540 PRINCIPLES OF SOIL MICROBIOLOGY 



Oxidation of sulfur and other minerals in the soil. The cycle of sulfur 

 transformation in the soil is treated in detail elsewhere (p. 600). It 

 is sufficient to indicate here that, when elementary sulfur is added to 

 the soil, it is oxidized to a limited extent chemically and to a much 

 greater extent biologically. A large number of organisms seem to be 

 capable, in the presence of various organic substances, of oxidizing 

 small amounts of sulfur, with the formation of various compounds. 

 Certain specific groups of bacteria seem to be most active in the proc- 

 ess, since these organisms utilize the sulfur as a source of energy. 

 This is true also of sulfides; the speed of oxidation of these depends 

 on their solubility. Hydrogen sulfide and alkali sulfides are oxidized 

 very readily and rapidly; alkali earth sulfides are oxidized more slowly; 

 while the biological oxidation of iron sulfide (iron pyrites) has not 

 yet been demonstrated. 



A detailed study of the oxidation and reduction of arsenic compounds 

 by microorganisms has been made by Van Zyl. 86 



Oxidation of organic compounds in the soil. Attention has been 

 called previously to the fact that various organic compounds are formed 

 in the soil as a result of the activities of microorganisms. These may 

 become toxic to the growth of higher plants unless further oxidized. 

 Conditions favoring oxidation processes stimulate the decomposition 

 of these substances and make conditions in the soil more favorable 

 for the growth of higher plants. 



Various oxidation processes are also essential for the liberation of a 

 sufficient amount of energy for the activities of microorganisms. The 

 maximum energy is liberated by organic substances only when they 

 are completely oxidized. Oxidation of amino acids and oxidation of 

 purine bases are important soil processes, especially in respect to the 

 formation of ammonia. 87 



Iron plays an important part as a catalytic agent in the oxidation 

 of various substances. In the auto-oxidation of cysteine, an inter- 

 mediate cysteine-iron complex is formed which is auto-oxidizable; this 

 process is inhibited by HCN. 88 



86 Van Zyl, J. P. Union S. Africa Dept. Agr. Repts. Div. Vet. Ed. and Res. 

 9-10: 727-808. 1923 (Exp. Sta. Reed., 82: 178). 



87 Dakin, 1922 (p. 483). 



88 Warburg, O., and Sakuma, S. Pflug. arch. ges. Physiol., 200: 203. 1923; 

 Sakuma, S. Uber die sogenannte Autooxydation dea Cysteins. Biochem. 

 Ztschr., 142: 6S-78. 1923; Harrison, D. G. The catalytic action of traces of 

 iron on the oxidation of cysteine and glutathione. Biochem. Jour., 18: 1009- 

 1022. 1924. 



