 5~gallon
caoacitv were prepared with about 2 inches of coarse sand in the bottom for
drainage, and the cores were transferred to them directly in the orchard.
Sand was tanped lightly into the space around the cores, and about 1 inch
of sand was added to the surface to prevent desiccation in the greenhouse
between irrigation periods.

Cores were taken in duolicate from different parts of the profiles
to represent both favorable and unfavorable soils for rooting, ranging
from an A horizon to dense and compact B and C horizons. Two cores were
taken from a dense sticky silty clay in the vicinitv of East Lansing in
order to have a soil of a distinctly different type for comparison with

the cores from the orchard.

Importance of Hoisture Control

As was stated earlier, the volume of non-capillary pores depends on
the moisture content of the soil in the higher range of soil moisture
values. In these compact subsoils with a small volume of large pores, a
slight excess of soil moisture above field capacity may be sufficient to dis—
place all the air from the few larger pores, and conditions for root
growth are definitely unfavorable especially when the tree is making rapid
growth. Under these conditions any roots which have penetrated such
horizons will be existing in an anaerobic environment. The absence of well-
developed cleavage planes, minute pores and other natural channels would
seem to be sufficient evidence that the air capacitz of compact subsoils in

Spring is quite small or nil, so that even Without considering the influence

(l?)
of ohvsical inoenetrabilitv, conditions for root growth have become
unfavorable.

It was not attemoted to have alfalfa roots in the cans exaosed to the
same conditions of moisture and aeration as were cherry roots in the or-
chard, but rather to have uniformly high soil moisture conditions
throughout the period of investigation. It was assumed that conditions
were similar to those which prevailed in these soils in spring. As was
shown later in this investigation, all cores were maintained at aooroxi-
metely field capacity, which probably represents a critical moisture level

in soils such as those used in this study with a small volume of large

pores.

Qetting_the Plants

 

One-year old alfalfa plants were dug from the orchard in October
while the leaves near the ground were still green. Only those plants
which had developed uniform well-branched root systems were used, the
tap root being pruned back to about 6 inches. The soil was removed by wash-
ing, and five plants chosen at random were set in each can. The manner

of setting the plants is illustrated in Fig. 4. Conditions were such that

 

 

 

(oar-5e 56nd

 

Soi/ (o-l'e

 

 

 

 

 

Fig. 4. Position of soil core and method of setting plants.

(13)
the roots could penetrate the core if the soil was favorable, but in the
event that the soil environment was unfavorable, the plants would produce
their roots in the coarse sand, obtaining their nutrient reouirements from
the nodules, and from the surface of the soil core. The plants were
allowed to grow for a period of 7 months, in which time it was considered

that there was ample Opportunitv for the roots to penetrate the cores.

Moisture Control

 

In controlline soil moisture, advantage was taken of the great ten-
sion exerted by heavy soils on water; it was therefore possible for the
cores to extract moisture from the sand surrounding them, but the reverse
process would not occur under the conditions of this study. The moisture
content of the cores seemed favorable for plant growth at the time the
cores were taken, but in all cases they absorbed water from the sand until
approximately field capacity was reached, and since the periods during
which free water appeared in the sand were not more than 30 minutes in dura-
tion, it is not likely that the moisture level in the cores rose much above
field capacity. At every irrigation, sufficient water was added to cause
drainage through holes in the bottoms of the cans, and it was attempted to
have the sand continually moist. Usually this was possible by applying

water every 2 or 5 days.

Artificial Soil-sand Cores
Unaerated Cores
Owing to Variations in nutrient level, compactness and other factors,
it was decided to construct artificial cores using finely ground soil, taken
from compact horizons, and coarse ouartz sand in different proportions by

weight; by mixing these components in the drv state and then adding water and

(14)

mixing until a puddling consistency was reached, it was hoped that a series
of cores could be constructed varying only in different amounts of pore
Space of a non—capillary size. Mixtures containing the higher proportions
of sand were expected to have the greater percentage of larger pores.

Cores were moulded in an 8—inch pot, jarring each pot several times
to reduce the size of pores, and consequentbv the volume of pore space.

It appeared later in the study that considerably more care should have been
given to the matter of puddling since a small variation in degree of pudd—
ling was sufficient to offset the influence of the soil-sand ratio upon
pore space relationships.

Plants were set as in the case of the undisturbed cores. Moisture
control was obtained in the same wav as mentioned above, and from data in
Table 2, it is evident that variations in soil moisture greater than 1 per
cent did not occur between duplicate samples of the same mixture.

Cores of the following soil-sand mixtures were constructed in dupli-

cate:
1/2 soil - 1/2 sand
5/1? " — 7/12 "
1/5 " - 2/5 "
1/4 " - 3/4 "

Aerated_Cores

 

Fssentialkv the conditions of this series were similar to those men—
tioned above, excepting that all cores received daily aeration. Only one
soil-sand mixture (5/12 - 7/12) was used in this series of cores, the con—
stituents being mixed and puddled as in unaerated cores. Aeration was

provided in three of these cores by running an air tube through the base

(15)
of the cans (Fig. 5a) into a layer of sand, the soil—sand mixture being placed
in the can like a horizon of a soil profile, with sand above and below it.
Air was removed through the tube in the base of the can, thus causing a move-
ment of gas within the core (the volumes of air removed were 500, 600 and
900 ml. daily, with each core receiving a different treatment). The method

of creating a flow of air is illustrated and described below.

 

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12% ',' FineGrere/ /.

(a)

 

 

 

 

 

 

 

 

Fig. 5. Method of aerating soil-sand mixtures

The other three cores of this series were moulded and placed in the
cans in the same manner as in the unaerated cores excepting that an air
tube (Fig. 5b) was run through the base of the can into the center of the
soil core. The ends of the glass tubing were covered with a small sack of
fine gravel to prevent clogging of the mouth of the tube with soil and also

to provide a more uniform distribution of air throughout the cores.

 

4 ”f 'll'u -.

 

 

 

501/ (are

 

 

 

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o ' l1. '_"

 

 

_.r d!__- Ian-AN.
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T to 6e Ream/ed

 

 

 

 

 

 

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Fig. 6. Sketch of apparatus for aerating soil cores.

