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thesis entitled

THE EFFECT OF WHEAT STRAW MULCH ON CORN GRAIN
YIELDS OF ERODED MARLETTE SOILS
IN SOUTH-CENTRAL MICHIGAN

presented by

Nd ibu Muamba

has been accepted towards fulfillment
of the requirements for

M.S. degree in C109 & Soil Science

 

WKW

Major professor

 

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MSU I: An Afflrmutlvo Action/Equal Opportunity Institution
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””1

THE EFFECT OF WHEAT STRAW MULCH ON CORN GRAIN YIELDS OF
ERODED MARLETTE SOILS IN SOUTH-CENTRAL MICHIGAN

By

Ndibu Muamba

A THESIS

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

MASTER OF SCIENCE

Department of Crop and Soil Sciences

1996

ABSTRACT

EFFECT OF STRAW MULCH ON CORN GRAIN YIELDS ON ERODED
MARLE'ITE SOILS IN SOUTH-CENTRAL MICHIGAN.

By

Ndibu Muamba

Water erosion has reduced crop yields in South-Central Michigan. Wheat (T riticum aestivum
L.) straw mulch was added to eroded Marlette soil (fine-loamy, mixed, mesic, Glossoboric
Hapludalt) to maintain or increase the productivity of corn (Zea mays L.). A paired
comparisons method with mulch and no mulch treatments of the plots was used to determine
com yields under slight, moderate, and severe phases of soil erosion. A mulch rate of 5 Mg/ha
was applied annually prior to emergence, over the five-year period of the study. Climatic data
including monthly precipitation and growing degree days during the growing season, and
dates of 50% emergence and 50% silking were determined to monitor the corn growth and
yields were measured at harvest. Results fi'om the 5-year period study showed that com grain,
plant population, and stover yields on the mulch plots were slightly less, but not significantly
different, than yields of the unmulched plots of the slightly eroded phase. However, corn
yields were reduced significantly for both the moderately and severely eroded mulched plots

in comparison with the unmulched plots (5% level). Corn grain yields on the mulched plots

were reduced 4%, 18%, and 19% of yields obtained from the unmulched plots for the slight,
moderate, and severe erosion classes, respectively. The individual year average corn yields
for mulched plots ranged fiom 96-100% of corn yields obtained on the unmulched plots for
the slightly eroded phase, 57-99% for the moderately eroded phase, and 68-98% for the
severely eroded phase. Grain moisture contents at harvest were greater for corn produced on
the mulch plots for both the moderately and severely eroded classes, but were not significantly
difl‘erent for the slightly eroded class. Plant population of mulched plots were reduced 4%,
9%, and 10% of those from the umnulched plots for the slight, moderate, and severe erosion
classes, respectively. Stover yields were reduced 4% for the moderately eroded and 6% for
the severely eroded mulched plots when compared to unmulched plots, but increased 4% for
the slightly eroded mulched plots. Dates of 50% emergence of mulched plots were delayed
3 days for moderately eroded plots, but none for either slightly or severely eroded plots.
Dates of 50% silking of mulched plots were delayed 1, 3, and 3 days for the slightly,
moderately, and severely eroded plots, respectively. Soil moisture contents of mulched plots
at the beginning of silking were significantly greater than those of the unmulched plots for all

three erosion classes.

Copyright by
N dibu Muamba
1996

To my mother Kankolongo Tshibangu
and my late father Muamba Kalemba:

for all their labor and love in raising me to be responsible
and successfiil in life.

"As long as the earth endures,
seedtime and harvest,
cold and heat,
Summer and Winter,
day and night
will never cease. "

(Genesis 8:22)

ACKNOWLEDGEMENT

I want to seize this opportunity to thank the Almighty God for allowing and enabling
me to carry this work on to the end. Above all, I thank him for being the source of wisdom
and understanding of everything that pertain to learning.

Achieving this goal was a result of help and assistance fi'om other people. My thanks
go to Dr. Mokma for accepting and advising me on my coursework, the collection and
interpretation of data, and the writing of this thesis. Coming fi'om the Italian educational
system and changing right away to the American system was not an easy task. It took me
some time to get adjusted, but he was very understanding and patient with me so that our
joint efl‘ort and determination were honored in the end.

Special thanks go to my brother Kabongo and many colleagues for their words of
hope and encouragement that kept me going even when I felt much pressure and uncertainty.
1 will also be thankfirl to Dr. Deliana Siregar for her kindness and patience in helping me run
the computer programs to meet the technical requirements of this work .

Finally, I will ever be grateful to each one of my family members, fiiends,
acquaintances and partners who, nearby or far away, have been a great source of support
morally and spiritually to me, during all the time of my graduate studies at Michigan State

University.

TABLE OF CONTENTS

INTRODUCTION .................................................... 1
LITERATURE REVIEW ............................................... 4
Effects of Erosion on Soil / Crop Productivity .......................... 4

Effect of Erosion on Soil Properties ................................. 6

Plant Rooting Zone ........................................ 6

Clay Content ............................................. 6

Soil Structure . . . .' ......................................... 7

Organic Matter Content ..................................... 8
Plant-Nutrients Loss ........................................ 8

The Effect of Soil Properties on Water-Holding Capacity .................. 9

The Efl‘ect of Moisture Content on Productivity ........................ 10
Productivity Restoration ......................................... 10

Mulch and organic matter ................................... 11

The effect of crop residues on plant growth ..................... 14

Allelopathy .............................................. 14

MATERIALS AND METHODS ......................................... 17
RESULTS AND DISCUSSION ......................................... 20
Corn Grain Yields .............................................. 20

The 5-year period ......................................... 20

Growing degree days (GDD) ................................ 23

Fifty percent emergence and fifty percent silking. ................. 29

Corn yields comparison between phase I and phase H of the study . . . . 32

Soil Moisture Content ........................................... 40

Grain Moisture Contents ......................................... 43

Stand Count ................................................... 46

Stover Yields .................................................. 49
SUMMARY AND CONCLUSION ....................................... 52
RECOMMENDATIONS ............................................... 54
LITERATURE CITED ............................................... 55

vii

LIST OF TABLES

Table 1. Corn yield (Mg/ha) of slight, moderate and severe erosion classes with

and without mulch for 1989-1993. ................................. 21
Table 2. Monthly precipitation (cm) for the growing season, 1989-1993. ........... 25
Table 3. Cumulative growing degree days (GDD) for the growing season. ......... 25
Table 4. Corn yield ratios of mulched to unmulcheded plots, 1989-1993. ........... 27

Table 5. Soil moisture (cm /90 cm thickness) of slight, moderate, and severe
erosion classes with and without mulch, 1989-1993. .................... 28

Table 6. Comparison of soil moisture contents (cm) at three different thicknesses
for the slight, moderate, and severe erosion classes, 1989-1993. ........... 29

Table 7. Fifty percent emergence and fifty percent silking in days after planting
for slight, moderate, and severe erosion classes. ....................... 31

Table 8. Comparison between means for corn grain yield (Mg/ha) prior and
after the utilization of wheat straw mulch for three erosion classes of
Marlette soils. ................................................ 34

Table 9. Corn grain yields (Mg/ha) for slight, moderate and severe erosion classes
with unmulched of Marlette soils, 1989-1993. ......................... 35

Table 10. Corn grain yield (Mg/ha) for slight, moderate and severe erosion classes
ofMarlette soils, 1985-1988 (Mokma and Sietz, 1992). ................. 35

Table 11. Grain moisture (%) of erosion classes with and without mulch, 1989-1993. . 43

Table 12. Plant population (plants/m2) of slight, moderate, and severe erosion classes
with and without mulch for 1989-1993. .......................... 47

Table 13. Stover yield (Mg/ha) of erosion classes with and without mulch
for 1989-1993. .............................................. 50

viii

LIST OF FIGURES

Figure 1. Mean Corn Grain Yield (Mg HA") for the S-Year Period ................ 33
Figure 2. Mean Soil Moisture Contents (CM Water / 60 CM Thickness)

Prior to Silking for the 5-Year Period. ............................. 42
Figure 3. Mean Grain Moisture Contents (%) for the S-Year period. .............. 45
Figure 4. Plant Population (N 0 Plants M2) for the S-Year Period. ................ 48
Figure 5. Mean Stover Yields (Mg HA") for the 5-Year Period. ................. 51

ix

INTRODUCTION

One factor that contributes to the decrease of crop yields on farmland is soil erosion.
Other factors such as annual weather variance or climatic trends for a given region, parent
material, and management can influence soil-crop productivity as well. Scientists around the
world have been studying erosion for more than 50 years now and have concluded that a
primary cause of the depletion of soil and crop productivity was soil erosion. This is primarily
due to the impact erosion has on soil physical and chemical properties. Reduced available
water-holding capacity and surface crusting, increased clay content and reduced organic
matter content of the plow layer, and shallower rooting depth were identified as factors
contributing to reduced crop yield (e. g. Langdale et al., 1979; McDaniel and Hajek, 1985;
White et al. 1985; Olson and Nizeyimana, 1988 ).

