422 MICROBIOLOGY OF SOIL 



the sulphur into sulphuric acid, which acts upon the insoluble rock 

 phosphate and makes it soluble and available for higher plants. The 

 best combination found at the New Jersey Agricultural Experiment 

 Station consists of 100 parts of soil, 120 parts of sulphur and 400 parts 

 of rock phosphate, inoculated with material from an old compost. 

 The bacteria causing the oxidation of sulphur were isolated at the New 

 Jersey Agricultural Experiment Station and were found to be short, 

 non-motile, Gram-positive rods. They are obligate aerobes and are 

 able to convert sulphur into sulphuric acid. 



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 hy- 

 drogen sulphide by these bacteria may be expressed by the following 

 equations: 



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

 S 2 + 2 O 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: 



SO 2 +0+H 2 O = 



We owe much of our knowledge concerning the sulphur bacteria to 

 Winogradski. This investigator showed that in places where hydrogen 

 sulphide is generated in considerable quantities sulphur bacteria grow 

 vigorously and accumulate granules of sulphur within their cells. 

 When the cells containing sulphur granules are removed to suitable 

 media, in which no hydrogen sulphide is present, the sulphur seems 

 to be gradually oxidized and disappears and the bacteria finally die of 



