360 MICROBIOLOGY OF SOIL 



probably there is also a small production of sulphates in soils due to 

 chemical action. 



It has been observed that soils differentiated by various treatments, 

 vary widely in sulphofying power, the presence of organic matter being 

 responsible for an increase up to a certain point. Aeration and mois- 

 ture must be optimum for favorable sulphofication while the addition 

 of carbohydrates to soils depresses the process. 



Sulphate Reduction. — The fact that sulphates may be reduced to 

 sulphides in the presence of organic matter has been known for many 

 years. In compost heaps, and at the bo.ttom of seas, lakes and rivers, 

 the reduction of calcium sulphate is of common occurrence. Similarly, 

 ferrous sulphate may be reduced in water-logged soils and in swamps 

 and may give rise to deposits of bog iron. But while sulphate reduction 

 is of common occurrence in certain localities, it has been shown by Bey- 

 erinck and also by van Delden, that the reduction can be accompUshed 

 in artificial media by specific microorganisms. Two species isolated by 

 these investigators have been named Sp. desulpTiuricans and Msp. 

 (Bstuarii. When grown under anaerobic conditions in culture media 

 supplied with combined nitrogen and organic nutrients these organisms 

 were found capable of reducing sulphates. The oxygen withdrawn 

 from the sulphates was used for the oxidation of organic matter in a 

 manner analogous to that in nitrate reduction where the oxygen is 

 derived from the nitrates. Apart from the two organisms that cause 

 the specific reactions just noted, there are many common soil bacteria 

 that may be responsible for sulphate reduction in a less direct manner. 

 Nadson has observed that when the supply of oxygen is limited calcium 

 sulphate may be reduced to sulphide by B. mycoides and by B. (Proteus) 

 vulgaris. The calcium sulphide according to him may react with car- 

 bon dioxide and water, giving rise to the formation of hydrogen sul- 

 phide. Thus: 



CaS + CO2 + H2O = CaCOs + H2S 



The hydrogen sulphide derived from sulphates or from proteins 

 becomes a source of energy to the sulphur bacteria as already noted in 

 the preceding pages. 



Potassium 



The Transformation of Potassium Compounds in the Soil. — 

 Potassium occurs in the soil largely in the form of silicate minerals. 



