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Bauder, James W.

Halsey, Clifton F.

University of Minnesota Digital Conservancy

Agricultural Extension Service 
University of Minnesota 
Extension Folder 477-1979 
Estimating the Effects of Crop Residue Mulches 
on Soil Erosion By Water 
Clifton F. Halsey 
James W. Bauder 
Most farmers know that conservation tillage is often benefi-
cial. The term can be applied to a variety of tillage methods, 
but usually not moldboard plowing. One goal of conservation 
tillage is to keep some crop residue on the field surface, after 
harvest, to reduce erosion. Trash management and mulch til-
lage are other terms associated with crop residue management. 
This publication describes the concept of crop residue man-
agement and should help farmers estimate the good .effects of 
surface crop residue in reducing soil erosion by water. 
Crop residue-stalks, stems, straw, and leaves-remaining on 
the field after harvest can help both farmer and soil. Residue 
on the field softens the blow of the striking raindrops. This re-
duces the amount of soil that is detached (broken loose) from 
larger soil clods and may be carried off by flowing water. Crop 
residue also slows the surface flow of water and reduces its 
power to detach and move soil particles. Crop residue on the 
field may reduce surface crusting and sealing which increases 
rainfall infiltration and may reduce evaporation. 
Uniformly scattered residue is more useful than that 
bunched by combines; spreader attachments are desirable on 
combines. Finely chopped and divided residue can be scattered 
more evenly and is more effective than larger pieces of stalk; 
chopped cornstalks are more effective than whole ones. 
To be most effective, crop residue should cover the soil sur-
face during the time of greatest erosion danger. Most erosion 
occurs between the beginning of the spring thaw and the time 
the crop canopy or leaves are developed enough to provide 
significant protection from raindrop impact-about the first 
week in July. The time of greatest erosion danger varies with 
the crop and cropping practices. For example, small grains give 
earlier protection than soybeans, but soybeans eventually pro-
vide a heavier canopy. Likewise soybeans, drilled or in 15-inch 
rows, will provide more and earlier protection than soybeans 
in 30-inch rows. 
Clifton F. Halsey is extension conservationist, Soils, and James W. Bauder 
is extension specialist, Soils. 
The proportion of the soil surface covered or protected by 
crop residue directly influences its effectiveness in reducing soil 
erosion. The quantity of residue remaining after harvesting 
varies with the yield per acre, the crop species, the amount of 
straw or stalks removed during harvest, and the type and 
amount of post-harvest tillage. The proportion of the soil sur-
face covered by residue depends on the amount, the fineness, 
and the uniformity of spreading. The optimum amount of crop 
residue on the surface for many fields is between 1-½ and 2 
tons per acre. Two tons of residue per acre is the amount re-
maining from small grain crops averaging 40 bushels per acre or 
corn crops averaging 70 bushels per acre. The lighter amounts 
are more desirable on the less erosive and more poorly drained 
soils; heavier amounts are more suitable on well-drained soils 
and are needed on the more erosive and steeper soi Is to ade-
quately control erosion. 
The type of tillage also influences erosion. There is less ero-
sion from no-till areas than from chisel-plowed or disked areas 
although all three areas have the same amount of surface cover. 
MULCH FACTOR 
Researchers use the term, mulch factor, to indicate the ef-
fectiveness of surface residue in reducing soil erosion. Mulch is 
crop or other plant residue left on the surface. The mulch fac-
tor is the ratio of (a) the amount of soil eroded from a surface 
having a given proportion of the area covered by mulch to 
(b) the corresponding amount of soil eroded from the same sur-
face when it is bare. Figure 1 illustrates how to determine the 
mulch factor. Assume a bare field where the amount of soil 
eroded is (b), 10 tons per acre. When that field is 10 percent 
covered by small grain mulch, the amount of soil eroded is 8 
tons per acre; alb= 8/10 = 0.8: the mulch factor is 0.8. When 
the field is 80 percent covered by small grain mulch, the amount 
of soil eroded is 1.2 tons per acre; alb= 1.2/10 = 0.12: the 
mulch factor is 0.12. The mulch factor literally describes the 
effectiveness of surface crop residues in reducing soil erosion 
by water. 
Figure 1. Determining the mulch factor 
Mulch factor for 1 O'\, cover by small gram mulch 
a __ s_._0_1,_a __ I \ Soil erosion when ' 10% of the soil sur-} face is covered by l small grain mulch. i,=---------------------G::J 
0.8 
{
1 
Soil erosion when 