Soil erosion-productivity relationships are complex since soil properties are not the
only factors controlling crop yields. Technological advances such as new crop varieties,
fertilizer technology, and more efficient agronomic practices frequently mask the efi‘ects of
erosion on yield (Rosenberry et al., 1980; Krauss and Allmaras, 1982). Landscape position,
climatic factors, loss of nutrients, and soils with either shallow Ap horizons or dense subsoil
restrictions, can influence the yield as well. Loss of organic matter through soil erosion is

particularly significant because it is the principal source of N in the soil. As organic matter

2

content decreases, soil loses its primary cementing agent and the stability of soil aggregates
is reduced. Lal (1987) found that the greater yield in deep soils was partly due to more
available soil water than in shallow soils. Moreover, he found that erosion increased the
frequency, intensity and duration of drought; and both landscape position and slope length
influenced the crop yield (drought in the upland and waterlogging in the lowland). Mokma
and Sietz (1992) found that increased erosion decreased corn yield, delayed corn maturity,
decreased the solum thickness and decreased the organic carbon (C) content of the Ap
horizon of Marlette soil, whereas clay content increased in the topsoil.

To reduce erosion and restore or maintain yields, various crop management practices
have been suggested. Engelstad and Shrader (1961) reported that in the absence of fertilizer
N, corn (Zea mays L.) yield was strongly dependent on the thickness of the topsoil. Increasing
the content of decomposed and partially decomposed organic residues in the soil resulted in
increased infiltration rates for most soils (Mannering and Meyer, 1961; Wischmeier and
Mannering, 1965). Plots with greater corn yields and correspondingly larger quantities of
residue material (stover yield) had substantially less runofl‘ than plots with lesser yields. Other
management practices used to increase yields of eroded soils include manure and organic
wastes, lime and fertilizers, irrigation, earth-moving and topsoil restoration, and mulching
(Olson et al., 1994).

The use of straw mulch was expected to compensate for the change of soil properties
that were afl‘ected by past erosion (Frye et al. 1985). Straw mulch decomposition would
contribute organic matter to the soil and improve soil aggregation and stability, increase the

infiltration rate and soil aeration, and facilitate a greater moisture retention by the soil,

3

especially on severely eroded plots. Organic matter also efl‘ects crop growth by tying-up or
releasing nitrogen in the soil, decreasing bulk density, increasing earth-worm and other fauna
populations, and changing soil pH (Lyon et al, 1952; Jamison, 1953).

Wheat (Triticum aestivum L.) straw mulch (about 5 metric tons/ha) was applied to
Mariette soils (fine-loamy, mixed, mesic, Glossoboric Hapludalfs) in south-central Michigan
over a 5-year period to provide protection from the impact of raindrops on bare soil, which
erodes valuable topsoil away, and to reduce the erosion potential. Beneficial efl'ects of
mulching include moderation of soil temperatures and evaporation, suppression of weeds and
reduction of competition with the crop, incorporation of organic matter to the soil after its
decomposition and mineralization, and increase of available water -holding capacity in the
soil. Other advantages of straw mulch are its low cost, availability and easy management on
farmland, and efliciency to control erosion if the right amounts were used (Mannering and
Meyer 1963; Meyer et al., 1970). The objectives of this study were to (1) determine the efi‘ect
of mulch treatment on maintaining or increasing corn yields of eroded Marlette soils in
comparison with the productivity of the unmulched plots and (2) compare soil moisture

contents between mulched and unmulched plots.

LITERATURE REVIEW

Efl‘ects of Erosion on Soil / Crop Productivity

Many studies show that soil erosion can reduce crop yields by reducing soil organic
matter, water retention capacity, plant rooting depth, and usually by increasing the clay
content on the topsoil. Other erosion -related factors affecting productivity include reduction
of plant nutrients, degradation of soil structure, and nonuniform removal of soil within the
same field (Williams, 1981; Pierce et al., 1983; Schertz et al., 1989; Weesies et al., 1994).

Erosion reduces soil fertility and alters soil properties, but the loss of productivity
usually results from poorer tilth with associated reduced infiltration rates, soil crusting, poorer
stands, and decreased water-holding capacity (Troeh et al., 1991). Water erosion is many
times greater on bare soils or under row crops than under continuous cover. Soil properties
that can be altered by erosion include organic matter content, pH, particle size distribution,
stability of aggregates, infiltration rate, soil water retention, root depth, porosity, and pore-
size distribution (Olson et al., 1994). Effects of erosion on productivity have been masked by
progressively increasing productivity as a result of improved technology and management.
However, it is not self-evident that this compensatory process can be maintained indefinitely

(Rosenberry et al., 1980; Krauss and Allmaras, 1982). Erosion often results in loss of organic

5
matter, nitrogen, potassium, and nricronutrients.

Langdale et al. (1979) found that soil erosion was the primary cause of low corn yields
under conventional tillage in the Southern-Piedmont. Mokma and Sietz (1992) found that
increased erosion delayed corn maturity, decreased solum thickness, depleted soil moisture,
decreased organic carbon (C) content of Ap horizon, whereas clay content increased.
Reduction of plant nutrients, degradation of soil structure, and nonuniform removal of soil
within a field of Miami soils were among the erosion-related factors afi‘ecting productivity
(Schertz et al., 1989). Topsoil depth and yield were positively correlated on some Typic
Hapludolls that are naturally productive and have no root restrictive layers (Engelstad and
Shrader, 1961).

Erosion also reduces productivity through nonuniform removal of soil within a field.
Erosion doesn‘t occur uniformly across a field mainly because of the runofl‘ flow network and
nonuniform topography. Proper timing, especially in planting, has an important impact on
productivity (Williams, 1981). In summary, the effect of soil erosion on crop yield is much
influenced by soil properties, particulariy the decrease of the available water holding capacity
(Frye et al. 1982), organic matter content, nutrients and plant rooting depth, whereas clay

content increases (Schertz et al., 1989).

6
Efl‘fit 9f Ergsign 9n Sgil Properties

Pl R in Z n
Efl'ects of erosion depend largely on the original thickness and quality of the topsoil

and on the nature of subsoil. Engelstad and Shrader (1961) reported that in the absence of
N fertilizer, corn yields mostly depended on the thickness of the surface soil. As soil erodes,
the rooting zone moves deeper into the profile. For soils with undesirable characteristics at
depth, productivity declines as erosion proceeds. However, erosion would not likely impair
the productive capacity of soils with favorable characteristics in the lower portion of the

profile (Pierce et al., 1983).

Clay Content

Infiltration rate and permeability to water are related to texture and bulk density of
the soil. As mixing takes place with tillage, several changes can occur depending on the
subsoil characteristics. Clay content of the surface layer may increase as the subsoil is mixed
with Ap horizon (Schertz et al., 1989). Frye et al. (1982) found that clay particles tended to
stick together and were difficult to detach (force of cohesion), but were easily carried away
great distances once separated from the soil mass. Fine to very fine pores common in medium
and fine-textured soils such as loams, clay loarns, and clays restrict water movement.
Compared to coarser-textured soils, the individual pores in fine soils are usually much
smaller, and both infiltration and permeability are slower. In general, a moderate storm

produces more runoff and erosion from fine-textured soils than fiom sandy ones (Troeh et al.,

7

1991). As the number of tillage operations increases, orientation of clay particles increases
too (more oxidation and drying taking place), which leads to a decrease in the stability of the

soil aggregates. That's why tilled soils usually have lesser soil aggregate stability compared
to sod soils (Woodrufl‘, 1939).

$21M

Degradation of soil structure increases soil erodibility, surface sealing, and crusting
and leads to poorer seedbeds (Williams, 1981). Large, stable aggregates make a soil diflicult
to detach and transport and rmke it more permeable to water. Troeh et al., (1991) stated that
while soils high in clay usually have low permeability and low infiltration rates, a well-
aggregated clay soil permits faster water movement than a poorly aggregated clay. Clay is
an aggregating agent. If the cation-exchange complex is occupied mainly by H+ or di- or
trivalent cations, the colloid will be flocculated, and individual soil particles will aggregate.
The higher the clay content, the larger and more stable the aggregates. Ofien, erosion results
in a shift of the pore size distribution toward smaller diameter soil pores (Troeh et al., 1991).
Raindrops destroy soil aggregates high in silt and very fine sand because they are relatively
unstable. These particles then plug surface pores producing a dense compact layer (crusting)
which reduces the infiltration rate and causes increased runoff and erosion. The type of clay
nrineral also influences the aggregation of soils. For example, tr0pical and subtropical soils
which contain large amounts of hydrous oxides of iron, aluminum and kaolinite, tend to be
better aggregated than soils high in expandable clay such as montrnorillonite and vermiculite

in temperate areas (Troeh et al., 1991).

W

Increasing soil organic matter content is crucial to the initiation of the reclamation
process (Lal, 1987). Erosion removes organic matter fi'om the soil which is the principal
source of soil N. Loss of of organic matter means greater requirements for N fertilizer and
greater costs of crop production (Lal, 1985). Organic matter is necessary to the formation of
soil aggregates of desirable size, and to promote favorable porosity, soil structure, and cation
exchange capacity (CEC) in the soil. As the organic matter content increases, there is a
proportionate increase in the stability of soil aggregation. The range of organic matter
contents in temperate soils is fi'om 1.5-3.5%, whereas in tropical soils there is about 0.1-
1.7% organic matter in the soil, mainly because of a rapid mineralization that occurs in the
topsoil (Lal, 1976). Finally, organic matter and soil biota are interrelated in maintaining soil

quality and recycling nutrients (Follet and Stewart, 1985).

