234 MODIFICATION OF THE SOIL POPULATION 



so far proven to be worthless, at best not better than a mere 

 infusion of some stable manure. Repeated, critical, controlled 

 tests of the effects of an inoculant under a variety of conditions are 

 necessary to establish justification for its use in agricultural 

 practice. 



The Estimation of Soil Fertility by Microbiological 

 Methods. — Even as the numerous and varied soil conditions 

 determine the intensity of plant development and the kinds of 

 plants which can be economically grown, so also the environmental 

 conditions are responsible for the microbiological activity. Growth 

 of both higher plants and the less conspicuous microbes are reflec- 

 tions of the soil conditions. The inanimate portion of the soil 

 itself is the product of diverse activities operating through long 

 periods of time. Different organisms are contained in some soils 

 than in others, but the difference in quantitative distribution of 

 organisms in various soils is more pronounced. It is this differ- 

 ence in the relative abundance of various species of microorganisms 

 which gives the soils some of their characteristic properties. 



The type and abundance of plant growth are important factors 

 determining the microbiological soil conditions. Also, the nature 

 of the activities of the soil microbes is important in determining 

 the kind and degree of development of the higher plants. Since 

 the two are so closely related, it is obvious that certain correlations 

 should exist between the biological activity of a soil and its fer- 

 tility. Based upon this assumption, numerous studies have been 

 made to determine what methods could be devised to estimate 

 conveniently the productive capacity of soils. Some of these 

 methods have been developed to further determine the desira- 

 bility of adding fertilizers or Hme to soils. 



The Activity of Physiological Groups of Soil Microbes. — 

 Some of the earliest methods which were used involved attempts to 

 determine the potential activity of such groups of the soil popula- 

 tion as could produce ammonia from organic compounds of nitro- 

 gen. The following procedure is typical of these studies. A 

 nutrient solution containing peptone, urea, protein, or other nitro- 

 genous organic substance is inoculated with soil or soil infusion and 

 then the speed of ammonia formation is determined. On the 

 assumption that the soils which contained the most organisms or 

 the most active organisms decompose the nitrogenous compounds 

 most rapidly, it was suggested that the rapidity of ammonia 