the soil surface is 
bare. 
Mulch factor for 80% cover by small grain mulch 
1.2 t/a { 
t ___ p 
Soil erosion when 
80% of the soil sur-
face is covered by 
small grain mulch. 
Soil erosion when 
the soil surface is 
bare. 
= 0.12 
Cover photo: Soybean stubble and straw being tandem disked. Right 
side before disking: 60 percent of the ground is covered. Left side 
after disking: 18 percent of the ground is covered. 
Figure 2 shows ranges of mulch factors for various propor-
tions of surface covered by residue. The figure is adapted from 
the reports of several researchers. Figure 2 indicates that the 
mulch factor and thus the amount of soil eroded decreases as 
the percentage of soil surface covered by mulch increases. 
Figure 2. Range of mulch factors for field crops 
bare soil 
1.0 
0.9 
0.8 
0.7 
B 0.6 
u 
~ 
~ 0.5 
.!:!. 
j 
~ 0.4 
0.3 
0.2 
\ ' 
\ \ 
\ \ 
\ ~ 
\ . ~ "-
" 
"' 
" 
" 
'
"-
" 
"' 
'-
"'-'" ~ 
0.1 ~ '--~, ~ 
0 10 20 30 40 50 60 70 80 90 100 
Percent of soil surface covered by mulch. 
Figure 3. Average residue yield for field crops 
Q 
u 
CD 
ai 
C. 
... 3 C 
0 
i 
"C 
ai 
.>-
2 Cl) 
j 
"C 
·;;; 
Cl) 
a: 
20 40 60 80 100 
Crop yield-bushels per acre 
Assumptions: 
1 pound of residue per pound of shelled corn 
1.6 pounds of residue per pound of sunflower seed 
100 pounds of residue per bushel of small grain 
45 pounds of residue per bushel of soybeans 
120 140 160 
Small grain stubble and straw being disked and chiseled. Right side after disking: 50 percent of the ground is covered. Left side after disking and 
chiseling: 24 percent of the ground is covered. 
The following example illustrates the usefulness of the mulch 
factor. A field with 70 percent of its surface covered by mulch 
would have between 0.15 and 0.20 as much erosion by water as 
a bare field with similar surface conditions such as roughness. 
If the mulch was partially buried by disking so that only 20 per-
cent of the surface was covered, there would be 0.50 to 0. 70 as 
much erosion by water as if the soil were bare. 
CROP RESIDUE 
To repeat, the amount of residue remaining after harvesting 
varies with the kind of crop grown. The amount of residue on 
the surface can be measured by collecting all the residue from 
a known sample area and weighing it. Figure 3 shows the aver-
age amounts of residue produced by several crops. Because of 
the smaller diameters of the stems of small grain and soybeans 
compared with cornstalks, a ton of soybean or small grain 
residue has many more stems to cover the ground and is as 
effective as 2 tons of corn or sunflower residue in reducing 
erosion by water. 
Table 1 illustrates the effectiveness of increasing amounts 
of residue from various crops in reducing soil erosion by' 
water. The table shows the percent of the surface covered by 
a given amount of crop residue and the resulting range of 
mulch factors. 
Table 1 applies only to conditions in which all the residue 
is left on the surface and is fairly evenly spread. Conditions in 
which portions of the stalks or straw have been removed or 
covered by tillage are not included. As a practical matter, the 
proportion of the soil surface covered by residue depends on 
the types and amounts of post-harvest tillage. Table 2 shows 
the approximate percent of preceding cover (proportion of the 
soil surface covered) remaining after various tillages. 
Two tandem diskings may leave about 25 percent of the 
preceding surface cover. Two passes with a chisel plow leaves 
about 40 percent. One chisel plowing and one disking may leave 
between 30-40 percent of the preceding surface cover. 
If a 100 bushel per acre corn crop produces about 2-¾ tons 
per acre of stalks covering about 85 percent of the surface, one 
Table 1. Relationship between amounts of residue, percent of surface 
protected, and mulch factors 
Small grain 
Chopped or soybean Percent 
cornstalk mulch- straw mulch- of surface Range of 
tons per acre tons per acre covered mulch factors 
0.5 30 0.40 - 0.58 
1.0 0.5 50 0.25 - 0.35 
1.5 60 0.20 - 0.28 
2.0 1.0 75 0.11 -0.17 
2.5 80 0.10 - 0.15 
1.5 85 0.C7-0.12 
3.0 2.0 90 0.04-0.10 
3.5 2.5 95 0.02 - 0.07 
Table 2. Influence of tillage on surface residue 
Tillage machine 
Moldboard plow 
Tandem off-set disk 
Field cultivator (16 inch sweeps) 
Chisel (2 inch points, 12 inches apart) 
No-till planter 
Till planter 
Percentage of preceding cover 
remaining after each 
tillage operation 
0-10 
35 - 85 
25 - 50 
50- 80 
100 
70 - 80 
llllll~il!imi~j[~lii111i~il~~~iii~llll 
3 1951 D01 927 498 M 
Figure 4. Stretch the line between two stakes at right angles to the rows 
or direction of tillage but from center to center of rows or ridges. 
chisel plowing and one disking may leave enough residue to 
cover about 30 percent of the surface. This amount of cover 
may reduce soil erosion by water to half of what would occur 