(17)

Air was removed at epprorimctetv the same rate from all cores in the
aerated series, but the duration and amount of dailv removal were varied.
Under each of the aerated soil cores, an apparatus was constructed
(fig. 6) to create a uniform flow of air, through reduction of air
pressure, over a considerable period of time. It consisted essentially
of a large bottle (the greater the diameter, the more uniform the flow of
air) connected by means of an air tube to the soil core above, and with
a siphon ending in a capillary glass tip to regulate the flow of water
from the bottle. It was found that a stop cock was required as well as
the capillary tip in order to reduce the flow of water to the rate of 2
to 5 drops a second. To adjust the apparatus to remove 300 ml. of air
daily from a core, it was only required to have this volume of water re-

0
moved from the bottle, the larger volumes being obtained by raising the

water level in the bottle. Hence both amount and duration of daily

aeration treatments were varied with rate of aeration constant.

U}

i

-- .I -4 .- .-

311191-93“. Eiadri-nnfepiup. “- .11 or
Owing to wide differences in types of rooting in different soil types
investigated, and it was frequently observed that this phenomenon occurred
even in different parts of an individual core, it was considered that
quantitative studies of root growth in the different cores would not give
significant results, even though all the roots from a core were removed
and measured. By making macroscopic observations on such characters as
amount of branching of roots, location of branches and penetration, and
supplementing these with binocular studies of certain minute details such
as the formation of root hairs, it was believed that some conclusions Could
be reached concerning the suitabilitv of a given type of soil for root

develOpment.

(18)

Results of Study

ngt Growth in Updisturbed Cores
£ear*0rchard (52 - 4O inchesl

Under field conditions, few roots were found penetrating the cores from
this horizon since they were taken from a depression where water frequently
remained for considerable time in spring and also following torrential rains
in summer. It is probable, however, that the moisture content of this
horizon is lower than in that immediately above, since there was no reason
to expect poor drainage conditions in a soil with the deve10pment of cleavage
planes and large pores that this soil possessed, although the presence of
a compact horizon below might create such a condition. The soil was a pale
yellow—brown silty clay loam containing gravel of various sizes admixed.

The pH was 7.4 to 7.6.

Cores broke readily along cleavage planes, which were most pronounced
in this soil, the horizontal planes being much better developed than those
running vertically. Many pores of various sizes and shapes were detected,
some of them being as large as 0.4 cm. in diameter. It was of interest to
note the small deposits of organic matter in some of these pores, and since
many of these were isolated, it is hardly possible that they represent
residues of old roots.

For the most part, roots followed planes of natural cleavage, as shown
in Fig. 7, with root hairs appearing clearly wherever root tips entered a
large pore, and it was also noted that there was little penetration into the
soil beyond the cleavage planes. Constrictions were noted near some of the
root tips where they penetrated the spaces between particles; these tips

usually enlarged again when resistance to their progress decreased. This

(l9)

 

 

 

 

Fig. 7. Root growth in core from subsoil of pear orchard-
(Note luxuriant root growth on plane of cleavage)
phenomenon is similar to that noted by Taubenhaus et a1 (19) in the case
of roots of the cotton plant which developed normally as long as the soil
was moist, but became strangled later in the season in a subsoil horizon
of hard, dry clay which prevented enlargement in diameter.
Cher Orchard 16-24 inche
The horizon from which these cores were removed was almost impenetrable
to cherry roots under orchard conditions. The soil was a pale yellow—brown
silt loam containing some gravel admixed, varying from prismatic columnar
structure and somewhat friable consistency to cloddy structure and compact
granules. The pH was 7.2 to 7.5. Cleavage planes were well developed
vertically as is illustrated in Fig. 8. Minute pores could be observed with
the naked eye, and excepting where the consistency was somewhat friable,

root development was largely restricted to planes of cleavage, a few root

(20)

channels and the minute pores.

 

Fig. 8. Root growth in compact core from cherry orchard.
(Note lateral branching and nodulation on cleavage
plane contrasted with relatively unbranched condi-
tion in lower part of core)

Lateral roots developed freely along cleavage planes, and even a few
single nodules were noted as shown in Fig. 8. The thread-like character
of the lateral roots and presence of nodules on roots in the cleavage plane

are illustrated in Fig. 9.

(?1)

 

Fig. 9. Dissected roots to illustrate (above) nodulation
and thread-like lateral branching on cleavage planes
and (below) large diameter and abundant branching
of roots in outer layer of Cores from A horizon.
A Horizon from Cherry Orchard
This soil is a dark gray-brown sandy loam containing considerable coarse
sand, with a friable consistency and compact structure. No organic residues
or fibrous materials were detected in October when the core samples were
taken and this was not unexpected since the amount of organic material added
to the soil annually through cover crops has been ouite small. It is
believed that the compact structure or absence of large pores is to be attri-
buted to the effect of continuous cultivation with only small returns of

organic materials, and there are many reports in the literature of such

degradation of pore space (4), (5).

(22)

It is entirely possible that following cultivation in spring, condi-
tions for rooting in this horizon are much more favorable in the orchard than
later in the season when the temporary effects of cultivation upon non—
capillary porosity have largely disappeared.

Few root or worm channels, or large pores were observed either with the
naked eye or with the aid of binoculars, and cleavage planes were entirely
absent. The pH of this horizon was 6.8.

In the outer layer of the cores, conditions were apparently favorable
for root develOpment, as shown by the dissected root in Fig. 9, but the
number of roots penetrating the interior of the cores was comparatively few.
Those roots which followed old root channels were characterized by their
thread-like form and free branching habit, while those which penetrated the
cores were whiter in color, larger in diameter and sparsely branched,
especially in the interior of the core.

It should not be inferred from these observations that roots would not
develoo in such horizons in the field. It is probable that the moisture
content under field conditions is somewhat lower than under the conditions
of this investigation, so that rooting might occur in the orchard, while
under such experimental conditions as these, there would be little or no
development. Further, it should be mentioned that under orchard conditions
the cherry roots had no alternative horizon from which they might obtain
moisture and nutrients more easily, while in the case of alfalfa roots in
cans in the greenhouse, a loose and open rooting medium was provided by the
surrounding sand. Apparentby the alfalfa roots were able to obtain all their

requirements from the sand and from the outside of the cores.

