Methods of StiidNing the Soil Population 39 



Tari.k 4. Influence of Soil Tkeatment on Numheh of Mkuooiu; anisms 

 IN THE Soil (from Waksman) 



Nniiilicrs 



Treat nu-nt of Soil * 

 Unfertilized 

 Lime alone 

 Minerals 



Manure ami minerals 

 Minerals and ammonium sulfate 

 Minerals, annnonium sulfate, an<l Hi 

 Minerals and sodium nitrate 



* Minerals = 320 pounds KCI and (i4() pounds acid ]>liosphate per acre every year. 



and Trichodcrma, and of bacteria, especially myxobacteria and spe- 

 cies of Cytopliaga; these organisms may be followed by the growth 

 of various spore-forming bacteria and finally by actinomycetes. The 

 sequence of various groups of microorganisms can best be studied 

 in composts, since the changes in temperature and the degree of 

 decomposition of the materials in the compost greatly influence the 

 nature of the organisms present: fiist bacteria begin to multiply 

 rapidly, accompanied by the nematodes, the protozoa, and certain 

 fungi, notably the mucorales; these are followed by other filamentous 

 fungi, such as penicillia and aspergilli; finally the actinomycetes ap- 

 pear, and certain bacteria, notably the thermophilic types. 



Methods of Studying the Soil Population 



To understand the theoretical and practical significance of the 

 results, a critical e\'aluation of the methods used in determining 

 the nature and abundance of this population is of the greatest im- 

 portance. 



Some of the methods, like the agar plate method and \arious liquid 

 culture methods, supply information concerning the abundance of 

 viable or reproducible members of the population; they also indicate 

 the nature of the biochemical processes for which these organisms 

 are responsible in the soil. These methods are based upon the 

 development of the living cells in the form of colonies on the agar 

 plate or upon their growth in specific cultme media. The numbers 

 of organisms obtained by either of these methods are commonly 

 reported to range from 1 to 50 millions per gram of soil. These num- 



