Legumes such as soybeans and alfalfa change atmospheric N 2 to plant available forms by way of a process involving Rhizobium bacteria and the plant roots. However, only a very small portion of the total N is available to plants within a growing season. Organic matter is replenished by returning crop residues to the soil or introducing other organic sources such as manures or animal bedding. Anhydrous ammonia is an efficient source of fertilizer N, but because it is under high pressure, it requires special handling and stringent safety precautions. Because of these requirements of anhydrous ammonia, other N products have increased in popularity.
Animal manures are important sources of N in the environment.
The quantity of N in the manure depends on the animal's species, age, and diet, and bedding materials. Manure begins contributing to plant nutrition and soil organic matter when added to agricultural soils, but not all the N within manure is immediately available to plants. Forms of Nitrogen in Soils Nitrogen is in organic and inorganic forms in soils. Nitrogen is in compounds identifiable as part of the original organic material such as proteins, amino acids, or amino sugars, or in very complex unidentified substances in advanced stages of decomposition. It is available to plants, but it will not leach.