 

Fig. 10. Foot growth in weathered core of heavy silty clay.
(Note minute pores throughout exposed surface and
rooting on planes of cleavage)

 

Fig. ll. Root growth in unweathered core of heavy silty clay.
(Note absence of minute pores and of roots on most
of exposed surface)

(24)

Heavy Silty Clay (24 — 52 inches) and (32 - 40 inches)

Observations are reported on both a weathered and an unweatlered core,
since the response of roots is quite different in each. The core illustrated
in Fig. 10 was taken at a depth of approximately 24 —52 inches, represent—
ing a core of weathered soil, as indicated by the reddish—brown oxidation
color, the considerable development of hortizontal cleavage planes, the
platy structure, and the presence of minute pores. The lower horizon
(Z2 — 40 inches) evidenced little weathering, shoving little development of
cleavage planes, and in most of the core, not even the smallest of pores
were visible as is shown in Fig. ll. It is significant that in both the
unweathered and weathered cores, there was no indication of impenetrable
properties such as existed in the orchard subsoils studied. The pH was about
8.0.

The differences in root distribution of the alfalfa between the two
cores is shown in Figs. 10 and ll. In the weathered core, there were many
roots ramifying throur. the core, and especially along cleavage planes. No
nodules were noted along the cleavage planes, but, as in the case of the
other cores, they appeared in profusion just beneath the surface of the cores.
In most of the unweathered core, there was a complete absence of roots, and
only where a slight degree of weathering appeared Was there any development
of roots. The white color of the roots in these cores was characteristic,

which is an indication that toxic conditions did not exist.

Nu t rien_t_f:ci1.dl. e s o f. :10: kl" .C.’ p is
In order to assist in the interpretation of root distribution of the
alfalfa plants in the different soil cores, some studies were made of the

nutrient level of the different soils under investigation. Pesults are

presented in Table l.

(1’5)

Table 1. Nutrient Studies of Soil Cores*

 

 

 

P205 C30

'fpjl ‘ _ ‘ppm. ppm. Carbonate nH
A Horizon 0.5 60 No test 6.8
Heavy Silty Clay 0.5 200 Very high 8.0
Pear Orchard (24-52") 0.5 125 I No test 7.9

" " (52-40") 0.5 100 No test 7.4
Cherry Orchard (16-94") 1.0 100 No test 7.4

" " (24-32") 1.0 100 High 7.8
Artificial Cores 0.5 100 No test 7.4

*
All tests for available nutrients made by Simpler Rapid Method

In interpreting the results of such studies, it is necessary to decide
whether or not there is an adequate supply of available nutrients, and
also whether any nutrient element or toxic constituent appears in exces..
It was considered that the test for phosohorus was especiallv important,
in that soluble arsenic shows up in the test for phosphorus, and instances
of arsenic toxicity have been reported in the surface horizon of orchard
soils. As far as the test for arsenic is concerned, it is of little signifi-
cance whether a low test for phosphorus is to be attributed to either
arsenic or phosphorus, or both, since in any case the arsenic content would
be too low to be of any significance. It was considered that arsenic
toxicity was most likely to occur in cores from the A horizon of the cherry
orchard, but the low test for phosphorus of 0.5 ppm. immediately dismisses
this possibility. It is not likely that phosphorus deficiency has been

responsible for absence of rooting in any of the cores, since there was

(96‘)

no indication of this in the artificial aerated cores in which a test of
0.5 pom was obtained.

The data for calcium are presented because of the high recuirement of
alfalfa for this element (19). It is significant that sufficient available
calcium appeared to be present in all soils, and even when the pH value
rose to 8.0 in the heavv silty clay, there was an abundant supplv of cal-
cium to meet the requirement of the plants.

High tests for iron were obtained in the most compact of the subsoils,
and it is probable that these horizons have been cemented by depositions

of iron and aluminum colloids.

Root Growth in Artificial Soilzggpd Cores

 

Unaerated Cores

 

Perhaps the most significant result of the entire investigation was
the almost complete absence of root penetration in all uneerated cores,
regardless of the proportion of soil to sand. There was a considerable
development of roots in the O - 1/2 inch layer on the outside of some of
the cores, with many large compound nodules just beneath the surface where
aeration conditions and nutrient supply were apparentlv quite favorable.
In onlv one core (1/2 soil - 1/2 sand) was there any penetration beyond
1 to 2 inches. It is at once apparent from a study of Fig. 12 that only
a few roots have penetrated inwards more than 1 inch, and no roots have
reached the center of the core during the 6-month period of growth.

Penetration of roots appeared to be more extensive in those cores in
which the pores w-re larger, the presence of the larger pores indicating
less puddling than where onlv smaller sizes were present. It is notable

that no relationship was to be found between proportion of soil to sand

(27)

 

Fig. 12. Cross section of unaerated soil-sand core
(Note very shallow penetration of roots on
all sides of coral)

k

l

Alfalfa roots were permitted to grow in the sand on all sides of the
cores so that penetration might occur anywhere on the surface.

 

Fig. 13. Cross section of aerated soil-sand core
(Note nodule in coarse gravel in center)

and extent of rooting, but there was evidence to show that the effect of
puddling was so pronounced that any influence of the constituents of
these artificial cores was masked. It is assumed that aeration conditions
Jere more favorable in those cores in which large pores were found.

In order to check the efficiency of the method of irrigating the
soil cores, duplicate moisture determinations were made of the unaerated

cores at the time of root examination, as was mentioned earlier. The

(29)

results of this study, together with the moisture equivalents of the same

mixtures are presented in Table 2. With the exception of the 1/5—2/5

Table 2. Per cent moisture in artificial unaerated cores
at conclusion of experiment

!
v ——

Per cent moisture

Mixture of
of Per cent moisture moisture equivalent
soil-sand (bv wt.) (aDDrox.)
1/4— 5/4 10.9, 11.2 12.1
1/5 -2/5 11.5, 12.4 12.8
1/2 - 1/2 12.9, 15.2 12.5

Moisture equivalent determined at pF 2.6

core, there was a tendency for high moisture values to be associated with
higher moisture equivalent, but further data concerning this core (Fig. 21)
have shown it to be somewhat abnormal. It is therefore believed that
moisture conditions were maintained uniformly in all cores of this series,

with approximately 1 per cent variation between duplicate cores.

