156 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



sulphides may give rise to such large amounts of hydrogen 

 sulphide as to constitute a nuisance. This is one of the 

 difficulties found, for example, in the operation of certain 

 types of septic tanks in sewage disposal. Where cities have 

 a water supply which is unusually high in sulphates, its 

 mixture with organic material and retention under ana- 

 erobic conditions is apt to lead to the formation of undesir- 

 able amounts of hydrogen sulphide. 



Certain bacteria already noted, particularly forms be- 

 longing to the group of so-called sulphur bacteria, are able 

 to oxidize hydrogen sulphide to free sulphur, and free sul- 

 phur to sulphuric acid or to sulphates. 



The sulphur cycle is completed by the assimilation of 

 sulphates by higher plants and the building up of the 

 sulphur into more complex organic compounds. As with 

 the nitrogen and the carbon cycles we have constant 

 changes, oxidation occurring in the presence of free atmos- 

 pheric oxygen, reduction in the absence of atmospheric oxy- 

 gen and assimilation by growing plants. Some authors, 

 particularly in soil bacteriology, have used the term sulpho- 

 fication in a sense analogous to nitrification, and desulpho- 

 fication for the reduction process, analogous to denitrifica- 

 tion. 



