SELMAN A. WAKSMAN 319 



In the decomposition of proteins, the sulphur is liberated first as cystin and thio- 

 compounds, then as hydrogen sulphide. The latter can be used as a source of energy 

 by certain specific bacteria which oxidize the sulphide first to elementary sulphur, 

 then to sulphuric acid. Elementary sulphur is also oxidized by certain specific bacteria 

 to sulphuric acid. Thiosulphates are oxidized to sulphates and thionates. Some bac- 

 teria are capable of reducing the sulphates to hydrogen sulphide, using the sulphate 

 as a source of oxygen. In the metabolism of heterotrophic organisms only a small 

 amount of sulphur is required. This is usually assimilated in the form of sulphate or 

 organic sulphur. 



When insoluble phosphates are introduced into the soil they are made readily 

 soluble by interaction with carbon dioxide as well as with the various organic and 

 inorganic acids produced by the soil micro-organisms. When a mixture of sulphur 

 and insoluble phosphate (the tri-calcium form) is added to soil or to a culture solution 

 and inoculated with the proper bacteria, the sulphur will at first be oxidized to sul- 

 phuric acid. This acid interacts immediately with the tri-calcium phosphate, changing 

 it to di-calcium, then to mono-calcium phosphate and finally to phosphoric acid. Phos- 

 phorus is a very important element in the nutrition of micro-organisms, and con- 

 siderable quantities of it are utilized in the synthesis of the microbial protoplasm. It 

 has been found that there is a definite ratio between the nitrogen and phosphorus 

 content of bacterial cells such as Azotohacter, This led to the introduction of a meth- 

 od for determining the amount of available phosphoric acid in a given soil, when a 

 certain amount of soil is used as the exclusive source of phosphorus in a medium in 

 which nitrogen fixation takes place. From the amount of nitrogen fixed, the amount 

 of available phosphate is calculated. 



Potassium compounds are changed in the soil in an entirely diflferent manner from 

 the phosphorus compounds. This, as well as the fact that potassium is not as indis- 

 pensable to all micro-organisms as phosphorus, makes the role of micro-organisms in 

 the transformation of potassium in soil somewhat different. The small amounts of 

 potassium added to the soil in organic combinations are liberated when the organic 

 matter is decomposed, some of it being again assimilated. Small amounts of potas- 

 sium may also be liberated from the insoluble silicates when these interact with the 

 various organic and inorganic acids formed by soil micro-organisms. 



Calcium is used to a limited extent as a nutrient by soil micro-organisms. It is 

 of greater importance in the neutralization of the acids produced by the various or- 

 ganisms; the reaction of the soil can thus be kept at or near neutrality, a reaction 

 which is essential for the growth of some very important groups of soil micro-organ- 

 isms. The formation of nitrates from ammonium salts would soon come to a stand- 

 still if it were not for the presence of calcium which neutralizes the acids formed. The 

 growth of some nitrogen-fixing bacteria takes place within a narrow range of reaction 

 optimum; this can be attained by the application of calcium in the form of its car- 

 bonate or oxide. Azotobacter, for example, will not develop in soil or in solution at 

 a pH of less than 6.0. The formation of acids by bacteria, notably nitrous, nitric, 

 and sulphuric, and their leaching out from the soil, in the absence of growing plants, 

 also leads to considerable losses of calcium from the soil. 



Magnesium plays a role similar to calcium although it is likely to be more toxic 



