SOIL ORGANISMS 403 



trient, being utilized by such crops as alfalfa, turnips, and 

 cabbage in much larger amounts than is phosphorus. Al- 

 though sulfur probably seldom becomes a limiting factor in 

 crop production (see par. 264), where rational methods of 

 soil management are practiced, its transformations in the soil 

 are of great importance. 



Sulfur is absorbed by the plant as the sulfate ion and con- 

 sequently all forms of soil sulfur must be changed to the sul- 

 fate before the plant may benefit to any degree. This trans- 

 formation of sulfur, both organic and inorganic, to the sul- 

 fate form, insofar as it is biological, has been termed by 

 Lipman, 1 sulfofication. The reactions involved after hydro- 

 gen sulfide or free sulfur are formed may be written as fol- 

 lows: 



2H 2 S + 30 2 as 2H 2 S0 3 



S + H 2 + 2 = H 2 S0 3 



H 2 S0 3 + CaH 2 (C0 3 ) 2 = CaS0 3 + 2H 2 + 2C0 2 



2CaS0 3 + 2 = 2CaS0 4 



While the oxidation reactions cited above are not entirely 

 biological, purely chemical changes occurring to a slight de- 

 gree, the decay processes preceding them are due wholly to 

 bacter logical and allied influences. The organisms involved 

 in sulfofication are probably many, including the higher forms 

 of fungi as well as bacteria. The organisms that function in 

 the carbon cycle no doubt are active in the sulfur transfor- 

 mations as well. 



The possible sources of the sulfur which is found in the 

 sulfur cycle are four: (1) plant and animals tissue, (2) fer- 

 tilizers, (3) rain-water, and (4) the inorganic sulfur of the 

 soil itself. The organic source is probably the most impor- 

 tant means by which the sulfur supply of the soil is aug- 

 mented in practice. The addition of farm manure and the 

 turning under of crop residues and green-manures will do 



1 Lipman, G. J., Suggestions Concerning the Terminology of Soil 

 Bacteria; Bot. Gaz., Vol. 51, pp. 454-460, 1911. 



