ACTIVITY OF PHYSIOLOGICAL GROUPS OF SOIL MICROBES 235 



formation be used to determine the microbial condition of the soils. 

 Such studies are not as valuable as had been anticipated for the 

 reason that bacteria develop so rapidly in the nutrient solutions 

 that the microbial conditions of the soils which are used as 

 inocula are quickly obscured. 



Somewhat more suggestive results are obtained when the soils 

 themselves are used as the media in place of nutrient solutions; 

 the organic nitrogenous materials are incorporated with the various 

 soils which are under investigation. Even these studies prove to 

 be of limited value, since so many and diverse types of organisms 

 have the capacity of forming ammonia from organic substances 

 that great differences in soils do not appreciably affect the rate of 

 transformation of the added nitrogenous compounds. 



There are certain other tests which have proved more useful 

 than the ammonification tests. Principal among these are deter- 

 minations of the rate of nitrate accumulation, which may be 

 studied in various ways: in solution media containing ammoniacal 

 nitrogen and inoculated with soil material; in the soil itself with- 

 out alteration other than creating proper moisture relationships; 

 in soil to which is added some source of nitrogen such as an ammo- 

 nium salt or some organic nitrogenous material. Since nitrifica- 

 tion is brought about by a limited number of organisms whose 

 activity is affected in a similar way to many cultivated plants by 

 similar degrees of reaction, temperature, moisture, and aeration, 

 it can be readily understood why the nitrifying capacity of fertile 

 soils may be greater than the nitrifying capacity of infertile soils 

 (Fig. 84). 



A study of the decomposition of cellulose may also be of use in 

 an interpretation of certain soil characteristics. For such studies 

 the cellulose is generally added to the soil in a finely divided condi- 

 tion and the rate of its disappearance is followed. Since large 

 numbers of various organisms may be associated with the decom- 

 position, and since different members of the group develop under 

 different environmental conditions, the rate of decomposition in 

 soils is not decreased by all of the conditions which limit develop- 

 ment of higher plants; the transformation is of more value for 

 determining the readily available nitrogen in the soil. The decom- 

 position of cellulose is accompanied by the assimilation of consid- 

 erable quantities of nitrogen, which are used by the organisms that 

 are effecting the decomposition in order to satisfy their nutritional 



