48 Microbiological Population of the Soil 



Among the \aiious factors which influence the abundance of 

 microorganisms in the soil, the most important are organic matter, 

 reaction, moisture, temperature, aeration, and nature of crop grown. 

 The distribution of microorganisms in the soil is, therefore, controlled 

 by numerous ecological factors, comprising climatic or atmospheric, 

 edaphic or soil, and biotic or living. Although no important relation 

 has been found between the numbers and kinds of microorganisms 

 and the climatic conditions, the edaphic and biotic factors are of 

 great significance. The amount of organic matter in the soil influ- 



FiG. 23. Colonies of fungi and bacteria, as determined by plate method (from 



Lohnis and Fred) 



ences markedly the numbers of all groups of microorganisms, whereas 

 the reaction governs largely the ratio of the fungi to the bacteria 

 and actinomycetes. According to Feher, the total number of bac- 

 teria in the soil diminishes as one proceeds northward; however, the 

 proportion of fungi to bacteria increases. This change was found 

 to be correlated with a reduction in pH value and a change in tem- 

 peratiue. There is also a decrease in numbers of bacteria with in- 

 creasing altitude. 



The effect of moisture is well illustrated in Table 6. With an 

 increase in moisture from a fairly dry state to 80 per cent saturation, 

 there is an increase in the number of bacteria; at the saturation point, 

 there is again a decrease. 



Among the other soil treatments that modify greatly the micro- 

 biological population of the soil, additions of organic matter are of 

 greatest importance. Addition of manure favors bacteria and actino- 

 mycetes. Addition of acid fertilizer, such as ammonium sulfate, 

 favors fungi and is detrimental to bacteria and actinomycetes; lime 

 has the opposite effect. 



