CHAPTER IX 

 MODIFICATION OF THE SOIL POPULATION 



The Soil Population Subject to Alteration. — The total 

 numbers as well as the relative abundance of the numerous kinds 

 of microorganisms found in the soil can be modified in various 

 ways by certain soil treatments. These treatments may be 

 primarily physical, chemical, or biological. The modifications in 

 the soil population thus brought about may either favor the 

 development of certain groups of organisms in preference to 

 others, or they may bring about the entire eUmination of certain 

 representative types of the soil population, permanently or tem- 

 porarily. 



The introduction into the soil of microbes which have not been 

 there previously may be another means of modifying the soil popu- 

 lation. When these organisms find in the soil a favorable habitat, 

 they will develop readily; when, however, the soil conditions are 

 not suited to the requirements of these microbes, they will fail to 

 become estabhshed, irrespective of inoculation by natural or 

 artificial means. The mere addition of microorganisms to soils 

 provides no assurance that they will develop there, or that they 

 will bring about the specific transformations which are expected 

 of them, even though they are able to grow. The changes in the 

 nature of the population brought about by any of the soil treat- 

 ments are accompanied by corresponding changes in the chemical 

 soil processes, which directly or indirectly influence the fertility of 

 the soil, as expressed by the response in the growth of higher 

 plants. 



By reason of the fact that microorganisms do not occur in the 

 same abundance in all soils and that they are generally favored by 

 conditions that lead to best plant growth, there exists a close 

 relationship between the biological activity of soils and soil fer- 

 tility. Since some of the microbial tests are comparatively 



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