3S8 MICROBIOLOGY OF SOIL 



in the soil, so phosphorus itself affects the growth and activities of 

 bacteria. As one of the essential constituents of living cells it reacts 

 on the growth of microorganisms and influences species relationships. 

 There are undoubtedly species whose phosphorus requirement is greater 

 than that of other species. Indeed, conditions may arise that favor the 

 rapid assimilation 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 meas- 

 ure dependent upon a proper supply of phosphorus compounds. 



Sulphur 



Sulphur Compounds in ihe 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, sul- , 

 phides and even elementary sulphur may be present in small amounts 

 as a transition 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 sxilphide 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 hy- 

 drogen sulphide by these bacteria may be expressed by the following 

 equations: 



2H2S -1- O2 = 2H2O + S2 

 S2 + 2O2 = 2SO2 



The sulphur dioxide produced is further changed into sulphuric acid 

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



