dry weight per acre. If a crop like winter rye is grown to
maturity, it can produce 3 to 5 tons of residue.
A five-year experiment with clover in California
reduce
erosion
showed that cover crops increased organic matter in the
suppress
suppress
nematodes
top 2 inches from 1.3% to 2.6% and in the 2- to 6-inch
weeds
add N
increase
enhance
layer from 1% to 1.2%. Some researchers have found that
(legume)
infiltration
mycorrhizal
of water
cover crops do not seem to increase soil organic mat-
numbers
ter. Low-growing cover crops that don’t produce much
decrease
organic matter may not be able to counter the depleting
add
nutrient
organic
cover
loss
effects of some management practices, such as intensive
crops
attract
matter
beneficial
tillage. Even if they don’t significantly increase organic
insects
matter levels, cover crops help prevent erosion and add at
least some residues that are readily used by soil organisms.
Figure 10.1. Cover crops have multiple benefits.
Cover crops help maintain high populations of
mycorrhizal fungi spores during the fallow period
bENEFITS OF cOVER cROPS
between main crops. The fungus also associates with
Cover crops provide multiple potential benefits to soil
almost all cover crops, which helps maintain or improve
health and the following crops, while also helping main-
inoculation of the next crop. (As discussed in chapter 4,
tain cleaner surface and groundwater (figure 10.1). They
mycorrhizal fungi help promote the health of many crop
prevent erosion, improve soil physical and biological
plants in a variety of ways and also improve soil aggre-
properties, supply nutrients to the following crop, sup-
gation.) Cover crop pollen and nectar can be important
press weeds, improve soil water availability, and break
food sources for predatory mites and parasitic wasps,
pest cycles. Some cover crops are able to break into
both important for biological control of insect pests. A
compacted soil layers, making it easier for the following
cover crop also provides a good habitat for spiders, and
crop’s roots to more fully develop. The actual benefits
these general insect feeders help decrease pest popula-
from a cover crop depend on the species and productiv-
tions. Use of cover crops in the Southeast has reduced
ity of the crop you grow and how long it’s left to grow
the incidence of thrips, bollworm, budworm, aphids,
before the soil is prepared for the next crop.
fall armyworm, beet armyworm, and white flies. Living
Organic matter. Grass cover crops are more likely
cover crop plants and their residues also increase water
than legumes to increase soil organic matter. The more
infiltration into soil, thus compensating for the water
residue you return to the soil, the better the effect on
that cover crops use.
soil organic matter. The amount of residue produced by
the cover crop may be very small, as little as half a ton
SElEcTION OF cOVER cROPS
of dry matter per acre. This adds some active organic
Before growing cover crops, you need to ask yourself
matter, but because most of it decomposes rapidly after
some questions:
the crop is killed, there is no measurable effect on the
• What type of crop should I plant?
total amount of organic matter present. On the other
• When and how should I plant the crop?
hand, good production of hairy vetch or crimson clover
• When should the crop be killed or incorporated into
cover crops may yield from 1 1/2 to more than 4 tons of
the soil?
102
Building SoilS for Better CropS: SuStainaBle Soil ManageMent