flowers and completes its life cycle in the second year.
A problem common to all the grasses is that if you
When used as a green manure crop, it is incorporated
grow the crop to maturity for the maximum amount of
into the soil before full bloom.
residue, you reduce the amount of available nitrogen
White clover does not produce as much growth as
for the next crop. This is because of the high C:N ratio,
many of the other legumes and is also less tolerant of
or low percentage of nitrogen, in grasses near maturity.
droughty situations. (New Zealand types of white clover
The problem can be avoided by killing the grass early
are more drought tolerant than the more commonly
or by adding extra nitrogen in the form of fertilizer or
used Dutch white clover.) However, because it does not
manure. Another way to help with this problem is to
grow very tall and is able to tolerate shading better than
supply extra nitrogen by seeding a legume-grass mix.
many other legumes, it may be useful in orchard-floor
Winter rye, also called cereal or grain rye, is very
covers or as a living mulch. It is also a common compo-
winter-hardy and easy to establish. Its ability to germi-
nent of intensively managed pastures.
nate quickly, together with its winter-hardiness, means
that it can be planted later in the fall than most other
Grasses
species, even in cold climates. Decomposing residue of
Commonly used grass cover crops include the annual
winter rye has been shown to have an allelopathic effect,
cereals (rye, wheat, barley, oats), annual or peren-
which means that it can chemically suppress germina-
nial forage grasses such as ryegrass, and warm-season
tion of weed seeds. It grows quickly in the fall and also
grasses such as sorghum–sudan grass. Nonlegume cover
grows readily in the spring (figure 10.3). It is often the
crops, which are mainly grass species, are very useful
cover crop of choice as a catch crop and also works well
for scavenging nutrients—especially N—left over from a
with a roll-crimp mulch system—in which the cover crop
previous crop. They tend to have extensive root systems,
is suppressed by rolling and crimping at the same time
and some establish rapidly and can greatly reduce ero-
and crops are seeded or transplanted through the mulch
sion. In addition, they can produce large amounts of
(see figure 16.7, p. 180).
residue and, therefore, can help add organic matter to
Oats are not winter-hardy. Summer or fall seedings
the soil. They also can help suppress weed germination
will winterkill under most cold-climate conditions. This
and growth.
provides a naturally killed mulch the following spring
and may help with weed suppression. As a mixture with
one of the clovers, oats provide some quick cover in the
fall. Oat stems help trap snow and conserve moisture,
even after the plants have been killed by frost. Black oat
is very popular with farmers in South America, where it
is mulched for no-till row crops.
Annual ryegrass (not related to winter rye) grows
well in the fall if established early enough. It develops
an extensive root system and therefore provides very
effective erosion control while adding significant quanti-
ties of organic matter. It may winterkill in cold climates.
Some caution is needed with annual ryegrass; because
Figure 10.3. Winter rye, which grows rapidly in the early spring.
it is difficult to kill, it may become a problem weed in
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Building SoilS for Better CropS: SuStainaBle Soil ManageMent