Sulfur in organic matter

In topsoil of temperate, humid regions 90-98% of the sulfur is present in organic forms. The exact forms of the sulfur in organic matter are not known. Some sulfur is carbon bonded (C-S fraction), mostly in amino acids and proteins. These materials are bound with organic and clay colloids, and are thereby protected from microbial decomposition. A somewhat transitory pool of organic sulfur is in the ester sulfate form (C-O-S), such as found in glucose sulfate. 

Over time soil microorganisms break down these sulfur compounds into soluble inorganic forms (mainly sulfate).

In arid and semi-arid region soils proportion of organic sulfur is lower than in humid-region soils. However, in arid and semi-arid region soils gypsum, which supplies inorganic sulfur, is often present in the subsurface horizons.