ABUNDANCE OF SOIL BACTERIA 397 



After wheat had been growing for two months 

 on these soils in the greenhouse, and maintained at 

 the same moisture content, they were again sampled. 



Character of Number of bacteria 



soil per 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 those 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 larger number. 



Bacteria are found chiefly in the upper layers of 

 soil, although not at the immediate surface of the 

 ground. The layer between the first and sixth or 

 seventh inches contains, in most soils, the great bulk 

 of the bacteria present. Below that depth they de- 

 crease in numbers, and below a depth of six to eight 

 feet there are usually none. 



267. Numbers. The number of bacteria in any 

 soil will naturally vary with the conditions that favor 

 or discourage their growth. In sandy soils, forest 

 soils, desert soils, acid soils, waterlogged soils and 

 soils low in humus, the bacteria are either absent or 

 very few in numbers. In soils very rich in organic 

 matter, especially where animal manure has been 

 applied, or where a carcass has been buried, the num- 

 ber becomes very large, as many as 100,000,000 per 



