794 PRINCIPLES OF SOIL MICROBIOLOGY 



microorganisms, due to many variables involved in a study of this kind 

 and the fact that the results are very difficult of duplication; it is also 

 difficult to differentiate between the direct influence of the growing 

 plant and the influence of the plant products. 



A larger number of organisms was found 109 under clover than under 

 grain crops, one gram of clover soil containing seven to eight millions of 

 microorganisms, barley soil, five to six millions, and soil growing sugar 

 beets, one to two millions. Higher numbers of bacteria were found 

 in soil under cowpeas than in fallow land. 110 Wilson 111 placed 

 300-gram portions of soil in large test tubes; these were plugged 

 with cotton and sterilized. Some were inoculated with sterile corn 

 and some not. A few of the tubes with and without corn were 

 inoculated with a nitrate reducing organism, a few with B. radicicola, 

 and a few left uninoculated. After 25 to 75 days incubation, the 

 numbers of bacteria were determined in the various soils. About 

 three times as many bacteria were found in the planted soil than 

 in the unplanted soil, irrespective of the organism and of the 

 nitrate added to the soil. Soils adjacent to the roots of various 

 plants were found to contain, in 27 out of 32 soils, a higher bac- 

 terial content than the soil at some distance away from the roots. 112 

 A particular plant continuously grown in a soil leaves residues 

 which will occasion a change in the chemical composition of the 

 soil; these in turn influence the bacterial flora. Certain plant species 

 will favor certain types of bacteria and inhibit others and thus disturb 

 the bacterial equilibrium in the soil. The new flora thus established 

 produces a specific change in the composition of the soil which affect 

 subsequent plant growth, favoring some plant species and retarding 

 others. This is true not only in case of favorable organisms, but also of 

 plant pathogens; the continuous growth of a single plant, such as 

 wheat, flax, clover, etc., will bring about the development of fungi 

 pathogenic to this plant, making the soil "sick" for the particular 

 plant. 



A number of contributions are devoted to the subject of influence 



109 Caron, 1895 (p. 772); Stoklasa and Ernest, 1905 (p. 34). 



110 Lechair, C. A. The influence of the growth of cowpeas upon some phys- 

 ical, chemical and biological properties of soil. Jour. Agr. Res., 5: 439-448. 

 1916. 



111 Wilson, J. K., and Lyon, T. L. The growth of certain microorganisms in 

 planted and in unplanted soil. Cornell Univ. Agr. Exp. Sta. Mem. 103, 1926. 



112 Hoffmann, C. A contribution to the subject of the factors concerned in 

 soil productivity. Kansas Univ. Science Bui. 9: 81-99. 1914. 



