The mineralization of N always is accompanied by the formation of new organic materials, either as new microbial cells or as by-products of microbial activities. When soil temperature and moisture are favorable, the availability of organic materials is the primary factor that determines how much microbial activity occurs. The nitrogen-richness of the residue, however, is a major factor that determines how decomposition of the organic materials influences N availability for the next crop. Immobilization refers to incorporation of nitrate and exchangeable ammonium into organic forms that are not available to plants. This occurs as microorganisms build new cells during growth supported by the decomposition of organic materials.
Immobilization is the opposite of mineralization, and both occur simultaneously in soils.
This release occurs because the organic material supplies more N than needed by micro-organisms during the growth of new cells. This initial release of N causes nitrogen-rich organic materials to behave like commercially prepared fertilizers. One major difference, however, is that only part of the N is released during the first growing season. Decomposition of nitrogen-poor organic materials usually has the opposite effect on N availability. During the initial stages of decomposition, soil microorganisms must use nitrate or ammonium from the soil to build new cells.