Plant-Nutrienfi Loss

 

Erosion affects the soil by contributing to the removal of plant nutrients (Langdale
and Shrader, 1982; Schertz et al., 1989; Schumacher et al., 1994). Eroded soil particles carry
attached nutrients fiom fields into streams and lakes. Included are both macro (N, P, K, Mg,
Ca)- and micronutrierrts which are important to plant growth. Selective removal of nutrient-
rich soil particles by erosion leaves behind a surface soil depleted, to various degrees, of
plant-available nutrients (Power et al., 1990). A consequence of this change can be a greater
potential for denitrification, thereby reducing nitrogen-use efficiency. Other soil environment

factors that afl‘ect microbial activity and subsequent nutrient cycling include temperature,

9
soluble organic carbon concentrations, and aeration. Therefore, (Power et al., 1990)
concluded that soil management practices used to control erosion also dramatically influence

soil nutrient balance and nutrients availability to the crop.

The Effect of Soil Properties on Water-Holding Capacity

 

Erosion reduces productivity first and foremost through loss of plant-available water
capacity (Williams, 1981). As the erosion process takes place, the thickness of the A horizon
is progressively reduced. Consequently, the near-surface soil exhibits an increased clay
content, a decreased organic matter content, and altered soil structure. All these may
contribute to lower available water-holding capacity of the soil (Schertz et al., 1989; Moknra
and Sietz, 1992; Weesies et al., 1994). Less soil water-holding capacity subjects crops to
more fi'equent and severe water stress. Water shortages severely afl‘ect crops at most stages
of development. Jordan (1983) found that water deficits reduced seed germination, seedling
emergence, photosynthesis, respiration, seed number, and seed filling. Longwell et al. (1963)
reported that soils with larger clay contents held water tighter than soils with smaller amounts
of clay or smaller surface area; thus, plants must exert more energy to extract water held at
greater tension in those soils having larger amounts of clay. On eroded soils, therefore, the
water-holding capacity increases while the available water-holding capacity decreases (greater
bulk density). Lal (1987) reported that greater yields in deep soils were partly due to more

available water reserves compared to shallow soils.

10

The Efl‘ect of Moisture Content on Productivity

The final effect of reduced available water-holding capacity is the reduction of crop
yields on eroded soils (Schertz et al., 1989; Adransky and Lowery, 1992; Weesies et al.,
1994). Lal (1987) reported that direct or on-site effects of erosion on crop yield included the
loss of rooting depth, the decrease in soil fertility, the reduction in organic matter, and the
reduced plant available water. F ollet and Stewart (1985) reported that reduced crop
productivity was caused by a complex set of ecological factors including organic matter,
nutrients, soil biota, and soil depth, interacting altogether. Olson and Nizeyirnana (1988)
concluded (I) that percent change in organic carbon related well to percent change in corn
yield; (2) that percent of stand count decrease was approximately proportional to clay
increase; and (3) that the primary reason for corn yield reduction was a loss of topsoil
(organic carbon) and the associated effects of increased clay content in the topsoil, restricted

rooting depth, and finally reduced plant available water storage in soil thereof.

Erosion has negative effects on soil properties which determine the reduction of the
crop productivity. Several methods such as commercial fertilizer, green manure, animal
manure and organic wastes, mulching with crop residue, and earth-moving (topsoil removal
and additions) have been used to maintain or restore soil productivity on cropland. Crop

residue management through conservation tillage is one of the most eflicient methods of

ll

controlling soil erosion. Maintaining a crop residue cover on the soil surface of at least 30%
after all tillage and planting operations will reduce water-caused erosion to about halfof what
it would be if the field was clean-tilled; a greater percentage left reduces soil losses even more

(Hill et al., 1989).

M h r ' tter

Erosion is more damaging to the quality and productivity of some soils than others.
On some soils, it may cause little or no permanent reduction in productivity, while in others
the productivity reduction may occur so slowly as to be unnoticed. Ifthe rate of reduction is
equal to or less than the rate of increase in crop production due to technological inputs such
as fertilizer management, irrigation, improved varieties, and more effective pest control, crop
yields may be maintained or increased during and alter erosion (Rosenberry et al., 1980;
Krauss et al., 1982; Frye et al., 1985). Restoring the organic matter content of eroded soil
doesn't necessarily restore the soils productivity to the level of the uneroded soil.
Nevertheless, it is one of the most effective and practical ways to help restore the productivity
of eroded soils (Frye et al., 1985).

Straw mulch and crop residues are used on cropland and have been reported to have
an efi'ect on soil temperature, soil water, bulk density, and even weed control (Van Wijk et
al., 1959; Unger, 1978, 1988; Radke, 1982; Wade and Sanchez, 1983) and to increase crop
yield (Moody et al., 1963; Wicks et al., 1994 ). Mulch materials available commercially
include bark, woodchips, pea gravel, river rock, red cinder, serpentine rock, compost,

manure, straw and other crop residues. However, they are more likely to be utilized for

12
gardening and horticulture than for agronomic use. Usually manure, straw mulch and other
crop residues are used for agronomic purposes.

Plant-cover decreases runofl‘ by slowing the flow of water over the soil surface and
protecting the soil from the erosive forces of water and wind. Straw mulch applied at rates
of 1 and 2 MW reduced runofl‘ velocities to about 50 % and 33 %, respectively, of the
velocity with no mulch (Mannering and Meyer, 1963). Infiltration increased by 2.5 and 7.5
cm, respectively, with 1 and 2 Mg/ha of mulch. Residues reduced runofl‘ even when turned
under. Runofl‘ was 40 % less where residues were returned each year rather than removed at
harvest (Wischmeier and Meyer, 1965). Mulch treatments of 1, 2, and 4 tons per acre almost
completely eliminated runofl‘ and controlled erosion (Mannering and Meyer, 1963). Lal
(1977) concluded that (l) the greater the mulch rate applied to the soil, the less the soil loss
rate; (2) the steeper the slope, the greater the mulch rate required to control erosion; and (3)
the effect of no-tillage on erosion control was equivalent to approximately 5 Mg/ha mulch
with conventional tillage. In a similar study on steep slopes, Meyer et al. (1970) found that
once the depth of runoff was as great as the depth of mulch, additional mulch would be of
little or no value in reducing velocity.

Frye et al. (1985) reported that restoring organic matter to the surface horizon of
erosiomdamaged soils over time may eventually return most of physical properties to near
their original condition, with the exception of soil texture. As the organic matter content of
the surface horizon increases, the degree of aggregation and the stability of the aggregates
increases. This in turn increases porosity, infiltration, and water-recharge capacity, while

decreasing bulk density and runoff. Increasing organic matter generally increases the total

13

water-supplying capacity of the soil, but apparently does not increase available water-holding
capacity except in sandy soils. Soil-management practices that use mulch tillage and cover
crops increase the amount of organic matter in the eroded soils (Black and Siddoway, 1979).
Volk and Loeppert (1982) found that yield potential increased by an average of 21% for each
1% increase in soil organic carbon (C). Vetch cover and rye mulch increased the soil
organic matter in the Ap horizon from 1.5% to 2% in ten years, while organic matter under
plowed check plots decreased from 1.5% to 1.2% over the same period (Beale et al., 1955).
Therefore, the degree of soil aggregation and the stability of the soil structure increased
during the ten years under mulched treatments, but was reduced considerably under the
plowed check treatment.

Wade and Sanchez (1983) reported that mulch decreased soil temperature, conserved
soil moisture in the topsoil during dry weather, prevented surface crusting, and decreased
weed growth However, mulch had little efl‘ect on increasing available plant nutrients. The use
of mulch without chemical inputs produced an average of 75% of the crop yields with
completely fertilized, bare soils in the humid tropics. In the contrary, no response to organic
additions were found when fertilizers were supplied. Lal (1976) reported similar results in
another part of the humid tropics. Black and Siddoway (1979) concluded that tillage and no-
till cropping systems that maintain crop residues on the soil surface increase dry aggregate soil
structure, reduce soil erosion, and increase soil water storage. The combined influence of
these factors is manifested in greater crop yields, greater crop residue production, and

extended periods of protective vegetative cover.

14

Th f r i e n l o

The use of mulch and other surface residues however, may lead to negative efl‘ects on
the soil properties. Generally, experiments comparing corn production under residue-covered
and bare soil conditions (Willis et al., 1957; Burrows and Larson, 1962; Moody et al., 1963)
and under conservation and conventional tillage (Grifith et al., 1973; Mock and Erbach,
1977; Tirnmons et al., 1986; Al-Darby and Lowery, 1986, 1987) have reported cooler soil
temperatures, delayed emergence, shorter plants and less above-ground dry weights during
the vegetative period, different whole-plant nitrogen concentrations, and delayed silking. In
general, the results have been summarized as residue-induced slower development and
depressed grth of corn, and have been attributed to cooler soil temperatures (Fortin and
Pierce, 1990). Gupta et al. (1983) reported that one of the major effects of crop residues on
the soil surface was the depression of soil temperatures at seed-zone and deeper depths which
would delay seed germination, slow the seedling growth and would ultimately reduce the crop
yield grown in areas with cool and wet springs, especially in the Northern Corn Belt. Residues
are assumed to delay com development via their effect on soil temperature. By decreasing the
soil temperature, residues are also thought to decrease the rate of cell division in the shoot
apical maistem during the period it is below the soil surface. However, a thorough analysis

of mulch's efl‘ect on corn leaf development has been lacking (Fortirr, 1989).

