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. Perennial legumes, such as alfalfa, can fix several hundred pounds of nitrogen per acre per year.
Manure contains an appreciable amount of nitrogen.
Cattle manure contains about 10 to 40 pounds of nitrogen per ton. About half of this nitrogen is converted to forms available to plants during the first growing season. Lesser amounts are converted during succeeding seasons. Each ton of applied manure is equal to about 5 to 20 pounds of commercial fertilizer nitrogen. All forms are equally effective when properly applied.