Aerated Cores
In contrast to the absence of rooting reported in all unaerated cores,
it was noted in all those receiving aeration treatment, that there was a
luxuriant development of roots as is shown in Fig. 15. Since the only
difference between this series and the unaerated series was the use of

artificial aeration, it seems reasonable to conclude that the differences

in rooting were associated with that factor. As far as it was possible.

(30)
to determine, the puddling was similar to that in the unaerated cores. At
no time during the course of the investigation was the sand surrounding
the cores allowed to become dry, for, although water was applied less heavily
than in the unaerated series, it was put on more frequently; hence it is
not likely that the moisture content of cores in this series differed
significantly from those in thexunaerated series.

After making detailed observations of the extent and habit of rooting
under the different aeration treatments, it was apparent that the lowest
treatment (500 m1. daily) was equally as effective in stimulating the
develooment of roots as were the higher degrees of aeration. Some concep~

tion of the ramification of fine roots can be obtained from Fig. 14.

 

Fig. 14. Ramification of roots as a result of aeration

(51)

As compared with roots from unaerated cores, these were larger in diameter,
more freely branched, and possibly whiter in color. There was an abundance
of root hairs near the root tips wherever the roots entered one of the
larger pores, which according to the investigations of Snow (18), is an
indication of satisfactory aeration conditions.

Since no detectable differences were noted in root response to the
different degrees of aeration, and since the sampling of roots for dry weight
or length determinations seemed to present many possibilities for error,
it was decided to resort to chemical measurements of the effect of aeration.
Providing the lowest rate of aeration was insufficient to meet the needs
of the plants, it seemed reasonable to expect that nitrification would be
greatest under the higher aeration treatments. The data presented in

Table 5 would seem to show that the higher degrees of aeration had caused

Table 5. Fffect of Aeration upon Nitrification

 

Aeration from center

of core Nitratesl
(Ml. per day) ppm.
300 6-12
600 10-15
900 10-20

Aeration from below

500 6-10
600 10—15
900 15-20
Check (Unaerated) 5-10

1 Nitrogen determinations made by Simplex Rapid Method

significantly large differences in nitrification to warrant conclusions
concerning the effect of different rates of aeration. Although duplicate
determinations were made, it is considered that the range of readings for
each treatment is too great to indicate more than a tendency for nitrifica-
tion to be related positively to degree of aeration. It can be stated,
however, that higher degree of aeration has tended to stimulate microbiologi-
cal activity more than the lower treatments, and it seems reasonable that
there should be a similar relationship between aeration and root growth.
Moisture determinations were not made as in the unaerated series,
and it can onlv be assumed from the results with unaerated cores, that
there was no greater fluctuation than is shown in the moisture values in
Table 2. It was realized, however, that a luxuriant development of roots
in this series must have reouired a considerable volume of water, and it is
not known whether or not the passage of water through the soil was suffi-
ciently rapid to maintain moisture conditions relatively constant. This
is an important consideration in view of the fact that lower soil moisture
values within the cores would make conditions for rooting more favorable.
At the time the experiment was in progress, it was considered that the
maintenance of a continuously high moisture content in the surrounding sand

would be sufficient precaution in moisture control.

DISCUSSICN 0F FOOT STUDIES

As far as root development in undisturbed soil cores is concerned,
it apneared that penetration of roots was favored by well-developed
cleavage planes, root and worm holes, minute channels, and other natural
passages through the soil. It is valuable, in this connection, to compare
the behaviour of alfalfa roots in soil with that noted by Thornton (55)
with nodule bacteria in agar culture. It is reported that the most
efficient nodules (those most active in nitrification) developed at the
surface of the culture or at some point where shrinkage or cracking of the
agar permitted free access of air. Plants with nodules embedded in agar
grew very poorly and gained no nitrogen, but the same plants were appar—
ently stimulated to new growth when cracks occurred in the agar, thus
exposing the nodules to air. It is considered that similarly favorable
aeration occurred in undisturbed soil along cleavage planes and other
natural passages, while conditions within the soil clods, or in the poorly
developed cleavage planes, were not suited to the growth of nodules.

In the more compact subsoils investigated, it is probable that defi—
cient aeration has not been the only factor limiting root growth, since
the gnarled and tortuous nature of roots found in such soil indicated that
it was difficult for the roots to penetrate it. It should be mentioned,
however, that white roots were found, even within the soil clods, in all but
the most compact of the soils studied in the greenhouse (Cherry orchard
24—52 inches), but there was little penetration beyond 2 inches from a
cleavage plane. It would be interesting and informative to apply aeration

to some of the undisturbed soil from these compact horizons and also from

(s4)
tight A horizons in future studies. It is noted that Heinicke and Boynton
(l?) by aerating the subsoil of a Dunkirk silty clay loam in New York
obtained significant responses in tree growth, leaf area, shoot growth
and in gain in cross section of trunk. It is not stated whether or not
root development was improved bv this treatment, but it can be inferred
from the higher oxygen concentrations that there was an effect upon roots
also.

When it is recalled that most of the non-capillary pore spaces of com—
pact horizons found in nature appear to have been filled with colloidal
material from the upper horizons, it is conceivable that aeration condi-
tions may become definitely unfavorable for root development, especialhv
under high soil moisture, since a slight excess of water above field capac~
ity is sufficient to displace the air from the limited volume of pores that
remain.

The influence of aeration in stimulating root development in
artificial soil-sand cores is unquestionable. As was stated earlier,
alfalfa plants have high oxygen reouirements, in which they are believed
to resemble certain of our tree fruits, and the profuse ramification of
roots indicates a favorable response to such conditions. The fact that
artificial aeration was necessary in order to obtain vigorous root develop-
ment in these mixtures is strong evidence of the fundamental importance of
aeration in permitting root penetration in the undisturbed soil cores
studied. It is further indication of the general necessitv for air exchange

in the development of roots of other plants of similar oxygen requirements.