Allelopathy
Another negative effect of surface residues is the allelopathy which is known to

depress plant growth and reduce crop productivity. Many weed plants have allelopathic

15

efi'ects on crops. Crop residue mulches reduce growth in several species through leaching or
microbial production of allelopathic chemicals. Phytotoxicity varies with the nature and
persistence of the residue (Guenzi et al., 1967; McCalla and Norstadt, 1974; Barnes and
Putnam, 1983; Yakle and Cruse, 1984; Lodhi et al., 1987). McCalla and Haskins (1964)
reported that microbial and plant toxins contributed to the allelopathic potential of crop
residues. The two inhibitors, ferrulic and p-cumaric acids were in much greater concentration
in soils where com residues had been maintained. Patrick and Koch (1958) and Patrick (1971)
found that greatest toxicity occurred under saturated soil conditions. Stage of maturity of the
plant residues also afl‘ected their toxicity. Residues fi'om young plants were toxic
immediately, whereas more mature plants required a longer period of time to release their
toxic substances. Generally, allelopathy refers to chemical compounds being released or
added into the environment ( Molisch, 193 7). It is thus separated from competition, which
involves the removal of or reduction of some factor from the environment that is required by
some other plant sharing the habitat. Factors that may be reduced include water, minerals,
food, and light (Rice, 1984).

Wheat straw has been found to have inhibitory effect on the growth of plants such as
cotton (Hicks et al., 1989), soybean and cereals (Rice, 1984). Other researchers have found
most cereal residues caused allelopathic effect on corn, including com itself (Y akle and Cruse,
1984; Fortin and Pierce, 1990; Martin et al., 1990). Fortin and Pierce (1991) also found oat
(Avena sativa L.) residue to have an allelopathic efl'ect that delayed corn development during
the early stage of growth. They stated that its occurrence was weather-dependent and that

cool soil temperatures gave the ideal conditions for allelopathy to take place. Wheat and other

16
cereals are said to release phenolic acids, cumaric acids (Blum et al., 1991), and hydroxamic
acids (HX) in amounts great enough that they may have potential for allelopathy. However,
the release of HX to the soil by the producing plant has not been determined (Perez and

Ormeno-Nunez, 1991).

MATERIALS AND METHODS

A study was started in 1985 to determine the effect of erosion on corn productivity
using a paired comparison method (Mokma and Sietz, 1992). This method compares paired,
naturally eroding plots to show the effect of the eroded phase on yields (Olson et al., 1994).
In 1989, the plots were split and mulch was applied to one—half of each plot. The effects of
wheat straw mulch on corn productivity of eroded soils were investigated during the period
1989 to 1993. Areas of Marlette soils (fine-loamy, mixed, mesic, Glossoboric Hapludalfs)
with slight, moderate, and severe erosion were located using the criteria established by the
Soil Survey Stafl‘ (1993). The study site is located within a Marlette fine sandy loanr, 2 to 6
% slope, mapping unit in a 24-ha field on the Michigan State University research farm in East
Lansing, Michigan ( in the SW 1/4 of Section 31, T. 4 N., R. 1 W.). Corn has been grown
continuously for more than 20 years on this field. A moldboard plow with secondary tillage
was used to prepare the seedbed. The field has been managed as one unit and represents
typical farm practices used in south-central Michigan.

Plots were located on backslopes with 2% to 5% slope gradients. The mean slope
gradients were 3% for the slightly eroded plots and 4% for the moderately and severely
eroded plots. Severely eroded plots tended to occur on upper backslopes, slightly eroded

plots on loWer backslopes, and moderately eroded plots on middle portions of backslopes.

17

18

Three separated IO-m by 10-m replications of each erosion class were located within the
southern 5 ha of the field. Plots of moderately and severely eroded soils were surrounded by
a border of the same erosion phase around each plot. The slightly eroded plots were randomly
distributed throughout the 5-ha portion of the field to provide unbiased data for comparison
with the eroded plots (Figure 1). Uniform fertilizer and pesticide application procedures were
used throughout the entire field. Fertility difl‘erences between erosion classes were masked
by high rates of fertilizer application. The thickness of the Ap horizon was approximately 25
cm for all plots. Soil properties measured in laboratory included particle size distribution, bulk
density of cores, organic carbon (C), soil pH in water, and cation exchangeable capacity
(CEC) by sum of cations (Soil Survey Staff, 1984).

Wheat straw mulch was added to one half of each plot to allow comparison of both
mulched and unmulched subplots of each replication in a split-plot design. The mulched
subplots received approximately 5 metric tons/ha of wheat straw annually shortly after
planting the corn. The mulch was redistributed when necessary, once or twice during the
first month after planting, to compensate for wind redistribution of the mulch and maintain
a fairly homogeneous distribution over the soil surface. The plots were weeded by hand as
necessary. The dates of 50% emergence and 50% silking were determined for each plot. Soil
moisture contents were determined gravimetrically for samples taken fi'om 0-15, 15-30, 30-
60, 60-90 cm depths fi'om each plot at the beginning of silking. Weather data were collected
from a weather station about one kilometer from the study site.

Corn was harvested by hand, shelled, weighed, and moisture tested. Com grain yields,

stand counts, and stover yields, were measured at the harvest. The Duncan's multiple range

19

test procedure (P=0.05) was used to determine whether or not the difl‘erences between the
means of com and stover yields, plant population per surface unit, grain moisture at harvest
and soil moisture at four difl‘erent thicknesses in the profile were statistically significant,

especially between the slightly and severely eroded phases of the field.

RESULTS AND DISCUSSION

Corn my Y1§1d§

Mamba

Unmulched plots on slightly, moderately, and severely eroded classes of Marlette soil
gave relatively greater com grain yields than mulched plots for each individual year and when
averaged over the 5-year period (Table 1). Corn grain yields on slightly eroded soils were
significantly greater than those on severely and moderately eroded soils fi'om 1990 to 1993,
both for mulched and umnulched plots. In 1989, yields were not significantly different,
probably due to optimum weather conditions. Corn grain yields on unmulched plots were
significantly greater than those on mulched plots for the slightly eroded plots in 1992, for the
severely eroded plots fi'om 1990 to 1993, and for the moderately eroded plots in 1991 and
1992. Yields of unmulched, moderately eroded plots were greater, but not significantly
difl‘erent, than the mulched plots in 1990.

When data were combined for all five years, corn yield reductions on mulched plots
were 4%, 18% and 19% for the slightly, moderately and severely eroded class, respectively,
compared to the unmulched plots. Yield reductions for both moderately and severely eroded

mulched plots were significantly different at the 5% level using Duncan's multiple range test.

20

21

However, yield reductions for the slightly eroded plots were not significantly difl‘erent. Thus
rrrulch and past erosion contributed to the reduction of corn grain yields on the mulched plots
(Table 1). Yield reductions for the moderate and severe erosion classes were statistically
significant (P = 0.05) whereas the reduction for the slight erosion class was not. This suggests
that erosion class had more efi‘ect on yield decrease than the addition of wheat straw to the
soil. It follows that yield reductions were almost 5 times greater for the severely eroded plots
when compared to the slightly eroded plots. Also, yield reductions were 4.5 times greater for
the moderately eroded plots when compared to the slightly eroded plots.

Table 1. Corn yield (Mg/ha) of slight, moderate and severe erosion classes with and
without mulch for 1989-1993.

 

 

Trgtment Class Year
1989 1990 1991 1992 1993 Mean Change (%)+
Slight

Unmulchfi 9.8a 8.5a 8.9a 8.8a 9.9 a 9.2a

Mulch 9.4a" 8.2a 8.6a 7.8b 9.9a 8.8a -4
Moderate

Unmggchfi 10.2a 8.1a 8.2a 9.6a 9.3ab 9.1a

Mggh 9.4a 7.8ab 5.8b 5.4d 9.2ab 7.5b -18
Severe

Unmulched 9.4a 6.9b 5.4b 6.5c 8.5b 7.3b

Mulch 9.2a 5.6c 3.9c 4.1e 6.8c 5.9c -19

 

+Percent reduction compared to the unmulched plots.
* Yields for a specific year not followed by the same letter are significantly different at the 5%
level using Duncan's multiple range test.
The reduced grain yields on the mulch plots when compared to unmulched plots

could be caused by the interaction of several factors. It is generally assumed that the lower

yields sometimes obtained with mulch tillage are associated with one or more of several

22

interacting factors including soil temperature, soil moisture, aeration, and nutrient availability
(Moody et al., 1963). The fluctuation of daily soil temperature between mulched and
unmulched plots may have had influence on the rate of plant growth (physiological change),
especially in cooler temperate climates. Mulched soil was cooler during the daytime and
relatively warmer during the night than bare soil (Willis et al., 1957; Unger, 1978; Fortin,
1989). Small changes in soil temperature can cause greater differences in plant physiology and
biology, especially for mechanisms related to nitrate and nutrients absorptions (Al-Darby and
Lowery, 1987; Swan et al., 1987 ). Because mulch did not significantly reduce corn yield on
the slightly eroded plots, one can not conclude that the addition of mulch produced cooler soil
temperatures, thereby reducing yields. The greater or longer moisture retention by soil under
mulch could have a positive effect on the activity or proliferation of soil microbial populations
and allow a better condition for plant rooting and productivity, especially when precipitation
is below normal conditions (Moody et al., 1963; Kladivko et al., 1986).

