236 



MODIFICATION OF THE SOIL POPULATION 



requirements. If appreciable amounts of cellulose are added to 

 soils, nitrogen is generally present in insufficient quantities to per- 

 mit most rapid decomposition. Consequently, the rate of decom- 

 position is regulated by the rate of liberation of nitrogen from the 

 soil organic matter. 



Fig. 84. — Relationships between crop ^yields, abundance of bacteria, and 

 microbial activities in soils of similar origin but differing in fertility due to 

 different fertilizer treatments extended over fifteen years (after Waksman and 



Stark ey). 



Abundance op Microbial Inhabitants of Soils. — One of 

 the methods first used to obtain information concerning the bio- 

 logical condition of soils was the plate technique for determining 

 the abundance of bacteria in the soil. Various other methods have 

 since been utilized to obtain additional information concerning 



