58 



THE SOIL POPULATION AND ITS DISTRIBUTION 



plate method. All these methods give, however, only a fraction 

 of the total bacterial population of the soil. 



TABLE 8 



Numbers of Bacteria in One Gram of Soil as Determined by Plate 

 AND Dilution Methods (from Duggeli) 



Soil type 



Numbers of aerobic bacteria 

 developing on gelatin 

 plate 



Numbers of anaerobic bac- 

 teria 



Aerobic nitrogen-fixing bac- 

 teria 



Anaerobic nitrogen-fixing 

 bacteria 



Nitrifying bacteria 



Anaerobic cellulose-decom- 

 posing bacteria 



Garden 



,116,000 



622,000 



2,620 



8,200 

 2,620 



401 



Field 



10,640,000 



820,000 



6,040 



4,420 

 2,201 



420 



Deciduous 



forest 



1,422,000 



44,020 







460 

 2,000 



Lowland 



1,010,000 



80,200 







440 

 2,020 



Influence of Soil Conditions and Treatment upon the 

 Distribution of Bacteria. — The physical and chemical composi- 

 tion of the soil, as well as the environmental conditions, determine 

 the relative abundance of the microbes present at any given 

 time in the soil. In general, light sandy soils, poor in organic 

 matter, contain fewer microbes than fertile clays and loams rich 

 in organic matter. The number of microbes need not necessarily 

 indicate their potential activity, since such processes of decom- 

 position and oxidation as are of the greatest importance for plant 

 growth may be as active, if not more so, in the lighter than in the 

 heavier soils. As soon as available nutrients are added, the 

 response in microbial development becomes even greater in sandy 

 soils than in heavy clay soils; there is a more rapid increase in the 

 numbers of microbes and quicker transformation of the microbial 

 foods. 



Undrained peat and water-logged soils contain considerably 

 smaller numbers of fungi, actinomyces and aerobic bacteria than 



