326 



Microorganisms and Soil Fertility 



3. An examination was made, by the plate or other suitable 

 method, of the abundance of certain organisms in the soil. An at- 

 tempt was then made to evaluate the fertility of the soil on the 

 basis of the numbers and biochemical potentialities of these or- 

 ganisms. 



4. Microorganisms were used for determining the concentration 

 of certain important plant nutrients in the soil. 



''^^j^ 



m 







Acttnomfcete 



-HP'" 



Actinomycete f'ungus 



Fig. 128. Effect of microorganisms on soil aggregation ( from S\\ab>- 



Without analyzing the voluminous literatiue that dealt with this 

 subject, it is sufficient to say that none of these methods yielded 

 results that could meet the test of severe criticism. They are now 

 largely abandoned, in spite of the fact that certain very definite cor- 

 relations have often been reported between the results obtained by 

 these methods and the fertility of soils. The major difficulty in- 

 volved in the use of such methods was that reactions brought about 

 by microorganisms in the soil under natural conditions are subject 

 to too many variables. These comprise not only inherent differences 

 in soil conditions, but also the effects of climate and soil manage- 

 ment. Not all these could possibly be taken into consideration in 

 the various laboratory studies. As long as a knowledge of the effects 

 of these variables was lacking, the information obtained by micro- 