following conventional tillage if the conventional plowing and 
the disking after chiseling were done at about the same time. 
One tandem-disking of grain stubble and straw may leave 
50 percent of the surface covered; a succeeding chiseling may 
leave half of the remaining 50 percent or 25 percent covered. 
The first tandem disking of a field of cornstalks which 
covers 85 percent of the ground may leave 70 percent of the 
ground covered by cornstalks. One tandem disking of soybean 
straw covering 60 percent of the ground may leave about 20 
percent of the ground covered. 
ESTIMATING AMOUNTS OF SURFACE RESIDUE 
Determining the amount of residue on the field by collecting 
it from representative areas and weighing it is time consuming 
and unnecessary. The following method is a variation from grid 
procedures that have been proven reliable by research. 
Take a piece of cotton clothesline or similar rope 10-12 feet 
long. Wrap pieces of plastic electrical tape at 25, 4-inch intervals 
around the line leaving a foot or more of unmarked line at each 
end. Tie firm loops about 2 inches in diameter at each end. 
To estimate the amount of residue in the field, pick four 
areas that appear to be representative of the amount of cover 
on the field. Stretch the clothesline between 2 stakes across 
each area at right angles to the direction of tillage or the crop 
rows. Adjust the line diagonally so that it runs from the center 
of a row or tillage ridge to the center of the row or tillage ridge 
at the other end of the I ine (figures 4 and 5). 
Look straight down along one edge of each of the tapes and 
note whether or not your line of sight intersects with a solid 
piece of leaf or stalk. Soybean leaves decay or break up rapidly; 
so ignore these and other thin leaves in counting. Count the 
number of sightings which intersect with significant residue at 
the same (either left or right) edge of each of the 25 points on 
the line. Do this at four locations adding up the number of 
intersecting points. The total will be a good estimate of the per-
cent of the surface covered by crop residue. 
Figure 5. Count the number of points which intersect with significant 
residue at the same edge (left or right) at each mark on the line. 
As an example, imagine you counted 5 intersecting points 
at the first location, 9 at the second, 6 at the third, and 7 at the 
fourth. Adding these indicates that about 27 percent of the sur-
face is covered. Figure 2 indicates that this amount of small 
grain straw would reduce soil erosion to about half of what it 
would be after moldboard plowing. 
CONCLUSIONS 
Crop residue, left on the surface as mulches, is very effective 
in reducing soil erosion by water. The stalks and leaves shield 
the soil from the beating action of raindrops which tear apart 
soil particles which can then be carried away by the flowing 
water. The amount of protection depends on the proportion 
of the surface covered; this proportion is influenced by the 
amount of residue present and how evenly it is distributed. 
Amounts present depend on the crop, the amount and coarse-
ness of the residue produced, and the effects of tillage on the 
residue after harvest. 
An estimate of the amount of protection provided may be 
determined by estimating the percent of surface covered. Per-
cent of cover also may be estimated by using a simple transect 
method which involves counting the number of points along a 
line which coincides with the location of pieces of residue. Per-
cent of cover may be translated quickly into a mulch factor or 
percent of erosion that would result when compared to occur-
rence on bare soil. 
The authors gratefully acknowledge the technical assistance and review 
provided by James Swan, extension specialist, Soils, and professor, 
Department of Soil Science, and Curtis Larson, professor, Department 
of Agricultural Engineering, both at the University of Minnesota; 
and by district conservationists and engineers, Howard Midji, Gerald 
Hildebrandt, and Charles Loggins, U.S. Soil Conservation Service. 
Issued in furtherance of cooperative extension work in agriculture and 
home economics, acts of May 8 and June 30, 1914, in cooperation with 
the U.S. Department of Agriculture. Roland H. Abraham, Director of 
Agricultural Extension Service, University of Minnesota, St. Paul, Min-
nesota 55108. The University of Minnesota, including the Agricultural 
Extension Service, is committed to the policy that all persons shall have 
equal access to its programs, facilities, and employment without regard 
to race, creed, color, sex, national origin, or handicap. 1 o¢ 


Estimating the Effects of Crop Residue Mulches on Soil Erosion By Water

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Estimating the Effects of Crop Residue Mulches on Soil Erosion By Water

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