Microbiological Population of the Soil 33 



to a gi\cMi ([iiantity of soil, the moisture adjusted to optimum, and 

 the soil kept in the laboratory for a given time. The chemical 

 changes that took place in the soil as a result of such treatment were 

 analyzed b\" simple teehniciues. These procedures were often called 

 "beaker" or "timibler" methods after the container in which the soil 

 was kept. The cliemieal ehanges brought about in the added sub- 

 stance by the soil microbiological population also gave an incom- 

 plete idea of the importance of certain groups of organisms; others 

 may not ]ia\e been recognized at all. 



The introduction of microscopic methods for the examination of 

 soil microorganisms proved to be a valuable tool in supplying infor- 

 mation concerning the distribution of microorganisms in the soil 

 and the nature and abundance of special groups of organisms. These 

 methods, usually based upon the staining and microscopic observa- 

 tion of a small amount of soil, were likewise subject to certain limi- 

 tations and did not necessarily convey a true picture of the popula- 

 tion, especially from the point of view of its importance in soil 

 processes. The greatest difficulty thereby encountered was that some 

 of these methods could not be developed as routine laboratory pro- 

 cedures. They could be used only by specialists, and the results thus 

 reported varied so considerably, especially when different modifica- 

 tions of the methods were employed, that the information obtained 

 proved to be of but limited value. This is brought out by the fact 

 that, after long experimentation, the conclusion was reached that 

 the direct microscopic methods can, at best, supplement but not 

 replace the plate and culture methods for evaluating the abundance 

 and activities of the soil population. Among the microscopic meth- 

 ods, the contact slide proved to be most valuable, since it could 

 present a picture of the microbiological state of the soil under given 

 conditions of treatment. 



Various discrepancies have been encountered in an attempt to 

 correlate the presence of certain groups of microorganisms with the 

 part that they were believed to play in certain soil processes. When 

 such organisms were isolated from the soil and grown in pure cul- 

 ture, it was found that the chemical reactions brought about by 

 them under those conditions did not necessarily correspond to simi- 

 lar reactions taking place in the soil itself. The importance of the 

 actual or potential actixities of such organisms in the soil under 

 various conditions of cultivation was, therefore, questioned. 



Studies made of the effect of various soil treatments and changes 

 in environment demonstrated that the microbiological population in 



