168 TRANSFORMATION OF MINERAL SUBSTANCES 



residues, rain-water, and fertilizing materials, in several distinctly 

 different forms, namely, as organic sulfur compounds, inorganic 

 sulfates, sulfides, and elementary sulfur. In the organic matter 

 of the soil, the sulfur is generally bound up in forms resistant to 

 decomposition and only after long periods of time liberated in 

 appreciable amounts as sulfate. The addition of elementary sul- 

 fur or sulfate in fertilizer materials may be responsible for rela- 

 tively large amounts of sulfate in the soil solution from time to 

 time, but, generally, sulfate makes up a small portion of the total 

 sulfur content of the soil. More commonly from 80 to 90 per cent 

 of the sulfur is present in organic combination, and only 10 to 20 

 per cent exists as sulfate. 



The plant residues commonly added to the soil contain sulfur 

 largely in organic combination, varying from 0.05 to 1.0 per cent. 

 Alfalfa hay, for example, contains 0.29 per cent sulfur; turnip 

 tops, 0.9 per cent ; and wheat straw, 0.12 per cent. The sulfur in 

 the organic compounds of such plant residues becomes liberated 

 as sulfate more rapidly than from the so-called humic matter of 

 the soil, just as ammonia is formed more rapidly from fresh organic 

 matter rich in nitrogen than from the residual organic matter in 

 the humus. 



Under natural conditions, many agents, both biological and 

 non-biological, carry the element through its course of changes. 

 Microbial effects are largely oxidations and reductions of the inor- 

 ganic sulfur compounds, and formation and decomposition of 

 organic compounds containing sulfur. Since sulfur enters into the 

 composition of all living cells, all forms of life become associated 

 with its transformation. In the decomposition of the organic 

 residues by microbes, the sulfur present in organic combination 

 is changed, after several transformations, into inorganic forms such 

 as hydrogen sulfide and sulfate. 



Some microorganisms utilize sulfate to satisfy their require- 

 ments for the element. In time, the microbial cells become 

 destroyed and mineralized. The assimilation of sulfate is more 

 pronounced when the organic matter which serves as food con- 

 tains little or no sulfur. Table 42 shows the rapid disappearance 

 of sulfate when an organism {Aspergillus niger) is feeding on a 

 sugar. 



Sulfur is present in the organic matter largely in the protein 

 molecule. This molecule contains an amino acid, cystine, which 