 

 

 

 

 

Fig. 15. Apparatus for measuring volume of pores evacuated
at different tensions.

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Fig. 16. Sketch of apparatus for measuring volume of pores
evacuated at different tensions.

POPURITY DFTFRVTNflTInNS

Procedure

The apparatus used for porosity determinations is similar to that

described bv Bradfield and Jamison (5), differing essentially in the

method of controlling tension in the position of the burette. The

apparatus designed for this study is represented in Figs. 15 and 16.

Essentially it consists of a porous plate fused in the bottom of a

straight-walled glass funnel, the lower face of the plate being in contact

with a free water surface. The connection between the moisture in the

(37)

Soil and this free water surface is continuous throurh the porous plate.
iater removed from soil placed on the plate is allowed to collect in a
burette where it can be conveniently measured. The partial vacuum is pro-
duced b7 a water pump connected through a vacuum tank to a sensitive mano-
meter. In making water extractions at low tensions, it was found necessary
to use water as the manometer fluid in order to adjust the tension accurately.
It was found desirable to have the water manometer on the one side of the
stand and a mercury manometer on the other.

The most important part of the apparatus is the porous plate or mem—
brane in the base of the funnel. In this study, a Jena sintered glass
plate supplied by the Coors Company was used. According to specifications,
this plate consists of a thin layer of sintered glass of 5 porosity over a
layer of 5 porosity, the pore diameters of the entire plate ranging from
1.0 to 1.5 u' Water is held in these small pores with such great force that
it is supposedly possible to exert tensions of 60 cm. of mercury (810 cm.
of water) without air passing through the plate. It was not possible,
however, even with prolonged boiling in water to use tensions above 36 cm.
of mercury without leakage of air. Since a tension of 50 cm. was suffi—
cient for the purposes of this study, the plate proved to be entirelv
satisfactory, and it had the advantage of permitting rapid percolation of
water. Tensiometer cups were not found to be practical for this study on
account of the extremely slow movement of water.

In conducting studies of pore space of sand or pulverized soil, a
Eiven volume, about 200 cc., is placed in the funnel and a tension of ?O to

l o o o
50 cm is exerted in order to renove the air from the internal pore spaces.

 

l
on. refers to height of mercury column.

(38)
After extracting at this tension for 5 minutes, boiled distilled water
from the reserve water supply is slowly drawn through the porous plate by
opening the three—way stop cock, and stop cock A, and closing B and C. The
tension is continued in order to insure complete wetting of the soil and
filling of all air spaces with water. When the water has completely
covered the soil, the flow of water is closed off, and after standing under
tension for a few minutes, it is assumed that all of the pores have been
filled with water.

When a determination of pore Spaces evacuated under the force of
gravitr is to be made, drainage of water is checked by closing stop cock B
at the moment the free water disappears from the surface of the soil, and
a reading of the burette is made. A second reading is made when no more
water drains from the soil under gravity, and it is Considered that this
volume represents the gravitational or free water. This extraction is
probably of little significance when pulverized soil is being studied
because the natural structure has been largely deranged and the volume of
free water would be of no significance. With undisturbed soil cores,
however, such a determination is of considerable significance because it
is a measure of the volume of the larger pores, and it is these which are
of most significance in aeration. It is probable that root and worm chan-
nels, natural fissures and cracks, and the major portion of the larger
pores are at least partly drained in this extraction, and that they are
almost completely drained under a tension of 1 cm. In the case of corun-
dum and 40-20 mesh sand, it is noteworthy that the volume of pores drained
at zero tension amounted to only 0.2 and 0.6 ml. respectively, as shown in
Fig. 18, while in some of the undisturbed cores, it amounted to as much

as 2 ml. The volume of the pores evacuated under the force of gravity has

(39)

been recorded in the moisture-tension graphs as a solid horizontal line
just above the base line. In this study, the force of gravity has been
represented graphically as zero tension, so that in making comparisons
between these data and what have been obtained by other investigators, it
is necessary to make a small correction since the tensions herein reported
are about 0.2 cm. greater than the actual force of gravity. Bradfield

and Jamison (5), for instance, place the graduated pipette on the same
level as the upper surface of the soil; with such apparatus, it is neces-
sary to exert a slight tension to remove the water which in this study

was removed under gravity.

After extraction under gravity has been completed, successive tensions
are exerted, and the volume of water evacuated at each increased tension
is recorded. The width of the tension classes (0-0.4 cm., 0.5-0.9 cm.,
1.0-1.4 cm., or 1.0-1.9 cm.) must be decided by the nature of the soil as
is discussed below. Mohsture-tension curves can be constructed more
accurately when narrow tension classes are used, and this is especially
important in plotting the hortizontal portion of the curve, as illustrated
in Fig. 18. It is unnecessary, of course, to make more than 2 determina-
tions in order to plot a straight line, so that some conception of where
the lines are likely to change direction is valuable. In the case of the
40-20 mesh sand, for instance, it was necessary to make extractions at
tensions of 0.5, 0.8, 0.9, 1.0, 1.4, 1.8, 2.5 and 30.0 cm. in order to
construct the curve accurately, while with some of the samples of finer
texture, it was sufficient to make extractions at tensions of 1.0, 2.0, 5.0,

4.0, 5.0, 10.0, 20.0 and 50.0 cm.

(40)

In preliminary trials with the apparatus, some difficulty was experi-
enced in determining whether or not extraction at a given tension was com—
plete. The capillary tip at the top of the burette affords a rapid and
simple test, for by exerting a slight pressure on the connecting rubber
tube, and then releasing the pressure, a small bubble of air appears in the
capillary glass tip. If equilibrium has not been reached, this bubble is
soon forced out into the burette, and extraction must be allowed to con-
tinue further. The danger of incomplete extraction of water is especially
great when heavy soils are being studied, since the rate of water move-
ment is very slow owing to the minute channels through which the water must
move.