Management systems that include the incorporation of crop residue into the soil is
beneficial for the improvement of the soil physical properties. However, this might have a
negative impact on the availability of plant nutrients, especially nitrogen. The carbon-nitrogen
ratio (C :N) primarily determines whether nitrogen is mineralized or immobilized in the soil
(Donahue et al., 1977; Stevenson, 1982a; McGuinness, 1993). Therefore, application of
wheat straw mulch might contribute to yield reductions on the mulched plots. Because mulch
did not cause a significant yield reduction on the slightly eroded plots, one can not conclude
that the addition of mulch produced a large C:N ratio which caused nitrogen to be

immobilized in the soil.

23

Corn yield reduction for the mulched plots also could be attributed to allelopathic
efl’ects of wheat mulch on corn plants during the growing season. Allelopathy is defined as
the release and interaction of biochemical compounds between all types of plants including
microorganisms. The reciprocal interactions can be both inhibitory and stimulatory (Molisch,
193 7; Rice, 1984). The efl‘ects of allelopathy have been reported to delay emergence, slow
plant growth, and reduce the yield (McCalla and Haskins, 1964; Gupta et al., 1983). It seems
the allelopathic effect (toxicity) took place under saturated soil conditions (Patrick and Koch,
1958) which would correspond with cooler. soil temperatures in the Northern Corn Belt, and
which have been reported to be the major factor for a poorly early grth and the lower yield
ofcom (Willis et al., 1957). Because mulch did notcause a significant yield reduction on the
slightly eroded plots, one can not conclude that the addition of mulch produced allelopathic
effect which reduced corn yields.

Finally, the soil type and weather trends are very important parameters in evaluating
the effect of straw mulch on corn yield. Marlette soil is well to moderately well drained and
physical properties have been described amply by Lowery et al. (1995). Weather data in terms
of monthly precipitation (rainfall) and air temperatures or growing degree days (GDD) for the

5-year period are presented in Tables 2 and 3.

win (1 DD
A growing degree day unit (GDD) is a representative index of accumulated heat,
normally derived from air temperatures at a given location. GDD are calculated on a daily

basis and summed for all or a portion of the growing season To calculate GDD for com, take

24

the day's minimum temperature and, if it is lower than 50 0 F, set it up to 50 ° F. Ifthe
maximumtemperatureis higherthan 86°F, setit downto 86° F. This is because corn growth
doesn‘tbeginurrtiltemperatureswarmtoabout 50°Fandgrowthbegins to slowat 86 °F and
higher. Next, calculate the average of the day by dividing the sum of the maximum and
minimum by 2. Finally, subtract the base temperature of 50 from this average to get the GDD
for the day. The GDD was accumulative from May 1 through September 30 (Table 3).
Cumulative growing degree days (GDD) for May and June were below normal (long term
average) in 1989, 1990 and 1992 and above normal in 1991. For May through July, GDD
were well below normal in 1990 and 1992 and near or well above normal in 1989, 1991 and
1993. A 95-97 day com hybrid in this field requires about 2200 growing degree days (GDD),

which didn't occur in 1992 due to a cooler than normal summer and a wetter July.

25

Table 2. Monthly precipitation (cm) for the growing season, 1989-1993.

 

 

Month m
1989 1990 1991 1992 1993 Normal+

May 12.3 8.0 4.3 1.8 2.8 6.5
June 8.3 5.5 7.5 4.5 10.0 8.8
July 4.5 7.5 9.0 18.3 8.3 7.0
August 17.3 6.0 6.8 3.5 10.3 7.5
September 14.8 8.0 2.0 5.8 14.0 6.3
Total 57 35 29.5 33.8 45.3 36

 

+Long-terrn average

Table 3. Cumulative growing degree days (GDD) for the growing season.

 

 

Month Ye_ar

1989 1990 1991 1992 1993 Normal+
May 270 219 499 309 328 314
June 744 720 817 703 799 811
July 1459 1296 1768 1276 1461 1468
August 203 l 1999 2364 1742 2085 2090
September 2394 2429 2742 2137 2346 2493

 

+Long-term average

When comparing yields between mulched and unmulched plots, average corn yields
for the individual years (or when averaged over the 5-year period) were significantly difl‘erent
at the 5% level, except for 1989. Greatest reductions occurred in 1992 for both mulched
severely eroded soils and mulched moderately eroded soils. The g'owing season in 1992 was
particularly cool. In 1993, only the slightly eroded class yielded almost the same amount of
corn gain on both mulched and unmulched plots. This indicates that wheat straw mulch had

little or no negative effect on corn growth and yield in that particular year. In general, the

26

slightly lower productivity observed on most mulched plots compared to unmulched plots
may have resulted from the combination of the degree of erosion, and the different gradient
in the warming of the soil between day and night. These factors may have altered the nitrogen
and nutrients uptake by the plant.

Another way to investigate the yield differences was to compare mulched and
unmulched plots using their mean ratios over the five-year period (Table 4). The reductions
were statistically difl‘erent when comparisons are made between the slight erosion plots and
both the moderate and severe erosion plots at the 5% level. Com yield ratios were statistically
similar for all three erosion plots in 1989. In 1990 and 1993, the yield ratio differences
between the slight and moderate erosion plots were not statistically sigrificant. Similarly, the
differences between the moderate and severe erosion plots were not statistically different in
1991 and 1992. Except for 1989, mulch productivity of the severely eroded plots during the
5-year period was less than mulch productivity of the slightly eroded plots. However, it was
slightly greater than the moderately eroded plots in 1989, 1991 and 1992. It appears,
therefore, that mulch had little or no effect on corn productivity on residue-covered plots
in those particular years.

The reduced com yields for mulched plots may be related to the interaction between
the soil under the mulch with fertilization and liming. Wade and Sanchez (1983) found that
mulch without chemical inputs produced an average of 7 5% of the crop yields achieved with
completely fertilized, bare soils. However, when complete fertilization was supplied there was
no significant response to organic additions. Although this work was done in different climatic

conditions and soil environment (humid tropics Ultisol) than those in the present study area

27
in Michigan, their findings may help us to understand why mulched plots generally tended to
produce less than unmulched plots on Marlette soil (temperate climate Alfisol). For the 5-
year period of this study, mulched plots produced an average 96% of the corn grain yield
produced by the unmulched plots for the slight erosion class, 83% for the moderate erosion

class, and 82% for the severe erosion class.

Table 4. Corn yield ratios of mulched to unmulched plots, 1989-1993.

 

 

Erosion olgss leg

1989 1990 1991 1992 1993 Mean
Slight 0.96a* 0.98a 0.97a 0.89a 1.00a 0.96a
Moderate 0.93a 0.97a 0.70b. 0.57b 0.99a 0.83b
Severe 0.98a 0.82b 0.77b 0.68b 0.83b 0.82b
Percent changel +2 -16 -21 -24 -17 -15

 

*Yield ratios for a specific year not followed by the same letter are sigiificantly
difl‘erent at the 5% level using Duncan's multiple range test.

‘Percent reduction or increase in corn yield ratio between the slight and severe
erosion classes.

The reduced yields on the severely eroded mulched plots, when compared to the
unmulched plots and when averaged over the 5-year period, suggested there was some
negative efl’ect fiom mulch during the growing season which reduced yield. Annual reductions
weren‘t statistically sigrificant for soils with slight erosion, except for 1992 (Table 1). These
reductions could have resulted from the combined efl‘ect of soil erosion and mulch. Adverse

weather conditions, i.e., lower soil temperatures during the growing season, or the depletion

of soil moisture can cause stress on plant growth and affect productivity. Lower soil

28

temperatures under mulched plots have inhrbited or retarded the corn growth and yield (Willis
et al., 1957; Wicks et al., 1994), but it is inconclusive for this study since soil temperature
data were not available.

Weather trends such as precipitation for a given region, have major modifying
influences on erosion-productivity relationships. Monthly precipitation during the growing
season was transformed in soil moisture contents. The available soil water during the
growing season is one major factor which determines crop yield; it is presented in Tables 5
and 6. Less soil moisture or below normal air temperatures in some specific individual years,
such as 1991, may have caused delays of the emergence and silking and therefore, would have
contributed to yield decrease. This fact though, was minimized when com yields were

averaged over the 5-year period.

Table 5. Soil moisture (cm I90 cm thickness) of slight, moderate, and severe erosion classes
with and without mulch, 1989-1993.

 

 

_ea_c_t_Tr tm n CM Year
1989 1990 1991 1992 1993 Mean Change (%)+
Slight
Unmoloheo 9.3d* 13.2b 9.9c 14.2bc 10.7b 11.4c
Mtgoh 10.0cd 14.0ab 9.50 15.6a 12.9a 12.4bc +9
Moderate
Whit! 11.0b 14.3a 10.9b 13.4c 9.7b 11.9c
Moloh 12.2a 14.8a 12.6a 14.9ab 13.2a 13.5a +13
Severe
Unm h 9.9cd 14.2ab 11.1b 13.7c 10.4b 11.9c
Mtr_loh 10.6bc 14.4a 13.1a 14.8ab 13.2a 13.2ab +11

 

‘Percent increase or decrease in soil moisture between mulch and unmulched plots.
*Soil moisture contents for a specific year not followed by the same letter are significantly
difi‘erent at the 5% level using Duncan's multiple range test.

