Chapter 6

Cover Crops for Organic Farms

Nitrogen Fixation by Legumes

Many plants in the legume family, such as peas, beans, vetch and clover, grow in cooperation with soil-dwelling rhizobia bacteria. These bacteria live in nodules on the roots of legumes and convert atmospheric nitrogen (N2) in the soil and transform it into ammonia (NH3) that converts to ammonium (NH4+) which can be used by the plant. The nitrogen fixation (N2-fixation) process between the legume plant and rhizobia bacteria is referred to as a symbiotic (mutually beneficial) relationship. Each organism receives something from the other and gives back something in return. Rhizobia bacteria provide the legume plant with nitrogen in the form of ammonium and the legume plant provides the bacteria with carbohydrates as an energy source. The primary pathways for nitrogen transfer from the legume to the soil are through decomposition of dead legume plant material. The root system and unused leaves and stems of annual legumes die at plant maturity and are decomposed by soil microbes over time. Nitrogen contained in this plant material is released over time and is available to other plants.

Seed Inoculation

In most cases when growing a legume cover crop the seeds will need to inoculated with the appropriate strain of Rhizobia (nitrogen-fixing bacteria). Arrowleaf clover, white clover, alfalfa, vetch, annual medics, cowpea, and lespedeza all require different strains of Rhizobium bacteria. There are some instances where an effective rhizobia strain will work on several legume species. There are numerous strains of native Rhizobium bacteria that occur naturally in different soils. Some of these rhizobia strains are capable of infecting a given legume species but will vary in their efficiency to fix nitrogen. Ineffective strains will form many small nodules on the legume root but fix little or no nitrogen.  Effective rhizobia strains that fix high rates of nitrogen form fewer but larger nodules that have dark pink centers. When the legume plant matures and dies, nodules on the root system decompose and release the rhizobia into the soil. If the same legume species is planted again the following year or volunteers from seed produced the previous year, sufficient numbers of rhizobia are usually present to provide good nodulation.

Nitrogen Fixing Capacity

Nitrogen accumulations by leguminous cover crops range from 40 to 250 pounds of nitrogen per acre (See Table 7.2). The amount of nitrogen available from legumes depends on the species of legume grown, the maturity at termination, the total biomass produced, and the percentage of nitrogen in the plant tissue. The highest nitrogen benefits will be realized by letting the legume grow to full term—through most of its lifecycle—before terminating it. The nitrogen content of legume cover crops is optimized at the flowering stage; no additional nitrogen gain will occur after that point. Nitrogen production is related not only to a later maturity stage but also to higher amounts of biomass production. An over-wintering legume fixes more nitrogen than the same legume planted in the spring.

Nitrogen Availability

Most of the biologically fixed nitrogen (about 80%) is in the top growth of the plant, and becomes available faster than the nitrogen content in the roots. Legumes break down relatively quickly after being incorporated into the soil, and as they do, the nitrogen is gradually released to the soil as nitrates, an available form for the next crop in the rotation.

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