Microbiological Population of tlio Soil 



31 



that certain groups of organisms occur in some soils and not in others, 

 as affected by \arious cultural and environmental conditions, have 

 all influenced the prexailing ideas concerning the nature and abun- 

 dance of the microbiological population of the soil and its importance 

 in the transformation of organic and inorganic materials. They have 



55 



45 



35 



25 



15 



5 - 



Organic 

 matter 



Mineral 



Air 



Water 



Fig. 18. The relati\e distribution of materials that compose a soil under con- 

 ditions fa\orable to plant growth (from Waksman and Starkey). 



modified considerably our understanding of the various processes 

 which go on in the soil and determine its fertility. 



At first, a general belief prevailed among agronomists and soil 

 chemists that it was sufficient to count the numbers of bacterial 

 colonies developing from a suspension of a given soil upon an agar 

 or gelatin plate to obtain an accurate idea not only of the relative 

 abundance of the particular organisms, but even of the composition 

 of the soil microbiological population as a whole. The introduction 

 of elective culture methods for the study of soil microbiological proc- 

 esses permitted a broader insight into the nature and activities of 

 this population. These methods had also certain serious limitations. 

 When artificial culture media are inoculated with small amounts 



