46 



THE SOIL POPULATION AND ITS DISTRIBUTION 



conditions, is followed by marked changes in the qualitative and 

 quantitative distribution of the soil microflora and microfauna. 



The microbes occur in greatest abundance in the surface layer 

 of the soil (Fig. 36). This layer varies in thickness from a few- 

 centimeters, as in the case of humid soils, to two or three meters, 

 as in the case of arid soils. The greatest abundance of individuals 

 and variety of microorganisms are found either at the very 

 surface of the soil, as in forests, meadows, and other shaded soils, 

 or just below the surface, as in the case of the open cultivated 



I Numbers in A Horizons 

 I Numbers in B Horizons 

 I Numbers in C Horizons 



25 1,000 5 



20 800 



15 o 



600 -2 3 



■5510 ^AOO ^2 



200 1 



NUMBERS OF 

 BACTERIA 



NUMBERS OF 

 ACTINOMYCES 



NUMBERS OF 

 FUNGI 



Fig. 36. — -Vertical distribution of microbes in soil 

 (after Brown and Benton). 



soils. The numbers of microorganisms diminish with depth, the 

 rapidity of the diminution varying with the soil conditions, espe- 

 cially the distribution of organic matter and degree of aeration. 

 The decrease in numbers and activities of microorganisms is very 

 rapid in the shallow, humid soils ; in the case of the deep arid and 

 semi-arid soils, the decrease is comparatively slow. 



Although there are certain differences in the abundance of 

 organisms from place to place in the soil, the general distribution 

 of microorganisms in any limited soil mass is more or less uniform. 

 However, the relative abundance of the different types of organ- 



