602 PRINCIPLES OF SOIL MICROBIOLOGY 



The sulfur does not remain long in the form in which it is introduced. 

 It undergoes in the soil a series of reactions involving the activities of a 

 number of microorganisms, the nature of which depends on the nature 

 of the compound containing the sulfur. If it is introduced into the 

 soil in the form of an organic substance, the organic matter is first 

 decomposed by various heterotrophic bacteria, fungi and actinomyces 

 and the sulfur bearing fraction is liberated. This is either assimilated 

 by microorganisms or it is decomposed by various bacteria, and the 

 sulfur is finally liberated as H 2 S. The part of the sulfur utilized by 

 the microorganisms for the synthesis of microbial protein has to be 

 decomposed again before the sulfur is liberated and made available 

 for higher plants. The H 2 S is oxidized by autotrophic and facultative 

 autotrophic bacteria to sulfur and then to sulfuric acid, which combines 

 with the soil bases to give sulfates. The latter are either assimilated 

 by higher plants or microorganisms and again transformed into pro- 

 teins or reduced to H 2 S by specific reducing bacteria under anaerobic 

 conditions. The H 2 S is of course again oxidized. 



When unoxidized or partially oxidized inorganic forms of sulfur, 

 such as thiosulfates, sulfides, including hydrogen sulfide, and elementary 

 sulfur, are added to the soil they are at first oxidized, if the soil aeration 

 and moisture are favorable. These substances may originate from 

 the decomposition of organic matter in the soil, in sulfur springs, in 

 river and sea waters, from the reduction of sulfates, from volcanic 

 eruptions, from the burning of coal or sulfide ore smelters. The oxi- 

 dation of sulfur may be both chemical and biological in nature result- 

 ing in the formation of sulfates. When soil conditions favor anaer- 

 obiosis, as in soils saturated with water, or when sufficient aeration is 

 lacking, the sulfates as well as the elementary sulfur may be reduced 

 to sulfides. Sulfates may be leached out from the soil into lakes 

 and seas, where they are reduced by other microorganisms to hydrogen 

 sulfide. 



The transformation of sulfur in the soil may thus be summarized 

 under four headings: (1) oxidation, (2) reduction, (3) synthesis (into 

 proteins), and (4) decomposition of proteins and protein derivatives 

 containing sulfur. 



The nature of oxidation of sulfur and its compounds in the soil. The 

 question of the chemical or biological oxidation of sulfur has been 

 the subject of various investigations. There is no doubt that some 

 quantities of sulfur, particularly in a finely divided or colloidal state, 

 as well as small amounts of H 2 S and sulfides, may be oxidized by chemi- 



