36 PRINCIPLES OP SOIL MICROBIOLOGY 



In the case of arid soils, the microorganisms are found to penetrate 

 much deeper into the subsoil layers, the activities of the organisms con- 

 tinuing in some cases undiminished through six feet of soil. The num- 

 bers of bacteria may even be greater at a depth of 24 inches than at 6 

 inches; 81 a wind-blown arid soil of Arizona, of a pH 8.6 to 9.0 and with an 

 average annual rainfall of 13 inches was found to contain 401 ,000 organisms 

 at a depth of 6 inches, and 1,898,500 at a depth of 12 inches and 916,500 

 at a depth of 24 inches. Fifty-two per cent of the organisms developing 

 on the plate were actinomyces. Better aeration and presence of organic 

 matter are no doubt largely responsible for this greater distribution of 

 microorganisms with depth of soil in arid regions. The favorable in- 

 fluence of irrigation upon bacterial numbers is marked not only at the 

 surface but also at lower depths, 82 as shown in table 3. Further informa- 

 tion on the influence of depth upon microorganisms and their activities 

 is given elsewhere (p. 48). 



TABLE 3 



Distribution of soil bacteria in different depths of soil in arid regions 



Numbers of specific physiological groups of bacteria in the soil. In 

 determining the numbers of specific morphological or physiological 

 groups of bacteria in the soil, the dilution method is largely employed, 

 as outlined above. 8384 The number obtained indicates a certain mini- 

 mum of cells of the particular organism, since often many of the cells 

 introduced into the specific medium may fail to develop. Hiltner and 

 Stormer found that a soil, which gave by the gelatin plate 1,270,000 

 bacteria, contained a much greater number of bacteria, when deter- 

 mined by the dilution method. 



81 Lipman, C. B. The distribution and activities of bacteria in soils of the 

 arid region. Univ. Cal. Publ. Agr. Sci. 1: 7-20. 1912;4:113-120. 1919. Snow, 

 L. M. A comparative study of the bacterial flora of wind-blown soil. I. Arroyo 

 bank soil. Tucson, Arizona. Soil Sci. 21: 143-161. 1926. 



82 Greaves, J. E. Agricultural bacteriology. Lea and Febiger. Phila., p. 

 164. 1922. 



83 L6hnis, 1905 (p. 13). 



84 Millard, W. A. Bacteriological tests in soil and dung. Centrbl. Bakt. II, 

 31: 502-507. 1911. 



