Legume - Nitrogen-fixing Ability

Nitrogen-fixing Ability

Many legumes (alfalfa, clover, peas, beans, lentils, soybeans, peanuts and others) contain symbiotic bacteria called Rhizobia within root nodules of their root systems. (Plants belonging to the genus Styphnolobium is one exception to this rule). These bacteria have the special ability of fixing nitrogen from atmospheric, molecular nitrogen (N₂) into ammonia (NH₃). The chemical reaction is:

N₂ + 8 H+ + 8 e− → 2 NH₃ + H₂

Ammonia is then converted to another form, ammonium (NH₄+), usable by (some) plants by the following reaction:

NH₃ + H+ → NH₄+

This arrangement means that the root nodules are sources of nitrogen for legumes, making them relatively rich in plant proteins. All proteins contain nitrogenous amino acids. Nitrogen is therefore a necessary ingredient in the production of proteins. Hence, legumes are among the best sources of plant protein.

When a legume plant dies in the field, for example following the harvest, all of its remaining nitrogen, incorporated into amino acids inside the remaining plant parts, are released back into the soil. In the soil, the amino acids are converted to nitrate (NO₃-), making the nitrogen available to other plants, thereby serving as fertilizer for future crops.

In many traditional and organic farming practices, crop rotation involving legumes is common. By alternating between legumes and nonlegumes, sometimes planting nonlegumes two times in a row and then a legume, the field usually receives a sufficient amount of nitrogenous compounds to produce a good result, even when the crop is nonleguminous. Legumes are sometimes referred to as "green manure".