Nitrogen Fixation
The biological conversion of atmospheric nitrogen gas (N2) into plant-available forms, primarily ammonium, carried out by specialized bacteria - most practically by Rhizobium bacteria in symbiosis with legume roots.
Nitrogen fixation is the conversion of atmospheric nitrogen gas (N2) into ammonia (NH3) or related compounds that plants can use. The atmosphere is 78% nitrogen, but plants cannot access it directly. Fixed nitrogen - converted to nitrate (NO3-) or ammonium (NH4+) by microbial activity - is what plants absorb through roots.
Biological nitrogen fixation is performed by specific bacteria, most significantly for agricultural purposes by Rhizobium and related genera (Bradyrhizobium, Mesorhizobium, Sinorhizobium) in symbiosis with legume plants.
How the Symbiosis Works
Legume roots secrete chemical signals (flavonoids) that attract compatible Rhizobium bacteria from the soil. The bacteria invade root hair cells and trigger the formation of root nodules - small pink or white bumps on the roots visible to the naked eye. Inside the nodule, bacteria differentiate into bacteroids that produce the enzyme nitrogenase, which catalyzes the conversion of N2 to NH3.
The plant provides the bacteria with carbohydrates (energy) from photosynthesis. The bacteria provide the plant with fixed nitrogen. The pink color of healthy nodules comes from leghemoglobin, a protein that regulates oxygen levels in the nodule to protect the oxygen-sensitive nitrogenase enzyme.
The process is energetically expensive for the plant - roughly 16 ATP molecules per N2 molecule fixed. When soil nitrogen is already abundant, plants downregulate the symbiosis: why pay to fix nitrogen you already have? This is why adding nitrogen fertilizer to legume crops often reduces nodulation and fixation rates.
Quantities Fixed
Under good conditions with compatible Rhizobium strains and appropriate soil pH, legumes can fix substantial nitrogen:
| Crop | Nitrogen fixed (lbs/acre/season) |
|---|---|
| Hairy vetch | 100-200 |
| Crimson clover | 80-150 |
| Field peas | 50-100 |
| Garden beans | 20-50 |
| Soybeans | 100-200 |
| Garden peas | 30-80 |
For home garden scale (1,000 sq ft = 0.023 acres), hairy vetch might fix 2-5 lbs of nitrogen per season. A heavy-feeding crop like corn needs roughly 4 lbs of nitrogen per 1,000 sq ft for a full season. A legume cover crop preceding corn can substantially reduce or eliminate supplemental nitrogen needs.
Inoculants
Rhizobium strains are host-specific: the bacteria that nodulate soybeans are different from those that nodulate garden beans or clovers. Soil that hasn’t grown the same legume species recently may lack adequate populations of the appropriate strain. Inoculant products - dry powder or peat-based liquid containing the appropriate strain - are applied to seed before planting to ensure nodulation.
Inoculants are inexpensive ($5-10 for a season’s worth of beans or peas), have short shelf life (refrigerate and use within the date), and are particularly valuable for legumes being grown in a new garden or in soil where that crop hasn’t grown in several years.
pH Requirement
Rhizobium activity drops sharply below pH 6.0. Acid soils limit nitrogen fixation even when the bacteria are present and the host legume is growing. Liming to pH 6.0-7.0 before establishing a legume cover crop or production planting is important to achieve full nitrogen fixation benefit.
Free-Living Fixers
Nitrogen fixation also occurs in non-symbiotic bacteria (Azotobacter, Azospirillum, cyanobacteria) living freely in soil or in association with non-legume roots. These organisms fix small amounts of nitrogen - typically 5-20 lbs/acre/year - compared to legume symbiosis. No-till practices and increased soil organic matter support higher populations of free-living fixers.