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Nitrogen fixation

Peas, like many legumes, contain symbiotic bacteria called Rhizobia within root nodules of their root systems. These bacteria have the special ability to fix nitrogen from atmospheric, molecular nitrogen (N) into ammonia (NH). The chemical reaction is:

Ammonia is then converted to another form, ammonium (NH + 4), usable by (some) plants by the following reaction:

The root nodules of peas and other legumes are sources of nitrogen that they can use to make amino acids, constituents of proteins. Hence, legumes are good sources of plant protein.

When a pea plant dies in the field, for example following the harvest, all of its remaining nitrogen, incorporated into amino acids inside the remaining plant parts, is released back into the soil. In the soil, the amino acids are converted to nitrate (NO − 3), that is available to other plants, thereby serving as fertilizer for future crops.