29

Table 6. Comparison of soil moisture contents (cm) at three difl‘erent thicknesses for the
slight, moderate, and severe erosion classes, 1989-1993.

 

 

Tr trn 11 films Thickness
0-30 cm 0-60 cm 0-90 cm
Slight
Unm h 2.9d* 7.1b 11.4c
Mu_loh 3.6c 8.4a 12.4bc
Percentage changel +24 +18 +9
Moderate
Unm lch 4.5a 7.2b 11.9c
Mulch 4.8a 8.7a 13.5a
Percentage change +7 +21 +13
Severe
Unmolohfi 3.2d 7.7b 11.9c
Mtgoh 4.0b 8.8a 13.2ab
Percentage change +25 +14 +11

 

*Soil moisture contents for a specific year not followed by the same letter are sigrificantly
difi‘erent at the 5% level using Duncan's multiple range test.

lPercent change in soil moisture between mulch and unmulched for each erosion class.
F' rnmrn dfi r tilkin.

Table 7 presents dates of 50% emergence and 50% silking for the five-year period.

Delays for the date of 50% emergence on mulch plots when compared to unmulched plots
were 1 day late for the slight erosion class in 1992 and 1993, but varied fi‘om 2 to 5 days for
the severe erosion class for the years 1991, 1992, and 1993. The delays were relatively
greater for the severely eroded class than for the slightly eroded class. There were virtually
no delays in 1989 and 1990 when weather conditions were cooler than normal during May.
When air temperature in May was warmer than normal, the date of 50% emergence was

delayed by the addition of mulch. When temperatures were cooler than normal, mulch did not

cause a delay in emergence. There was more variability for the moderate erosion class during

30

the five-year period. When averaged over the five-year period, dates of 50% emergence of
mulched plots were delayed 3 days for the moderately eroded, plots but none for either the
slightly or severely eroded plots. Mulch does not appear to sigrificantly delay the date of 50%
emergence, probably because the straw had not decomposed into the soil yet to afl‘ect the
corn seedlings.

Over the 5-year period, mulch delayed the date of 50% silking for all three erosion
classes. The delays between mulch and unmulched plots varied from 1 to 2 days for the slight
erosion class fiom 1990 to 1993, 5 to 6 days for the moderate erosion class, and 2 to 4 days
for the severe erosion class, respectively, during the same period. No delay occurred in 1989
on any erosion class. The individual year 1991 had a geater delayed silking which contributed
to the delayed maturity of corn and consequently the lowest grain yield, especially on the
mulched plots. However, 1991 had the greatest number of GDD during July and August,
but had the lowest rainfall of the 5-year period. As in the case of the date of 50% emergence,
mulch did not delay the date of 50% silking in 1989 and 1990 but did so in 1991, 1992 and
1993. When averaged over the five-year period, dates of 50% silking of mulched plots were
delayed 1, 3, and 3 days for the slightly, moderately, and severely eroded plots, respectively.
The delays, therefore, appear to be related to temperature and possibly precipitation trends

during the growing season.

31

Table 7. Fifty percent emergence and fifty percent silking in days after planting for slight,
moderate, and severe erosion classes.

 

Emma Trgtment Year
1989 1990 1991 1992 1993 Mean

 

50% Emergence
Slight
Unmulched 7 7 7 8 12 8
Mulch 7 7 7 7 13 8
lawsuits '
Unmulched 6 7 7 8 12 8
Mulch 6 8 1 1 15 14 1 1
Severe
Unmulched 7 8 12 14 19 12
Mulch 7 8 10 19 17 12
50% Silking
Sham
Umnulched 67 74 67 82 76 73
Mulch 67 75 69 83 77 74
Madam
Unmulched 67 74 67 81 74 73
Mulch 67 75 73 86 79 76
Savors
Unmulched 68 74 72 86 79 76
Mulch 68 76 76 88 85 79

 

Early soil temperatures in Michigan (Northern Corn Belt) are often too low for
optimum germination. Delayed planting often reduces yields and, crop residue applied on the
topsoil often aggavates the conditions of soil temperature early during the growing season.
Precipitation and GDD data are usefirl indicators of weather trends. Delayed emergence and
silking will definitely delay corn maturity, and the latter will cause high moisture con tent in

corn grain which will afl‘ect profitability. Com maturity occurred generally later on the

32
severely eroded plots than on the moderately and the slightly eroded plots, especially on the

nrulched plots (Table 7). The consequence of this would be a difl‘erent grain moisture content
at time of corn harvest for the same field. This will bring additional costs for the farmer in
terms of money and also in terms of degradation of soil physical properties. Delayed harvest

may cause soil compaction when operating combines and trucks in fields.

rn'l m 'nbetweenhaseland IIfh

Means for overall corn yields were compared between plots with and without mulch
treatment to determine whether the mulch improved or hindered the productivity. Yield
reductions for the moderately eroded class and the severely eroded class were 6% and 39%,
respectively, during the 1985-1988 period (Mokma and Sietz, 1992). Also, reductions for
the moderately eroded class and the severely eroded class were 1% and 21%, respectively,
during the 1989-1993 period (Table 8). Corn yields were greater for all three classes of soil
erosion during the second phase than those during the first phase (Tables 9 and 10, Figure 1).
The comparison between the two periods showed a 18% increase in corn yields for the
severely eroded class of the second period (1989-1993). During this same period, yield
increases for the moderately eroded class were 5% g'eater than those of the previous period.
The yield reduction for the severely eroded class (21% for unmulched) is only slightly greater
than that found by Schertz et al. (1989) and Weesies et al. (1994) for the Miami soils (Typic
Hapludalfs, fine-loamy, mixed, mesic) which are similar to the Marlette soils used on this
study. Yield reductions for the severely eroded class when compared to the slightly eroded

class were 15% for a six-year period (Schertz et al., 1989) and 18% for a ten-year period

33

 

nose I

 

85:. 85

 

 

 

mmjo szOmm
226m 2223: Eu=m

  
 

 

 

.ooEma m<m>.m
m_.:. mo... :.<: as: ad; 2.56 2:00 z<ms_ .- manor.

(,yH 5w) $013M vauo

34

(Weesies et al., 1994). These findings don't agree with those of Volk and Loeppert (1982)
who reported a 21% potential yield increase on eroded soils for each 1% increase in the soil
organic carbon Even though straw mulch has been applied to the topsoil for five years in this
study, it appears to have had little or no effect on soil properties, not enough yet to increase
corn productivity on the eroded soils. In the experiment of Beale et al. (1955), ten or more
years were needed for the crop residue to have some impact on soil properties of the eroded
soil. However, this type of comparison is weak since it involves factors that are unpredictable
or diflicult to control such as differences in the date of planting and harvest, and the weather

conditions that may have changed from year to year.

Table 8. Comparison between means for corn grain yield (Mg/ha) prior and after the
utilization of wheat straw mulch for three erosion classes of Marlette soils.

 

 

Period Erosion classes Rgugion (%)+
Slight Moderate Severe Moderate Severe
1985-1988 6.9a* 6.58 4.2b -6 -39
1989-1993 (unmulched) 9.2a 9.1a 7.3b -1 -21
1985-1993 8.1a 7.9a 5% -2 -27

 

+Percentage reduction in corn yield of severe and moderate erosion classes compared to the
slight erosion class.

*Numbers for a given yield not followed by the same letter are significantly difl‘erent at the
5% level using Duncan's multiple range test.

35

Table 9. Corn grain yields (Mg/ha) for slight, moderate and severe erosion classes with
unmulched of Marlette soils, 1989-1993.

 

Erosion class lea;
1989 1990 1991 1992 1993 Mean

 

Slight 9.8ab“ 8.5a 8.9a 8.8b 9.9a 9.2a
Moderate 10.2a 8.1a 8.2b 9.6a 9.3a 9.1a
Severe 9.4b 6.9b 5.4c 6.5c 8.5b 7.3b
Reduction‘ -4 -19 -39 -25 -14 ~21

 

*Numbers for a given yield for a specific year not followed by the same letter are significantly
difi‘erent at the 5% level using Duncan's multiple range test.
lPercent reduction in corn yield between the slight and the severe erosion classes.

Table 10. Corn grain yield (Mg/ha) for slight, moderate and severe erosion classes of
Marlette soils, 1985-1988 (Mokma and Sietz, 1992).

 

Erosion class Year
1985 1986 1987 1988 Mean

 

Slight 7.2a* 7.8a 8.4a 3.4a 6.9a
Moderate 5.5b 7.7a 8.7a 3.9a 6.5a
Severe 3.2c 3.8b 7.4b 2.5b 4.2b
Reduction1 -56 -51 -12 -26 -39

 

*Numbers for a given yield for a specific year not followed by the same letter are sigiificantly
different at the 5% level using Duncan's multiple range test.
‘Percent reduction in corn yield between the slight and the severe erosion classes.

