In most areas of the world, the nitrogen found in soil minerals is negligible. In the soil, nitrogen can undergo a number of transformations. Rainfall adds about 10 pounds of nitrogen to the soil per acre per year. The nitrogen oxides and ammonium that are washed to earth are formed during electrical storms, by internal combustion engines and through oxidation by sunlight. The extent of this possible damage has not been substantiated.
Crop residues decompose in the soil to form soil organic matter.
This organic matter contains about 5 percent nitrogen. An acre-foot of soil having 2 percent organic matter would contain about 3,500 pounds of nitrogen. Generally, about 1 to 3 percent of this organic nitrogen is converted per year by microorganisms to a form of nitrogen that plants can use. Legumes fix atmospheric nitrogen through their symbiotic association with Rhizobium bacteria. If plant roots are well nodulated, the legume plant does not benefit from the addition of fertilizer nitrogen.