Legumes are popular cover crops between seasons as well as active inter-plantings in the garden. This is because they have a symbiotic relationship with rhizobium bacteria that allows them to convert atmospheric nitrogen into a form that plants can readily use. Growing legumes reduces or eliminates the need to purchase and apply expensive and potentially harmful fertilizers to crops, gardens and pastures.
Rhizobium bacteria infect specialized nodes in the roots of legumes. These nodes grow and reach into the surrounding soil, taking in atmospheric nitrogen and producing ammonium nitrogen. The legume uses the ammonium nitrogen as food, and in return provides the rhizobium bacteria all the nutrients and energy they need to survive.
Despite the fact that 80 percent of Earth's atmosphere is nitrogen, organisms can starve from nitrogen deficiency because it is not in a form that they can absorb as food: ammonia. Whether produced synthetically or naturally, converting atmospheric nitrogen into ammonia takes a great amount of energy. But whereas synthetically produced ammonia depends entirely upon non-renewable fossil fuels, bacterially converted ammonia is powered by sun energy that plants convert into food for the rhizobium.
Symbiotic bacteria are usually specific to their host plant. While there may be a small presence of the correct rhizobium bacteria already in the soil, this is not usually the case. So it is important not only to inoculate the seeds or soil with rhizobium bacteria, but also to know which type of rhizobium inoculation you need for your garden, forage or crop. After successful inoculation and plant establishment, the rhizobium bacteria can live in the soil without their host plant for a time. Invasive or chemically intensive cultural practices, however, may deplete rhizobium populations.
The use of legumes as cover crops or forage crops to build the structure and nutrition value of soil goes back to early Rome, where legumes were used as "green manure" to enrich the soil in the absence of animal manure. Thomas Jefferson instituted crop rotations with leguminous red clover to replenish depleted tobacco fields in early America. While synthetic fertilizers seemed to be a useful replacement for legumes, the lack of sustainability and soil degradation caused by these fertilizers is encouraging many growers to go back to legume crop rotation.
In Australia, food forests are being cultivated. A degraded stretch of land is cut with swales for water, then planted with a field of short-lived legumes. Legume shrubs and trees are introduced, inter-planted with food shrubs and trees of various sizes. As the longer-lived legumes grow and fix nitrogen, they are occasionally cut back. This causes them to shed their nitrogen-rich roots for the food trees, and the cuttings are dropped around the base for mulch and compost.