Chapter 6

Cover Crops for Organic Farms

Types of Cover Crops

Many types of plants can be used as cover crops. Legumes and grasses (including cereals) are the most extensively used, but there is increasing interest in brassicas (such as rape, mustard, and forage radish) and continued interest in others, such as buckwheat. Some of the most important cover crops are discussed below.

Legumes

Legumes are broad-leaved, annual or perennial species known for their ability to fix atmospheric nitrogen (inert gas) into usable forms. Nodules on the roots are the “factories” that house nitrogen-fixing bacteria (Rhizobium spp.) that form a symbiotic relationship with legume roots. In organic farming systems, legumes are integral for maintaining soil fertility and are often the primary nitrogen input to the system.

Carbon-to-Nitrogen Ratio

Legume-based cover crop systems can also play an important role in the short- and long-term cycling of soil nitrogen and carbon in an agroecosystem. Legumes are generally lower in carbon and higher in nitrogen than grasses. This lower carbon-to-nitrogen (C:N) ratio results in faster breakdown of legume residues. Initially, this can result in a faster release of mineralized residue nitrogen (converted from organic to plant-available forms) to the system that is available for the subsequent crop.

Cool Season and Warm Season Legumes

Cool season (winter) annual legumes are generally planted in the fall and provide forage in late fall and spring. These plants flower and produce seed in late spring and die soon after seeds mature. Cool season annual legumes differ substantially in their preferred soil characteristics, growth distribution, cold tolerance, bloat potential and reseeding potential. These species can be used for grazing (typically the best use) or for making hay, silage or grass-legume mixtures. Most cool season annual legumes are susceptible to several disease problems, nematodes and insects. When available, species and varieties with good pest resistance should be chosen to minimize management problems.

Life Cycles of Common Legumes

The following are examples of annual and perennial legumes:

Grasses

While grasses do not grow a taproot that helps break up compaction like the brassicas, the quick-growing grasses have a more fibrous root system with outstanding abilities to hold soil firmly in place and promote water infiltration. Further, the grasses like to scavenge vulnerable nitrogen in the soil profile. Grasses tend to produce large amounts of residue.

Carbon:Nitrogen Ratio

Grass-based cover crop systems can also play an important role in the short- and long-term cycling of soil nitrogen and carbon in an agroecosystem. Grasses are generally higher in carbon and lower in nitrogen than grasses. This higher carbon to nitrogen (C:N) ratio results in slower breakdown of grass residues. Initially, soil nitrogen availability may be substantially decreased following their incorporation into the soil.

Cool Season and Warm Season Legumes

Grasses are often described by their growth cycle through a year: cool season and warm season grasses. The main growing period for cool-season grass is in spring and fall when soil temperature is 50 to 65 degrees F (10 to 18ºC), and the air temperature is 60 to 75 degrees F (16 to 24ºC).

Life Cycles of Common Grasses

The following are examples of annual and perennial grasses:

Brassicas

Brassicas used as cover crops include mustard, rapeseed, and forage radish. They are increasingly used as winter or rotational cover crops in vegetable and specialty crop production, such as potatoes and tree fruits. Rapeseed, also known as rape, grows well under the moist and cool conditions of late fall, when other kinds of plants are going dormant for winter. Rape is killed by harsh winter conditions but is grown as a winter crop in the middle and southern sections of the United States.

Buckwheat

Buckwheat is a summer annual that is easily killed by frost. It will grow better than many other cover crops on low-fertility soils. It also grows rapidly and completes its life cycle quickly, taking around six weeks from planting into a warm soil until the early flowering stage.

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