CHAPTER XXVII 



Microbiological Analysis of Soil as an Index of Soil Fertility 



Soil fertility and microbiological activities. The measure of soil fer- 

 tility is the crop itself. This is influenced by the physical and chemical 

 conditions of the soil, such as texture, moisture content, aeration and 

 presence of nutrient elements essential for plant growth. The avail- 

 ability of these nutrients is variously affected by the activities of the 

 soil microorganisms. By modifying the soil, as by manuring, liming, 

 cultivation, etc., the activities of the microorganisms are modified, 

 which is followed by a modification of the availability of plant nutrients. 

 In addition to the beneficial microorganisms, the soil harbors various 

 plant parasites, which are greatly influenced by the treatment of the 

 soil. 



The very occurrence and abundance of various specific organisms 

 can be modified by soil treatment. By keeping the soil well aerated 

 and properly limed, in the presence of sufficient available energy, we 

 stimulate the development of Azotobacter, nitrifying bacteria, and 

 most strains of Bad. radicicola. When a soil is more acid than pH 6.0 

 and when it is not properly aerated, due to excess of moisture or com- 

 pactness, Azotobacter is inactive, but the Bac. amylobactcr predominates 

 and may take an active part in the process of nitrogen fixation in the soil. 

 By adding an abundance of organic matter, low in nitrogen, to the soil, as 

 well as by adding fertilizers which tend to make the soil reaction acid, 

 the development of fungi is favored, over and above that of bacteria 

 and actinomyces. Fungi rapidly decompose the organic matter, 

 assimilating the available nitrogen compounds in the soil for the syn- 

 thesis of their mycelium; this nitrogen is liberated again only when 

 the fungi are decomposed by bacteria. Nitrification and denitrifica- 

 tion, decomposition of proteins with the liberation of ammonia, or 

 utilization of the latter by microorganisms with the building up of 

 proteins, reduction of the supply of available nitrates in the soil, sulfate 

 reduction or oxidation of sulfur to sulfates, are among the phenomena 

 which can thus be controlled by soil management. 



By heating the soil or by treating it with various disinfectants, the 



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