430 SOILS: PROPERTIES AND MANAGEMENT 



After wheat had been growing for two months on these 

 soils in the greenhouse, the soils being maintained at the 

 same moisture content, the samples showed the following 

 count : — 



Character of Number of bacteria 



soil to a gram of dry soil 



Good 760,000 



Poor 1,120,000 



Another reason why this relation between the number 

 of bacteria and soil productiveness does not hold is that 

 the bacteria having the same functions in relation to plant- 

 food do not always have the same physiological efficiency. 

 In other words, they do not have the same virulence, a 

 small number in some cases being able to bring about the 

 same changes that in other cases require a much greater 

 number. 



Bacteria are found chiefly in the upper layers of soil, 

 although not in large numbers at the immediate surface 

 of the ground. In humid regions the layer between the 

 first inch and the sixth or the seventh inch contains, in 

 most soils, the great bulk of bacteria present. In arid or 

 semiarid regions, bacteria are found at greater depths 

 and the densest population is located at lower levels than 

 in humid regions. This is largely because of the deeper 

 penetration of the air and the conditions that accom- 

 pany it. 



352. Numbers of bacteria. — The number of bacteria in 

 a soil will naturally vary with the conditions that favor or 

 discourage their growth. In very sandy soils, forest soils, 

 desert soils, water-logged soils, and soils low in humus, 

 the bacteria are either absent or comparatively few in 

 numbers. In soils very rich in organic matter, especially 



