288 MICROBIOLOGY OF SOIL. 



doubtedly species whose phosphorus requirement is greater than that of 

 other species. Indeed, conditions may arise that favor the rapid assimi- 

 lation of soluble phosphates by bacteria. In that case the microorganisms 

 would act as competitors to the higher plants. Among the species 

 favorably affected by an abundant supply of phosphates Azotobacter is 

 quite prominent. Hence nitrogen-fixation is in a measure dependent 

 upon a proper supply of phosphorus compounds. 



SULPHUR. 



SULPHUR COMPOUNDS IN THE SOIL. Sulphur occurs in the soil in 

 the form of sulphates and in that of organic compounds. In ill- 

 aerated soils the reduction products of sulphates; viz., sulphites, sulphides 

 and even elementary sulphur may be present in small amounts as a transi- 

 tion stage. According to Berthelot and Andre the protein compounds 

 of the soil humus are quantitatively more important than the sulphates. 

 However, this is not true of arid and semi-arid soils in which sulphates 

 represent a larger store of combined sulphur than is contained in organic 

 substances. 



SULPHUR BACTERIA. In the decomposition of protein compounds 

 with a limited supply of air, hydrogen sulphide and mercaptans are evolved. 

 The quantities of hydrogen sulphide produced may be large enough to 

 become perceptible to the sense of smell, as happens in the putrefaction 

 of eggs. At the bottom of seas, rivers, lakes and ponds (in canals, ditches, 

 swamps, etc.) as well as in finer-grained soils the production of hydrogen 

 sulphide goes on almost uninterruptedly owing to the activities of a great 

 variety of bacteria. The hydrogen sulphide thus generated serves as a 

 source of energy to a group of organisms known as sulphur bacteria. 

 The oxidation of the hydrogen sulphide by these bacteria may be ex- 

 pressed by the following equations: 



2 H 2 S+O 2 =2H 2 O + S 2 



S 2 +2O 2 =2SO 2 



The sulphur dioxide produced is further changed into sulphuric acid 

 in the presence of oxygen and water. In its turn the acid reacts with some 

 base, usually calcium carbonate, resulting in the formation of calcium 

 sulphate. Thus: 



H 2 SO 4 + CaCO 3 = CaSO 4 + H 2 O+ CO 2 



