The contribution of arbuscular mycorrhizal fungi to the success or failure of agricultural practices - Microbial Ecology in Sustainable Agroecosystems - page 142

Table 7.1 Aboveground Biomass and Nitrogen Input from Overwintering Cover Crops Grown at
the Rodale Institute, Kutztown, Pennsylvania

Biomass

Seeding
rate
(kg ha -1 )

Planting
date

Sampling
date

N input
(kg N ha -1 )

Replicate
years Cover crop kg ha -1 %N

Winter rye 157 Early

November

Mid-April 114-1,147 2.4-4.5 5-27 4

Winter rye 157 Early

November

Early May 240-2,283 1.6-3.6 7-57 7

Red clover 11 Late March Early May 1,185-3,710 2.7-4.1 43-135 8

Hairy vetch 25-35 Late

August

Early May 2,581-5,595 3.5-6.2 116-281 16

Cover crops can be annual or perennial species and primarily include legumes,
grasses, and brassicas. A winter cover crop is planted in late summer or fall in temperate
climates to provide soil cover during the nongrowing season. A range of species is avail-
able to use as winter cover crops ( Table 7.1 ) . Small grains (barley, oats, rye, and wheat) grow
rapidly, they have good winter hardiness, and seed is readily available. With their fast
growth, they are strong competitors against weeds, and some (such as rye) can be allelo-
pathic, emitting chemicals that inhibit weed seed germination. Legumes, such as clovers,
vetches, and peas, also may be used as winter cover crops. They are less winter hardy than
grasses, grow less rapidly, and are not as effective in preventing erosion or reducing leach-
ing loss of leftover nitrogen. However, they add significant amounts of nitrogen to the soil
( Table 7.1 ), which is made available gradually to the following crop, a nitrogen availabil-
ity pattern that is more adapted to plant growth and needs than most mineral fertilizers
(Sarrantonio, 2003).
Adding cover crops to a farm plan may be a challenge; therefore, a farmer needs first
to identify the main reason for planting a cover crop (e.g., to prevent erosion or to provide
nitrogen) and then determine which cover crop best fulfills that criterion and where it can
fit into the rotation. Typical planting and termination dates of the chosen cover crop have
to be coordinated with the planting and harvesting dates of the cash crop to ensure a wide
enough growing window for the cover crop. In northern regions, the cover crop needs to
be cold tolerant to survive hard winters. Hairy vetch and most small grains are able to
withstand low temperatures, and winter rye can be planted later in the fall than most other
grasses. Legumes and grasses can also be planted in a mix and can either be established
by aerial seeding into maturing cash crops in the fall or by drilling or broadcasting seed
immediately following harvest.

7.2.3.1.2 Effects of cover crops. Similar to conservation tillage and crop rotation, the
purposes of cover crops in any farming system are multifold: They prevent soil erosion by
wind or water (Hargrove, 1991); increase yields, especially if legumes are used (Kramberger
et al., 2009); reduce nitrate leaching and conserve water resources (Hargrove, 1991; Salmeron
et al., 2010); enhance soil organic matter and nitrogen storage (Harper et al., 1995; Stivers
and Shennan, 1991); and reduce insect and pathogen damage (Fritz et al., 1995; Laub and
Luna, 1992). Although conservation tillage and cover crops overlap in some of their benefits
(for example, erosion reduction, soil carbon enhancement), a trial by Veenstra et al. (2006)
showed that of the two practices cover crops had the greater impact. In a cotton-tomato
rotation in California, cover cropping increased soil organic matter, regardless of the tillage