recommended for most direct-seeded vegetable crops,
this works well for many agronomic crops.
Management Cautions
Cover crops can cause serious problems if not man-
aged carefully. They can deplete soil moisture; they can
become weeds; and—when used as an intercrop—they can
compete with the cash crop for water, light, and nutrients.
In drier areas and on droughty soils, such as sands,
late killing of a winter cover crop may result in moisture
deficiency for the main summer crop. In that situation,
the cover crop should be killed before too much water
is removed from the soil. However, in warm, humid
Figure 10.8. A wide cover-free strip and living mulch, which is also used climates where no-till methods are practiced, allowing
for traffic.
the cover crop to grow longer means more residue and
better water conservation for the main crop. Cover crop
crimping in the same operation, or naturally by winter
mulch may more than compensate for the extra water
injury. In many cases it is a good idea to leave a week or
removed from the soil during the later period of green
two between the time a cover crop is tilled in or killed
manure growth. In addition, in very humid regions or on
and the time a main crop is planted. Studies have found
wet soils, the ability of an actively growing cover crop to
that a sudex cover crop is especially allelopathic and that
“pump” water out of the soil by transpiration may be an
tomatoes, broccoli, and lettuce should not be planted
advantage (see figure 15.8, p. 168). Letting the cover crop
until six to eight weeks to allow for thorough leaching of
grow as long as possible results in more rapid soil drying
residue. This allows some decomposition to occur and
and allows for earlier planting of the main crop.
may lessen problems of nitrogen immobilization and
Some cover crops can become unwanted weeds in
allelopathic effects, as well as avoiding increased seed
succeeding crops. Cover crops are sometimes allowed
decay and damping-off diseases (especially under wet
to flower to provide pollen to bees or other beneficial
conditions) and problems with cutworm and wireworm.
insects. However, if the plants actually set seed, the
It also may allow for the establishment of a better seed-
cover crop may reseed unintentionally. Cover crops
bed for small-seeded crops, such as some of the vegeta-
that may become a weed problem include buckwheat,
bles. Establishing a good seedbed for crops with small
ryegrass, crown vetch, and hairy vetch. On the other
seeds may be difficult, because of the lumpiness caused
hand, natural reseeding of subclover, crimson clover, or
by the fresh residues. Good suppression of vetch in a
velvet bean might be beneficial in some situations.
no-till system has been obtained with the use of a modi-
Finally, thick-mulched cover crops make good habi-
fied rolling stalk chopper. Farmers are also experiencing
tat for soil organisms—and also for some undesirable
good cover crop suppression using a crimper-roller that
species. Animals like rats, mice, and snakes (in warm
goes ahead of the tractor, allowing the possibility of no-
climates) may be found under the mulch, and caution is
till planting a main crop at the same time as suppressing
recommended when manual fieldwork is performed.
the cover crop (see figure 16.7, p. 180). Although not
111
Building SoilS for Better CropS: SuStainaBle Soil ManageMent