224 MODIFICATION OF THE SOIL POPULATION 



influence a variety of soil organisms, the kinds of organisms affected 

 and the degree of the effects being determined by the nature of 

 the fertihzers. 



Influence of Plant Growth. — The type of plant grown in a 

 soil will also considerably influence the microbial population, as 

 shown in detail in the previous chapter. The influence of the 

 plant is due to several factors. Considerable material becomes 

 added to soils from plant roots, either as excretions or as parts of 

 the roots themselves. The influence of plants on soil moisture, 

 salt concentration, and physical conditions is largely responsible 

 for the changes in the population. Subsequent to the death of the 

 plant, appreciable amounts of organic residues may find their 

 way into the soil and serve as food for the microbial inhabitants. 

 Influence of Partial Sterilization of Soil upon the Soil 

 Population and its Activities. — Soils may be treated in numer- 

 ous ways for the control or elimination of certain insects and soil- 

 harbored plant diseases; they may be steamed, treated with vola- 

 tile antiseptics, as toluene and carbon bisulfide, and also with some 

 non-volatile substances. These treatments are referred to as 

 partial sterilization, since they destroy certain members of the 

 soil population without killing all of the organisms. Some species 

 or even groups of organisms might be entirely eliminated during 

 the process, while most of the other microbes become affected to 

 some extent at least. As a result of studies of the diverse effects 

 of partial sterilization treatments upon the soil and its population, 

 it has been shown that plants respond to these treatments in ways 

 which cannot be explained completely by merely assuming the 

 ehmination of plant parasites. A brief survey of some of the fac- 

 tors involved will aid in understanding the phenomenon. 



Partial sterilization of soil not only eliminates certain organisms 

 but greatly modifies the soil as regards its capacity to support plant 

 growth. The soil acts much as if it had been fertilized with nitro- 

 genous substances. The extent of the response of plant growth is 

 apparent from Fig. 79. The effects vary greatly with different 

 soils as well as with different treatments. Fertile soils rich in a 

 great variety of organic constituents appear to be most suscep- 

 tible to changes. 



The changes in the soil population are numerous and pro- 

 nounced; these changes are reflected in the transformations with 

 which the microbes are associated. There is at first a depression 



