Small quantities of soil N are provided by residue from plants that do not fix atmospheric N. Organic matter in Mississippi soils typically ranges from 0.5 to 2.0 percent by weight of the upper six inches. 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 specialized handling and stringent safety precautions. Because of theses requirements for 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 species, their 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. Over 90 percent of soil N is associated with soil organic matter. 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 the electrostatic attraction protects it from leaching.