about 500 pounds per acre per year and four years of
4
alfalfa increased organic matter by 800 pounds per
acre per year. (Keep in mind that these amounts are
grass sod
small compared to the amounts of organic matter in
3
most soils—3% organic matter represents about 60,000
pounds per acre to a depth of 6 inches.)
2
Two things happen when perennial forages are
corn
(moldboard plow)
part of the rotation and remain in place for some years
organic matter (%) 1
during a rotation. First, the rate of decomposition of
soil organic matter decreases, because the soil is not
continually being disturbed. (This also happens when
0
using no-till planting, even for nonsod-type crops,
25
50
75
100
years
such as corn.) Second, grass and legume sods develop
extensive root systems, part of which will naturally die
Figure 11.2. Organic matter changes in the plow layer during long-term
each year, adding new organic matter to the soil. Crops
cultivation followed by hay crop establishment.
with extensive root systems stimulate high levels of soil
when the cropping system is changed from a cultivated
biological activity and soil aggregation. The roots of a
crop to a grass or mixed grass–legume sod. However,
healthy grass or legume-grass sod return more organic
the increase is usually much slower than the decrease
matter to the soil than roots of most other crops. Older
that occurred under continuous tillage.
roots of grasses die, even during the growing season,
A long-term cropping experiment in Missouri com-
and provide sources of fresh, active organic matter.
pared continuous corn to continuous sod and various
Rotations that included three years of perennial forage
rotations. More than 9 inches of topsoil was lost during
crops have been found to produce a very high-quality
sixty years of continuous corn. The amount of soil lost
soil in the corn and soybean belt of the Midwest.
each year from the continuous corn plots was equivalent
We are not only interested in total soil organic
to 21 tons per acre. After sixty years, soil under continu-
matter—we want a wide variety of different types of
ous corn had only 44% as much topsoil as that under
organisms living in the soil. We also want to have a good
continuous timothy sod. A six-year rotation consisting
amount of active organic matter and high levels of well-
of corn, oats, wheat, clover, and two years of timothy
decomposed soil organic matter, or humus, in the soil.
resulted in about 70% as much topsoil as found in the
Although most experiments have compared soil organic
timothy soil, a much better result than with continuous
matter changes under different cropping systems, few
corn. Differences in erosion and organic matter decom-
experiments have looked at the effects of rotations on
position resulted in soil organic matter levels of 2.2%
soil ecology. The more residues your crops leave in the
for the unfertilized timothy and 1.2% for the continuous
field, the greater the populations of soil microorgan-
corn plots.
isms. Experiments in a semiarid region in Oregon found
In an experiment in eastern Canada, continuous
that the total amount of microorganisms in a two-year
corn led to annual increases in organic matter of about
wheat-fallow system was only about 25% of the amount
100 pounds per acre, while two years of corn followed
found under pasture. Conventional moldboard plow
by two years of alfalfa increased organic matter by
tillage systems are known to decrease the populations
117
Building SoilS for Better CropS: SuStainaBle Soil ManageMent