Some important modifications of the above procedure must be introduced
when undisturbed soils are under consideration, for it would be hardly
possible to prepare a core which exactly fits the funnel. Baver (4)
describes a core sampler which permits the insertion of a brass cvlinder
within itself; after taking the core, the cylinder containing the soil
column is clamped onto the porous plate. Such methods of core sampling
can onlv be used, however, when the soil is free from gravel, roots and
other such coarse material. In the soils under investigation, small stones
occurred frequently, so that it was necessary to shape the cores with a
knife; such a procedure also eliminated the danger of compacting the cores
during swnpling, a precaution which is especially important in this study,
since it is the larger pores that are most affected by such compaction.

In preliminary trials, undisturbed cores were used without covering
of any kind. It was found, however, even when water was introduced quite
slowlv into the funnel, that slaking and erosion occurred on the surface

of the cores, and an accurate measure of the volume of pores drained at each

(41)
tension was impossible. There was also the disadvantage that it was never
possible to calculate the exact volume or weight of the core after material
was lost from the surface.

It was found that, by covering the base and sides of the core with a
single layer of cheesecloth with as little overlapping as possible, the
error caused by slaking and erosion could be eliminated. Since a consider-
able amount of water is retained by the cheesecloth after saturation of
the core with water, it is necessary to make a correction for the volume
of water removed from the cheesecloth at each tension. This was done by
covering waxed soil cores with the same piece of cheesecloth and making
triplicate determinations of the water drained from the cheesecloth. By
this procedure, it was found that an accurate correction could be made for
water removed from the cheesecloth at tensions above zero, but there were
variations of 0.1 to 0.5 ml. on draining under gravity. It is assumed
that this variation is the error inherent in the method of measuring gravi—
tational water and that no greater error is introduced when the undisturbed
soils core is used. Further, it is not likely that the error involved in
the use of the cheesecloth is as great as the error of sampling, since a
single root or worm channel in the core could easiLy cause a variation of
several times this amount.

In conducting a pore space determination of an undisturbed soil core,
all changes in tension must take place gradually in order to prevent
derangement of the natural structure. When air is removed suddenly from
the core, there is danger of fracturing the walls of the pores owing to the
sudden release of air from isolated pores. Likewise, water must be admitted
slowly in order to prevent similar alterations in structure, especialty
in soils of unstable aggregates. Both the period of extraction of air

preceding saturation with water, and the period of wetting must be extended

(42)
somewhat longer in the case of undisturbed cores than when pulverized
soil is used, and especially when compact subsoil is being studied.

In order to measure the volume of the pores evacuated under the force
of gravity, it is necessary to remove the water from around the core as
quickly as possible so that any free water in the large pores or cracks
in the soil does not drain into the funnel before measurement of water in
the burette has begun. By connecting a rubber hose and drainage bottle
to the tension line (illustrated in Fig. 15), it was possible to remove
almost all the free water from around the core in less than 5 seconds.
Determination of the volume of pores extracted at each increased tension
was made as in the case of the pulverized soil.

After extraction of water was complete, all cores were weighed and
a determination of volume was made by dipping the cores in melted para-
wax at 700 C. and then measuring the amount of water they diaplaced on
immersion in water. The parawax was heated until soft, and it was possi-
ble to remove it without removing soil from the core. A determination
of drv weight was then made by heating in the oven for 24 hours at 97° C.

and all moisture percentages were based upon this oven dry weight.

Expression of Tension

The relation of moisture content to tension can be plotted graphi—
cally without converting the tension values in centimetres of mercury or
water to logarithms, but the space required for such a representation makes
this method impracticable when a wide range of tensions is considered.
By using the logarithm of the capillary potential, pF, it is possible to
represent moisture-tension curves on one graph which covers a wide range
of tensions. The term pF was introduced by Schofield (16) to express

the logarithm of the height in centimetres of the column of water that is

(4-4)

 

 

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(45)

necessarv to produce the desired suction or tension. For examole, a
force which supports a column of water 1000 cm. high is said to have a nF
of 5 since the logarithm of 1000 to the base 10 is 5. Similarly a column
of water 10 cm. high must be supoorted by a force whose pF is 1, since the
logarithm of 10 is 1. Forces measured by mercury columns can be converted
to equivalent water columns by multiplying the height in centimetres of
mercury by 13.54. The relationShip of the height of mercury and water
columns to pF is illustrated bv the graph in Fig. 17.

The greatest tension at which water was extracted in this study was
DF 2.6, which, according to Russell and Richards (15), is only slightLy
below the pF of the moisture equivalent.

According to Bradfield and Jamison (5), the diameter of pores evacu-
ated at a given tension can be calculated from the formula d I .30/h,
where h represents the tension in centimetres of water. For examole,
at a tension of 60 cm. of water, it is considered that all pores are
drainedwhich have effective diameters larger than sou. From the curves
of Fig. 18, it is seen that pores in corundum are largely about 60u in

diameter, while those of the 40-20 sand are largely in the 2'70u class.

Significance of Moisture—tension Curves.

Curves for corundum and sand have been presented for the purpose of
illustrating the significance of certain characteristics of moisture-
tension curves. The short horizontal lines, to which reference has already
been made, indicate that the volume of pores drained under the force of
gravitv is relatively small. The nearby vertical lines show that there

are few pores in the corundum ebove sou and above 270u in the sand, and a

(46)

large number of the pores fall within relativelv narrow class limits.
For the purposes of this study, the portion of the curve above the flex
point (the more or less horizontal portion of the curve at the point where
it begins to swing upwards) is not of much significance since it has
already been well established (4) that it is only the non-capillary pores
(those evacuated from zero tension to the pF of the flex point) that are
instrumental in soil aeration. 0n the other hand, when soil moisture move-
ment is considered, these small pores may have considerable significance.

As far as this study is concerned, the volume of the pores drained
under the force of gravity, and from zero tension to pF 1.0 and perhaps
the height of the flex point, are considered to have special significance
in indicating or measuring the aeration capacity of soils. It is evident
that the length of the horizontal line, together with the slope of the curve
from zero tension to pF 1.0 is also a measure of the volume of pores which
contribute towards aeration. Fever (1) has found that the tension of the
flex point, and the volume of water removed from zero tension to the pF
of the flex point, have considerable influence upon the rate of permea-
bility of water. The lower the pF of the flex point, and the larger the
volume of pores drained at low tensions, the greater was the permeability,
and since aeration and rapid permeability of water are conditioned by the
same or similar factors, it is quite possible that these characteristics of
moisture-tension curves are also indicative of the aeration to be expected
in a soil in which free drainage is permitted. In this s udv, special
emphasis has been placed upon the volume of pores drained from zero tension
to pF 1.0 since pores drained at tensions above this point are too small to
influence aeration to any considerable extent, even when moisture condi-

tions are optimum.

