INFLUENCE OF REACTION 



63 



in regions of heavy rainfall and may appear more abundantly in the 

 deep soil layers in regions of scant precipitation. The factor of 

 major importance in determining the conditions in the dry regions 

 is the lack of sufficient moisture at the surface to support a vigor- 

 ous growth of either the plant or the microbial population. In 

 compact soils the physical condition is such as to resist the deep 

 penetration of roots and, even though there were an abundance 

 of organic matter at considerable depth, the material would be 

 incompletely disintegrated on account of the lack of oxygen pen- 

 etration. 



The influence of climate is quickly impressed upon the bac- 

 terial activities in the soil. Even when soils are brought from one 

 region to another there is a rapid modification, chemically, bio- 

 logically, and physically (Table 10). The numbers of bacteria of 

 arid soils usually increase when the soils are placed under humid 

 conditions, and bacterial numbers in humid soils decrease when soils 

 are placed under arid conditions. 



TABLE 10 



Influence of Climate on Bacteria in Soils (from C. B. Lipman) 

 Numbers of bacteria per gram of soil in the surface foot 



In California 

 813,000 



Califoryiia Soil 

 In Kansas 

 5,067,000 



In Maryland 

 5,467,000 



In California 

 1,507,000 



Kansas Soil 



In Kansas 



5,167,000 



In Maryland 

 1,370,000 



In California 

 6,140,000 



Maryland Soil 



In Kansas 



880,000 



In Maryland 

 546,000 



Influence of Reaction. — Among the environmental factors 

 influencing the development of bacteria in soil, the reaction, 

 that is, the degree of acidity or alkalinity, is of particular impor- 

 tance. When a humid soil is treated with acid-forming fer- 

 tilizers, such as ammonium sulfate which on oxidation by nitri- 

 f3dng bacteria gives nitric and sulfuric acids, or with sulfur 

 which on oxidation by sulfur-oxidizing bacteria gives sulfuric acid, 