 

36

In generaL corn yields for the severe class of erosion were always less than those of
both the slight and moderate erosion classes and were also statistically different (Figure 1).
The greatest yields for the 9-year period occurred in 1989 with the maximum given by the
moderate erosion class and in 1993 when weather conditions were at or near the optimum.
The smallest grain yield occurred in 1988 with the minimum given by the severe erosion class
when the weather was very dry during the growing season (Mokma and Sietz, 1992). In
1986 and 1992, theweatherwasvery cool duringthe gowing season and may explain at least
partially the great yield differences for the severe erosion class compared to both the
moderate and slight erosion classes. In 1991, however, the air temperature was high above
normal whereas the soil moisture was one of the lowest during the 9 year-period.
Mean corn yields between the slightly and moderately eroded classes were not sigrificantly
different during the 9-year period except in 1985, 1991, and 1992 (Tables 9 and 10).
The relationship between the growing season precipitation (X) in cm and corn yield
(Y) in Mg/ha (Tables 1 and 2) is expressed by the following linear equations:
Y = 7.295 + 0.047X R2= 0.68
for slightly eroded unmulched plots
Y = 6.545 + 0.056X R2 = 0.51
for slightly eroded mulched plots
Y = 6.536 + 0.063X R2 = 0.60
for moderately eroded unmulched plots
Y = 1.742 + 0.144X R2 = 0.73
for moderately eroded mulched plots

Y = 1.703 + 0.141X R2 = 0.94

37
for severely eroded umnulched plots
Y = -1.808 + 0.193X R2= 0.96
for severely eroded mulched plots.

The correlation between corn yields and precipitation was higher for slightly eroded
unmulched plots than that of mulched plots, lesser for moderately eroded unmulched plots
than that of mulched plots, and slightly higher for severely eroded mulched plots than that of
umnulched plots. The correlation for both slightly and moderately eroded soils was relatively
poor for either mulched or unmulched plots. The correlation for severely eroded soils was
relatively high for both mulched and unmulched plots. The correlation for slightly eroded
unmulched plots was within the range of R2 = 0.61 found by Schumacher et al. (1994) on
glacial till (Marlette soils) in Michigan. The relatively high correlation of severely eroded plots
compared to that of both slightly and moderately eroded plots indicates that increased erosion
and water shortage (decreased water holding-capacity) reduced corn productivity. The
negative slope in the equation for severely eroded mulched plots indicates that straw mulch
further contributed to depletion of corn yields on these soils. Corn on uneroded plots had
sufficient soil moisture during the growing season and consequently gave greater yields
compared to those of eroded soils.

For prolonged droughts during the growing season, mulch has the advantage of
allowing a longer moisture retention in the soil. Other advantages include increasing the
infiltration rate, reducing the bulk density, and increasing the soil porosity (better drainage for
excessive water in the topsoil). These factors contribute to reducing the impact of raindrops
that destroys the soil structure (stability and aggregation of soil particles), to decreasing the

runoff rate, and to reducing the amounts of standing water on the soil surface, especially

38

following large rainfalls as it often occurs in some growing seasons. It is evident that greater
infiltration and reduction of runoff was a major factor in maintaining a greater supply of soil
moisture under mulch conditions, although reduction of evaporation through the season no
doubt was also an important factor (Moody et al., 1963). As an illustration of this fact, while
walking on the plots after a large storm in July, the soil under mulch supported well my
weight, allowing easy movement, but it was more diflicult to walk on the unmulched plots
because the soil structure was weaker and easily breakable. This would mean that under heavy
loads, such as those of combines and trucks during field operations, there could be much
more damaging impact on the soil structure. Another positive factor that may occur between
rainfall events of high intensity in certain years could be a prolonged period of excessive
wetness (waterlogging) on the umnulched soil (saturation and plant anoxia) which would slow
the growth and eventually kill the crop.

In addition, about 45 metric tons/ha of straw mulch or other crOp residues are
required to raise the soil organic matter content by 1% to a 0.3 m depth (Unger et al., 1975;
Allison, 1973 ). This is 9 times greater than the rate of 5 metric tons/ha used in this study to
avoid adverse efl’ects on com growth and productivity (slow decomposition, insulating effect
on the soil surface, greater toxicity in saturated conditions), especially in the somewhat

cooler temperate climate of the study area in south-central Michigan.

39

From Table 1, it can be seen that mulch reduced corn grain yields during the 5-year
study period. The yields for eroded mulched plots were sigrificantly reduced compared to
those of the unmulched plots. The reasons for these reductions could be that 5 years of mulch
addition were not suficient to bring about a change in the levels of organic matter which
would have improved soil properties. Since the reduction was not sigtificantly difl‘erent on
the slightly eroded sites, it can be assumed that the reduced corn yield on the eroded plots
was due to a combination of the negative efi‘ects of both mulch addition and soil erosion.

The second approach to explaining this reduction of corn gain for mulched plots
would be to consider the effect of climatic conditions for individual years (short term scale).
For example, 1989 and 1993 yielded greater corn grains for all three classes when soil
moisture contents were above normal, whereas 1991 and 1992 had lesser yields, especially
the eroded plots, when precipitation was below normal. In 1991, mean air temperatures were
above normal, whereas in 1992 they were below normal. Thus, corn yields in 1991 were less
than corn yields in 1992 for all three classes of erosion. This indicates that soil moisture more
than air temperature was the important factor in determining corn yields (Tables 2 and 3).

Com yield ratios of mulch to unmulched plots (Table 4) showed a slight increase in
1989 between the slight and severe erosion classes, whereas a sigiificant reduction (16 to
24%) occurred fi'om 1990 to 1993. Comparison between the slight and severe erosion classes
(Table 8) shows that com yield reductions were above the range of the threshold of 25%
reduction found by other researchers such as Olson and Nizeyimana (1988) on Kentucky
soils, Schertz et al. (1989) on Miami soils in Indiana, and Weesies et al. (1994) on the same

Indiana soils.

40

Soil Moime Content

Table 5 shows the average soil moisture contents in the 0-90 cm thickness from 1989
to 1993, for mulched and unmulched plots. Gravimetric water content determinations were
obtained by 30 cm increments to the 90 cm depth taken shortly before silking for all plots.
When comparison was made between mulched and unmulched plots within the same class,
soil moisture content increases were 9, 13, and 11% for the slightly, moderately, and the
severely eroded classes, respectively. Both moisture content difl‘erences for moderately and
severely eroded mulched plots were statistically significant (P = 0.05), but were not sigrificant
for the slightly eroded plots. Therefore, soil moisture was always greater with mulch than
without mulch treatment and increased with the degree of soil erosion . It also means that
mulch is needed more on eroded soils than on uneroded soils in order to maintain or increase
the available water capacity in the soil.

Mulched plots had greater soil moisture contents than unmulched plots for all three
erosion classes when averaged over the 5-year period. However, there were exceptions in
some individual years, such as 1991 for the slightly eroded plots and 1990 for the moderately
eroded plots, where mean moisture contents were not statistically different (P = 0.05). The
greater moisture contents on mulched plots could be attributed to the progressive
decomposition and mineralization of wheat straw and addition of organic matter to the soil.
The addition of organic matter would have a positive effect on soil physical properties such
as increased soil aeration, the improvement of porosity, and the provision for a better and
homogeneous water distribution in the profile. Mulch reduces runofl‘, regulates soil

temperature and evaporation, and inhibits soil compaction and surface crusting. Mulch can

 

41

mitigate declining soil moisture when precipitation is below normal conditions, which
occurred in May and June of 1991, 1992, and 1993 (Table 2). However, the yield reduction
on mulched plots suggests that mulch did not sigrificantly increase the organic matter in the
soil during the test years.

Soil moisture contents increased with sample thickness regardless of erosion class
and the mulch plots always exhibited a geater moisture content than unmulched plots (Table
6). Figure 2 is the summary of soil moisture contents at 60 cm thickness prior to silking over
the 5-year period. The soil moisture trend varied within each thickness. Both the severely
eroded and the slightly eroded classes had greater moisture contents in the upper part of the
profile. At a greater thickness though, the severe erosion class contained a slightly greater
soil moisture content than the slight erosion class. However, they both held slightly less
moisture contents when compared to the moderate erosion class. The difi‘erence in percentage
change decreased with the increased thickness for the slight and severe erosion classes. This
indicates mulch had little efl‘ect on soil moisture at greater depths in the profile. Yields can
be related to soil thickness and maximum yields have been attained with thicker soils. Soil
thicknesses between 60 to 90 cm are considered as the useful zone for root proliferation and
development (Power et al. 1978; Power et al. 1981; Schuman et al. 1985). The shallow
rooting depths of the severely eroded plots corresponded with sigrificantly reduced yields
except when there was enough moisture in the soil (as occurred in 1989). Deeper soil and
adequate rooting zone thickness on the slight and moderate erosion classes explain the greater
yields obtained on these plots compared to the severe erosion class. At depths greater than
90 cm, root proliferation and activity decrease as the bulk density increases (Pierce et al.,

1983; Mokma and Sietz, 1992).

42

 

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43

'Mi out

On mulched plots, the mean grain moisture content tended to increase from the
slightly to the moderately to the severely eroded classes (Table 11). This agrees with the
findings of Mokma and Sietz( 1992). “Widely variable grain moisture contents occurred on
unmulched plots for all three classes, both in individual years and when averaged over the 5-
year period as well. The difference between means was statistically sigiificant at the 0.05 level
(using Duncan's multiple range test) except in 1989 for all three classes, and in 1991 and 1992
for the slightly eroded class. Figure 3 represents the grain moisture contents over the 5-year

period of the study.

Table 11. Grain moisture (%) of erosion classes with and without mulch, 1989-1993.