<47)

Owing to the length of time reeuired to make pore space measurements
by this method, it was impossible to make more than one determination with
most of the soil cores. The characteristics of the entire moisture-tension
curve are interesting, but it seems probable that the careful determination
of that portion of the curve from zero tension to pF 1.0 by decreasing the
interval between determinations, and representing this portion in detail
on the graph, would give a better representation of the aeration character-

istics of a soil; measurements at higher tensions could then be omitted.

Porositv Studies of Undisturbed Cores
The moisture-tension curve of the core from the A horizon (Fig. 19),
is similar to that of the 1/5-2/3 soil-sand mixture. The small volume of

1
1

pores drained under the force of gravity, the nearly vertica nature of the
urve from zero tension to the pF of the flex point, and the hi?” pF of the
flex point, are all factors which would indicate that the structure of this
soil is unfavorable for root development. As was mentioned earlier, an
inspection of this core revealed very few root or norm channels, the absence
of fissures and cracks, and verv few pores, either large or small, and these
observations were confirmed by the pore space measurements.

Moisture-tension curves of undisturbed cores from three soils are
presented in Fig. 21. In the case of the cores from the 16-24 inch hori-
zon of the cherry orchard, and the 32-40 inch horizon of the pear orchard,
there was a considerable drainage of pores under the force of gravity, and
from zero tension to pF 1.0. Further, the pF of the flex point was very

low in both soils (about 0.9), all of which are believed to be favorable to

aeration and root penetration.

(48)

The heavy siltv clay soil has certain characteristics, notablv the
tendency to swell on wetting, which make this method of measuring pore
space of questionable value for such soils. Soil pore space is a dynamic
property, owing to a large extent to changes in moisture content. When
such highly colloidal soils become saturated with water, either under
natural or laboratory conditions, not only do the larger cleavage planes
and pores become filled with water, but there is also a decrease in the
volume of these spaces. Hence, on measuring the volume of pores drained
at a given tension, it is found that the results are considerably lower
than expected from observations made on the soil in the drv state or at
the time of sampling in the field. It is the author's opinion, and like—
wise that of Baver (4), that a measurement of the volume of pores in a
soil in the swollen concition more closelv represents the conditions in the
soil, especially in spring when moisture is high in the available range,
and when most of the root growth occurs.

The small volume of pores drained under gravity and the vertical
nature of the curve from zero tension to the pF of the flex point, are
indications that this is a soil of poor internal drainage, yet the con-
siderable degree of rooting observed (Fig. 15) would seem to show that

his is not the case.

It is noteworthy that in this soil, a considerable time was required
for ecuilibrium to be reached at each tension. In making the extraction
at pF 1.4, for instance, it was necessary to continue the tension for 2
hours in order to obtain complete drainage of pores. The small size of the
pores retards the rate of movement of water through the soil, so that it
was found impracticable at tensions above pF 1.4 to attempt to obtain

complete extraction at any given tension.

Pp Tie :71? firjmclisrrfiamifiaialjo5-1-8 and flame

An inspection of the curves presented in Fig. 20 shows that there was
considerable variation in the pore space relationships in the artificial
soil-send cores, and, as was mentioned earlier, it would appear that the
degree of puddling has had more influence upon the amount of moisture held
at all tensions, and upon the pF of the flex points, than have the relative
proportions of soil to sand. The 1/9-1/2 and 5/12-7/12 mixtures, have
lower moisture values almost throghout the range of tensions, than have the
1/5—2/3 and 1/4-5/4 mixtures, and the pF of the flex points Show similar
discrepancies. It is believed that the degree of puddling has contributed
largelv to these variations, although it is possible that failure to per—
mit complete saturation of the cores with water before applving tensions
has also had an effect.

No porosity studies were conducted of aerated cores.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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(53-)

DISCUSSION OF POPOEITY DETEFWINPTIONS

Large differences were found in the total amount and size distribu-

tion of pores in the soils studied, as is shown in Table 4.

Table 4. Percentage of pores drained from zero tension to oF 1.0

 

 

 

 

Vol. of oores drained Per cent of pores
from drained from
Soil “HMZfiILiQHRF 1.0 1;:ng 101*" 1.0
Undisturbed
A-horizon 1.9 6.8
Cherry orchard
(16-24") 3.9 12.1
Pear orchard
(52-40") 5.8 1604.
Heavv siltv clay
(32-40") 5.5 7.0
Artificial
1/8- 2/5 4.2 9.2
5/12- 7/12 4.1 10.9
1/2 - 1/2 5.0 10.0

The small volume of the pores (6.8 per cent) drained from zero ten-
sion to pF 1.0 in the core from the.AIbrizon, and the large volume of
pores (16.4 per cent) of the same size from the 32-40 inch horizon of the
pear orchard are noteworthy with respect to root response. hhen only
undisturbed soils are considered, it is believed that differences between
volumes of the larger pores were associated with differences in type and

amount of rooting in the different cores. Although the small number of

(54)

cores studied does not warrant the statement of definite conclusions, still
the differences between degree of rooting in the different trpes of soil

and those illustrated in Table 4 concernina the volume of the laraer pores
would seem to be sufficient evidence for the existence of this associa-
tion. Further, the absence of rooting in the unweathered core of heavy
silty clay, and the very sparse development in the A horizon from the

cherry orchard were associated with small volume of pore saaces of all sizes
(as determined from observation), while in the weathered core of silty

clav and in other cores in which minute pores and planes of cleavage

could be detected, there seemed to be favorable conditions for rooting.

