268 AGEICULTUBAL AND INDUSTEIAL BACTEEIOLOGY 



phosphates. Most soils are somewhat higher in sulphates 

 than in phosphates and consequently additions of sulphates 

 are not as frequently required for soils as are the phos- 

 phates. In some cases, however, they are deficient. 



Organisms Reducing Sulphates to Sulphides. Under 

 anaerobic conditions, in the presence of organic matter, 

 many species of microorganisms are known which will 

 change sulphates to sulphides, in many cases releasing free 

 hydrogen sulphide. This power particularly is possessed 

 by many of the soil bacteria and by forms belonging to the 

 genus Vibrio, particularly the species Vibrio desulphuri- 

 cans. 



Bacteria of this type may be of significance in soils which 

 are water-logged and which contain an abundance of sul- 

 phates and organic matter. Hydrogen sulphide is quite 

 poisonous to many plant roots. The reduction of the 

 sulphates to sulphides, therefore, will make conditions par- 

 ticularly unfavorable for the development of many kinds of 

 plants. Processes such as the reduction of sulphates to sul- 

 phides and of nitrates to nitrites are among the agencies 

 which are important in causing plants such as corn to turn 

 yellow when the soil in which they are growing becomes 

 water-logged. These organisms are also important in 

 sewage disposal. When organic matter, such as sewage, is 

 mixed with a city water supply high in sulphates, such as 

 magnesium sulphate, the reduction to sulphides and the 

 formation of hydrogen sulphide may be so great as to 

 create a nuisance. Special methods of sewage disposal have 

 been adopted in some cases in which the sewage is more 

 thoroughly aerated than is necessary with sewage which is 

 lower in sulphates. 



THE CYCLE OF IRON 



Iron in nature occurs in organic form and in the inor- 

 ganic form in the oxidized and reduced condition. Micro- 



