In contrast to the relatively rapid decomposition of fresh organic residues in soil, degradation and release of sulfur from the large humus fraction are limited and slow. Mineralization of sulfur depends on the sulfur content of the decomposing material in much the same way that nitrogen mineralization depends on the nitrogen content.
Soils generally mineralize more sulfur in the presence of growing plants than in their absence.
Numerous transformations of sulfur in soil occur as it converts back and forth from inorganic to organic forms due to the presence of microbes. Immobilized sulfur is bound in soil humus, microbial cells, and in by-products of microbial synthesis. Sulfur reactions in the soil are very similar to those of nitrogen, which are dominated by the organic or microbial fraction in the soil. Sulfur is needed by plants in about the same quantities as phosphorus. Crops grown on coarse-textured soils are generally more susceptible to sulfur deficiency, because these soils often have low organic matter contents and are subject to leaching.