In the case of the cores from the A horizon, it is possible that the
absence of connecting channels between the pores may be of even greater
significance in determining root response than the small volume of large
pores, since in this horizon only very few root and worm channels were
observed, and there was a complete absence of cleavage planes. Owing to
the fact that rooting in many of the other undisturbed cores was largely
restricted to the natural channels in the soil, it is postulated that the
presence of such connecting channels, by virtue of their effect upon
water and air movements, may be of extremely great importance in deter-
mining the suitability of a soil for root growth.

In the light of this hypothesis, it is of interest to study the re-
lationship between volume of pores drained from zero tension to pF 1.0
in the different artificial unaerated soil-sand mixtures and the absence
of rooting in them. In the case of the 1/4-3/4 mixture, there has been

a drainage of 15 per cent of the pores which would seem to indicate that

(55)

aeration conditions were satisfactory for abundant root growth, yet this

did not occur. Owing to the fact that the pores in the artifiCial soil—sand
cores were largely isolated from one another, it seems reasonable to

suppose that the absence of connecting channels between isolated pores

has been responsible for relatively stagnant pockets of air. In all
probabilitv active microbiological development soon depleted oxygen supply
of these pockets, and since there was no connection established with the

atmosphere, the conditions for root development were decidedly unfavorable.

Ill: 1|" II. III] 11!; ‘ I‘ll all I

 

( 5 6)
smmm

Root observations of some Hontmorenqy cherry trees growing in dense
compact subsoil indicated that certain soil horizons were definitely un~
favorable for root development. Freezing injury in January, 1940, was
most severe where the surface soil was tight and somewhat compact, a
condition which apparently allowed deep penetration of frost.

Rooting of alfalfa in undisturbed soil cores taken from these and other
profiles showed considerable variation. Wherever cleavage planes or
other natural passages were well-developed, conditions for rooting seemed
to be favorable, as was indicated by the deveIOpment of nodules and abun-
dant lateral branching in such locations. Very sparse rooting was ob-
served in the cores from the A horizon, and this is to be attributed to
the small volume of non-capillary pores, and to the absence of connecting
passages between pores.

Aeration was necessary, under greenhouse conditions, to enable alfalfa
roots to penetrate artificial soil-sand cores, apparently on account of
the high oxygen requirement of this plant, and the absence of connecting
passages between isolated pores of the soil. Artificial cores had higher
volumes of non—capillary pores than certain of the undisturbed soil cores,
yet no rooting occurred without artificial aeration, apparently due to
the lack of air exchange within these cores.

In many compact subsoils, lack of aeration in spring may be of greater
significance in determining root behaviour of certain tree fruits than

physical impenetrability.

10.

(57)

BIBLIOGRAPHY

Baver, L. D. Soil permeability in relation to non—capillary
porosity. Proc. Soil Sci. Soc. Amer. 5: 52-56. 1958.

Baver, L. D. Soil Physics. John Wiley & Sons, Inc., New York. 1940.

and Farnsworth, R. B. Soil Structure effected in the growth
of sugar beets. Proc. Soil Sci. Soc. Amer. 5: 45-48. 1940.

and Reuther, W. A may of sampling soil gases in dense subsoils
and some of its advantages and limitations. Proc. Soil Sci.
Soc. Amer. 3: 37-42. 1958.

and Jamison, V. C. Soil structure ~~ attempts at its quanti-
tative characterization. Proc. Soil Sci. Soc. Amer. 5: 70—78.
1958.

Buehrer, T. F.. The movement of gases through the soil as a criterion
of soil structure. Arizona Experiment Station Tech. Bull. 39.
1932.

Cannon, W. A. Root growth in relation to a deficiency of oxygen or
an excess of carbon dioxide in the soil. Carnegie Inst. Wash.
Year Book 20: 48-51. 1921.

Coville, F. V. Directions for blueberry culture. U. S. Dept. Agri.
Bull. 9741. 1921.

Dean, 8. E. Effect of soil type and aeration upon root systems of
certain aquatic plants. Plant Phys. 8: 205-222. 1955.

Fred, E. 8. Root nodule bacteria and leguminous plants. Univ. of

Wise. Studies. 1932.

ll.

12.

15.

16.

17.

18.

19.

20.

(58)

Gourley, J. H. and Hewlett, F. 8. Modern Fruit Production. The
Macmillan 00., New York. 1941

Heinicke, A. J. The effect of submerging the roots of apple trees
at different seasons of the year. Proc. Amer. Soc. Hort.
Sci. 29: 205-207. 193?.

and Boynton, D. The response of McIntosh apple trees to
improved sub—soil aeration. Proc. Amer. Soc. Hort. Sci. 38:
27-31. 1940.

Bobbins, W. W. The Botany of Crop Plants. 2nd ed. P. Blakiston's
Son & 00., Philadelphia, 1924.

Russell, M. B. and Richards, L. A. The determination of soil
moixture energy relations by centrifugation. Proc. Soil Sci.
Soc. Amer. 5: 65-69. 1938.

Schofield, R. K. The pF of the water in soil. Trans. 3rd Int.
Soil Cong. 2: 57-48. 1955.

Schuster, C. E. and Stephenson, B. F. Soil moisture, root distri-
bution and aeration as factors in nut production in Western
Oregon. Ore. Sta. Agr. Exp. Sta. Bull. 572. 1940.

Snow, L. M. The effects of external agents on the production of
root hairs. Bot. Gas. 57: 145-145. 1904.

Taubenhaus, J. J., Ezekiel, W. N. and Rhea, B. E. Strangulation
of cotton roots. Plant Phys. 6: 161—186. 1951.

Thornton, H. G. The influence of the host plant in inducing para-
sitism in lucerne and clover nodules. Roy. Soc. (London)

Proc. Ser. B. 106: 110-122. 1950.

(59)

21. Tisdale, W. H. and Jenkins, J. M. Straighthead of rice and its

control. U.S.D.A. Farmers' Bull. 1212. 1921.

(60)

ACKNOWLEDGMENTS

The author wishes to acknowledge the direction and assistance
given by Dr. N. L. Partridge and Director V. R. Gardner of the Depart-
ment of Horticulture of Michigan State College in planning and conduct-
ing this investigation, and to Oliver M. Neal for assistance in

constructing the apoaratus for porosity determinations..

 

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137