222 MODIFICATION OF THE SOIL POPULATION 



isms, the direct effects of inorganic artificial fertilizing materials 

 on the abundance and activities of microbes are nevertheless quite 

 evident. This is apparent from the data presented in Tables 54 

 and 55. Of the various salts mentioned, phosphates exerted a 

 somewhat more marked influence than sulfates. Since most of 

 the inorganic compounds are present in the soil solution, the 

 greatest stimulating effects might be anticipated when the salts 

 are added to soils which contain the materials only in small 

 amounts. Under certain conditions, as in alkali soils, they may 

 occur in such abundance as to be quite toxic to microbial devel- 

 opment. 



TABLE 54 



Influence of Phosphates and Sulfates on the Number of Bacteria 

 IN Soil (from Fred and Hart) 



Bacteria per gram of soil 

 Treatment (Average during 12 days 



of treatment) 



Untreated 12,330,000 



0.5% CaS04-2H20 14,212,000 



0.5% K2HPO4 23,680,000 



1 . 0% Ca3(P04)2 12,702,000 



1.0% CaH4(P04)2 16,922,000 



The effects of inorganic materials are exerted in a variety 

 of ways: (1) Some constituents of the fertilizing materials may 

 furnish a supply of elements which are deficient in the soil and may 

 be consumed by the microorganisms in their nutrition. In the 

 presence of an abundance of undecomposed plant residues, artificial 

 fertilizers increase the activities of the various fungi and bacteria 

 to a much greater extent than would be the case where only the 

 organic materials were present. The nutritional demands of the 

 organisms for inorganic substances become multiplied propor- 

 tionally with the increases in organic food supply. This applies 

 particularly to available forms of nitrogen, phosphorus, and 

 potassium. (2) The inorganic materials may affect the physical 

 properties of the soil, making it a more or less favorable environ- 

 ment for development of the organisms. (3) The inorganic com- 

 pounds may affect the composition of the soil solution by either 

 exerting solvent effects on the insoluble soil minerals, or by pre- 

 cipitating substances from the soil solution, or by increasing the 



