Nitrogen Fixation

Atmospheric nitrogen makes up 78% of the air but is unusable by plants in its gaseous form because the N≡N triple bond is extremely stable. Rhizobium bacteria produce the nitrogenase enzyme that breaks this bond and converts N₂ into ammonia, which the plant assimilates into amino acids and proteins. In return, the plant supplies the bacteria with carbohydrates from photosynthesis — a textbook mutualistic symbiosis.

Typical fixation rates: soybeans 100–200 kg N/ha, clover 80–200 kg N/ha, alfalfa 100–300 kg N/ha, common bean 30–80 kg N/ha. Actual rates depend on bacterial strain, soil pH, molybdenum availability, and nitrogen already in the soil (high residual N suppresses fixation).

Crop rotations that include a legume year every 3–4 seasons can supply 30–60% of the following crop's nitrogen demand for free — worth €150–400/ha/year at 2026 fertilizer prices. Farmers sometimes boost fixation by inoculating seeds with commercial Rhizobium strains before planting, especially on fields that have not grown that legume recently. The nitrogen is released gradually as legume residue decomposes, so legumes are typically followed by heavy nitrogen feeders (corn, wheat) to capture the benefit.