424 MICROBIOLOGY OF SOIL 



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 accomplished 

 in artificial media by specific microorganisms. Two species isolated by 

 these investigators have been named Sp. desulphur leans and Msp. 

 cestuarii. 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 The oxygen withdrawn 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 + CO 2 + H 2 O = CaCO 3 + H 2 S 



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. 

 Smaller amounts occur as nitrate, carbonate and in organic compounds. 

 The portion present as silicates is often very large in clay-loam soils, 

 amounting not infrequently to 22,679 kg. to 34,019 kg. (50,000 to 

 75,000 pounds) per acre-foot. Unfortunately for the farmer, the grow- 

 ing crops fail, in many cases, to secure sufficient quantities of available 

 potash for their rapid development, notwithstanding these enormous 

 stores of potassium compounds. However, when sufficient quantities 

 of readily fermentable organic matter are present and the generation of 

 carbon dioxide is rapid the silicates weather sufficiently fast to meet 

 the demands of maximum harvests. The part played by carbon dioxide 