 

 

1mm Class M
1989 1990 1991 1992 1993 Mean Change (%)+

Slight

Unmolohg 31.9a* 54.4a 29.8c 54.2b 28.1b 39.7b

Mtgoh 30.8a 52.9ab 30.3c 55.2b 29.3ab 39.7b 0
Moderate

mom 30.4a 38.3d 30.1c 40.8c 26.6b 33.2c

Muloh 31.8a 46.8bc 42.6ab 57.4b 29.5ab 41.6ab +25
Severe

Unmflohod 32.0a 42.2cd 38.6b 53.5b 31.4ab 39.5b

Moloh 33.2a 53. lab 47.8a 64.6a 34.8a 46.7a +18

 

*Numbers for a given grain moisture percentage for a given year not followed by the same
letter are sigrificantly difl‘erent at the 5% level using Duncan's multiple range test.
+Percent change in moisture contents comparing no mulch with mulch plots.

 

44

Grain moisture contents increased when comparing mulched and unmulched plots at
the erosion class level. There were 0%, 25%, and 18% grain moisture content increases for
the slightly, moderately, and severely eroded class, respectively. The increase of these
moisture contents was statistically significant (P = 0.05) except for the slightly eroded class.
Also, the increase was much greater for the severely eroded class and the moderately eroded
class, respectively when compared to the slightly eroded class (Table 1 1). High grain moisture
content is indicative of a delayed maturity which may eventually affect corn yield and post-

harvest operations.

45

 

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46

mm

Plant populations of mulch and unmulched plots over the 5-year period for the
slightly, moderate, and severely eroded classes are shown in Table 12. Figure 4 shows the
mean plant population for the 5-year period. Population reductions on mulched plots were
4%, 9% and 10% for the slightly, moderately and severely eroded classes, respectively, when
compared with the unmulched plots. Population reduction on mulched plots for the slightly,
moderately and severely eroded classes were not statistically sigiificant (P = 0.05) compared
to respective unmulched plots. Population reduction for severely eroded class and the
moderately eroded class were 2.5 and 2.25 times greater, respectively, than those of the
slightly eroded class.

Plant population almost repeated the same trend as observed for the corn grain
yields. The lowest stand counts were registered in 1991, 1992 and 1993 on mulched plots
for the severely eroded class, but in 1991 and 1992 for the moderately eroded plots. Plant
population was only once reduced sigrificantly for the slightly eroded plots in 1991. The plant
population is closely associated with grain yield. In fact, fewer healthy corn plants per
surface unit with larger ears per plant may give higher yields than numerous smaller corn
plants per hectare but with multiple smaller corn ears. The latter was the general tendency that
often occrured on the severely eroded soils and especially on the mulched plots. Better crop
spacing facilitates the plant pollination especially in grarninacea species, and reduces both
intraspecific and interspecific competition for water, nutrients, and sun, which are

indispensable for the plant growth and yielding.

47

Table 12. Plant population (plants/m2) of slight, moderate, and severe erosion classes with
and without mulch for 1989-1993.

 

 

_tm_tTrea on Class Yer
1989 1990 1991 1992 1993 Mean Change (%)+
Slight
nm lch 5.7a 5.6a 5.6a 5.7a 5.5a 5.6a
Muloh 5.6a* 5.4ab 4.9b 5.6a 5.2ab 5.4a -4
Moderate
nmulch 5.8a 5.6a 4.8b 4.9b 5.3ab 5.3ab
Muloh 5.8a 5.4ab 3.9cd 3.9c 4.9ab 4.8bc -9
Severe
Unmulohoo 5.7a 5.0b 4.4bc 3.9c 4.8b 4.8cd
Moloh 5.8a 5.0b 3.8d 2.7d 3.9c 4.3d -10

 

*Numbers for a given population for a specific year not followed by the same letter are
sigrificantly difi‘erent at the 5% level using Duncan's multiple range test.

+Percent reduction between mulch and unmulched plots.

Plants on the severely eroded soils are likely to be more stressed than plants on less eroded
soils. This stress is explained by a greater runoff on the topsoil, more droughty conditions,
and the high clay content in the Ap horizon resulting from tillage (mixing of the plow layer

with the subsoil) and erosion. These factors interacting together afl‘ect the water-holding

capacity which will definitely deplete the soil-crop productivity.

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49

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Stover yields were slightly greater for the mulched slightly eroded plots than for the
unmulched ones. However, they were slightly less for moderately and severely eroded
mulched plots than for the unmulched plots, when averaged over the 5-year period
(Table 13). Figure 5 shows mean stover yields over the 5-year period. Reductions on
mulched plots for the severely eroded class and moderately eroded class were statistically
sigrificant (P = 0.05) when compared to the respective unmulched plots. For stover, yields
increased 4% when comparison was made between mulched and unmulched plots, for the
slightly eroded class; but they decreased 4% and 6% for the moderately and severely eroded
class respectively. Stover yield reductions of mulched plots on each erosion class were not
statistically sigrificant (P = 0.05) in certain years (1989). However, fl'om 1990 to 1993, there
was much variability in yield reductions for the difl‘erent erosion classes. Stover yield decrease
for both moderately and severely eroded mulched plots was minus 1 and 1.5 times,
respectively, greater than the slightly eroded mulched plots. Stover yields were greater on
mulched plots than on unmulched plots for the slightly eroded class, but tended to decrease
constantly with the degree of erosion for both moderately and severely eroded mulched and
unmulched plots. The trend was the same as observed for grain and plant population yields.

Stover yields in terms of plant height and dry matter production was depressed and
decreased progressively with erosion class, especially severely eroded mulched plots. Usually
though, appropriate amounts of crop residue on the soil would be efficient on controlling soil

erosion and allowing necessary amounts of heat to the soil under mulch during the growing

50

season The stover yield is closely related to plant population and affects, therefore, the grain

yield at harvest.

Table 13. Stover yield (Mg/ha) of erosion classes with and without mulch for 1989-1993.

 

 

Treatmoot gag M
1989 1990 1991 1992 1993 Mean Change (%)+

Slight

Unmulched 3.8a 6.5a 5.4a 5.9a 5.3a 5.4ab

Mulch 4.1a* 7.3a 5.0ab 5.8a 5.6a 5.6a +4
Moderate

Unmolchod 4.2a 5.0b 4.6bc 5.3a 5.5a 4.9ab

Mulch 4.0a 6.5a 3.4d 4.5b 4.9a 4.7b -4
Severe

Unm l h 3.9a 6.7a 4.2c 4.2b 5.1a 4.8ab

Muloh 4.7a 7.2a 4.1cd 3.0c 3.5b 4.5b -6

 

*Numbers for a given yield for a specific year not followed by the same letter are significantly
difl‘erent at the 5% level using Duncan's multiple range test.
+Percent reduction or increase compared to no mulch.

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SUMMARY AND CONCLUSION

For most severely eroded soils, efl‘orts to restore productivity should be aimed at
increasing the water supplying capacity of the soil. This includes increasing the organic matter
content in the soil which will efl‘ect the infiltration rate, the grade of soil aggregation, and
soil available-water for the crop. However, restoring crop productivity is often very
expensive and requires long term efi‘orts.

The use of wheat straw mulch in this study showed a general tendency toward
reducing corn yields on eroded Marlette soils when compared with unmulched soils. The low
corn yields and high grain moisture contents of these mulched plots over the 5-year period
suggested that soil erosion combined with mulch addition to the plots negatively affected the
yields when compared to unmulched plots.

Soil erosion reduced corn yields on Marlette soils, especially severely eroded
conditions. This reduction in corn yield between erosion classes was statistically sigrificant
and consistent through the years of cultivation. Mulch was used in an attempt to maintain or
increase productivity on the eroded soils, but results showed generally less productivity
compared to unmulched soils. This pattern was repeated on both yearly corn yields and when
averaged over the 5-year period. However, corn yields were not significantly difl‘erent
between nrulched and unmulched soils for the slightly eroded class, except for 1992. Because

yield reduction on eroded soils was always greater than reduction on uneroded soils, we can

52

53

assume that the degree of soil erosion had more impact in reducing com yields than the
addition of mulch. The amount of organic matter added with straw mulch to the soil during
this 5-year period was not sufficient yet to stabilize soil properties quantitatively and
qualitatively on these eroded soils. It still may take several years of mulch application to the
topsoil on this field before we begin to see some real and effective impact of the organic

matter on the soil properties.

RECOMMENDATIONS

This study has left me with some unanswered questions when trying to understand the
possible causes of the low corn yield under mulch. The objective approach in studying the
crop productivity would be to consider other factors than those based solely on soil properties
in relation to soil erosion.

Allelopathy under certain conditions can significantly reduce crop productivity. More
study is needed to determine whether or not wheat straw mulch has an allelopathic effect on
corn productivity on Marlette soils and under Michigan climate. Studies should be done both
under field conditions as well as under controlled ambient conditions (laboratory, growth
chambers, and green houses).

Soil temperature measurements at different depths are needed to determine whether
or not straw mulch has an insulation efl‘ect on the soil surface that could have a negative
impact on corn yields on Marlette soils here in Michigan. Further research is needed to
determine whether wheat straw mulch has an allelopathic effect or whether the yield
reductions are related to lower soil temperatures or large C:N ratios which influence corn

development and yield.

54

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