Methods of Stuchiiig the Soil Population 



47 



By use of the plate method, it was possible to demonstrate that 

 an extensive microbiologieal population is found in field and garden 

 soils, in forest soils, in peat bogs, and even in the ash of volcanoes 

 and in desert sands. 



Fig. 22. Plate preparation, slun\ing dc\ clopiiiLiit of spreading colonies of bacteria 



(from Lipman). 



The results obtained by the plate method, fully substantiated by 

 other methods, emphasize the fact that the various groups of micro- 

 organisms are largely concentrated in the surface layer of the soil. 

 This is true particularly of podzol soils, in which the surface layer 

 corresponds to the Ai horizon. In cultivated soils, the changes 

 in the numbers of microorganisms with depth of soil are more 

 gradual. Although bacteria and actinomycetes diminish with an 

 increase in depth, the proportional reduction is far greater for the 

 bacteria than for the actinomycetes; whereas at a depth of 1 inch the 

 actinomycetes make up only 9.2 per cent of the organisms develop- 

 ing on the plate, their proportion may be increased to 65.6 per cent 

 at a depth of 30 inches. 



