TRANSFORMATION OF SULFUR IN SOIL BY MICROBES 167 



more favorable moisture conditions and the soluble salts fail to 

 accumulate in appreciable amounts. What amounts are not 

 fairly rapidly absorbed by growing plants are soon removed in the 

 percolating waters. The various acids which are so important in 

 these weathering processes are not removed from the soil in the 

 form of acids but are combined with various bases which arise 

 from the compounds in the soil. It is the acceleration of the re- 

 moval of basic substances in humid regions which so quickly causes 

 the development of acidity and impoverishment in plant nutrients. 



Mineral Assimilation by Microorganisms. — Minerals are 

 of direct importance to the life of the soil organisms in that they 

 enter the composition of the cells. Some of the mineral elements 

 enter into organic compounds of the cell, while others play impor- 

 tant parts as physiological agents in the cell fluids. Although 

 required in smaller amounts than nitrogen, these minerals are 

 nevertheless quite essential in certain amounts to the development 

 of the living cells. In the decomposition processes carried out by 

 microorganisms, much of the mineral content of the substances 

 which undergo decomposition (such as plant residues) becomes 

 liberated in forms available for subsequent development of higher 

 plants. The phosphorus, sulfur, potassium, calcium, magnesium, 

 and other minerals previously absorbed by the plant from the soil 

 and stored up in its tissues again become liberated and made avail- 

 able for utiHzation by other plants. On the other hand, a part of 

 these minerals becomes locked up in the microbial cell substance 

 which has been synthesized during the decomposition process. 

 This involves only a fraction of the minerals liberated in the 

 decomposition of the plant residues, and represents the nutritional 

 requirements of the microbial cells. The amounts liberated and 

 not assimilated by the cells represent excess over these require- 

 ments and may be considered as waste products of the cells. 



A detailed analysis is given here of the transformation of four 

 minerals in the soil, namely, sulfur, phosphorus, iron, and potas- 

 sium. From what has been said previously, one may conclude 

 that many of the other minerals are also subject to various changes 

 in the soil as a result of the activities of microorganisms, and that 

 many of these minerals play important roles in the growth of 

 microbes and higher plants even if required only in mere traces. 



Transformation of Sulfur in Soil by Microbes. — Sulfur 

 exists in the soil and is added to it constantly in plant and animal 



