DISTRIBUTION OF MICROBES IN SOIL 



47 



isms found in different soils varies considerably, depending upon 

 the composition of the soil and the environmental conditions. 

 The physical characteristics of the soil (whether it is sand, loam 

 or clay, porous or compact, well drained or poorly drained), the 

 reaction, abundance and nature of organic matter, the amount 

 of rainfall, average temperature, period of the year, nature of the 

 vegetation, type of cultivation, and numerous other factors — all 

 influence the nature, abundance, and activities of microorganisms 

 in the soil. Largely as a result of the influences of these factors, 



2.1 35 



1.8 30 



= 1.5 25 - 



§ 1.2 = 20 - 



h 0.9 § 15 



■G 0.6 10 - 



0.3 5 - 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 



Fig. 37. — Abundance of bacteria and carbon dioxide in soils at different seasons 

 of the year (after Russell). 



marked differences are found in the numbers of different groups of 

 microorganisms during different seasons of the year (Fig. 37). 

 Differences may be found even within a few days or a few hours. 

 It is important to keep in mind the fact that, since the physical 

 and chemical soil conditions even at the same depth of soil are not 

 homogeneous, the distribution of microorganisms throughout the 

 soil as a whole is not uniform. There are pronounced differences 

 in the abundance of microorganisms in different samples of soil 

 from an apparently uniform field. Twice as many organisms may 

 be found in one gram of soil taken at one spot of the field than in 



