34 PRINCIPLES OF SOIL MICROBIOLOGY 



thaw are important factors, in determining the extent of the breaking 

 up process. The work of Lochhead 71 also tends to refute the idea that 

 there are in the soil two groups of bacteria, winter and summer forms. 

 Although the numbers of bacteria in frozen soil are high, there is no 

 phenomenal increase, as a result of freezing, while thawing of the soil 

 brought about a great increase in numbers. A typical winter flora is 

 absent and the bacteria are to be regarded as cold-enduring rather than 

 psychrophilic in the true sense. There was also no indication of a rise 

 of organisms from unfrozen levels to the frozen surface layers. 



Distribution of bacteria at various soil depths. In humid soil, most of 

 the bacteria are concentrated in the upper 2 feet of soil and the highest 

 numbers are found just 1 to 3 inches below the surface. The germicidal 

 effect of the rays of the sun and the rapid evaporation of the moisture 

 tend to diminish the numbers right at the surface. Much fewer bac- 

 teria are present in the subsoil; in sandy soils, due to better aeration, 

 the numbers do not diminish as rapidly. Since most of the earlier work 

 was limited to the use of the plate method, the results are of necessity 

 too low. 



Proskauer 72 was the first to point out the fact that the numbers of 

 bacteria in humid soils decrease with depth: at a depth of about one 

 meter the soil was found to be almost free from bacteria. Frankel 73 

 observed a decrease of bacterial numbers with depth of soil, from 90,000 

 to 300,000 at the surface, to 100 to 700 at a depth of 2.5 meters. The 

 change was not gradual, but sudden and irregular. The bacteria were 

 found to go deeper in cultivated than in uncultivated soils, but no in- 

 fluence of the crop upon the number of organisms could be demonstrated. 

 A decrease from 2,564,000 bacteria at the surface to none at a depth of 

 six feet was observed 74 in a stony soil, and from 524,500 at the surface 

 to 5,800 at a depth of 1.5 meters in a wet meadow soil. Clover soils 

 contained 6,000,000 bacteria at a depth of 20 cm. and 1,500,000 at 50 

 cm. depth. 75 A change from 8,000,000 bacteria per gram of soil at the 

 surface to sterility at a depth of one meter was also recorded. 76 The 



71 Lochhead, A. G. Microbiological studies of frozen soil. Trans. Roy. Soc- 

 Canada. 18: 75-96. 1924; Soil Sci. 21: 225-232. 1926. 



72 Proskauer, 1882 (p. 12). 

 "Frankel, 1887 (p. 12). 



74 Reimers, J. tlber den Gehalt des Bodens in Bacteria. Ztschr. Hyg. 7: 

 319-346. 1889. 



76 Caron, A. Landwirtschaftlich-bakteriologische Probleme. Landw. Vers. 

 Sta., 45: 401-418. 1895. 



75 Stoklasa, J., and Earnest, A. tjber den Ursprung, die Menge und die 

 Bedeutung des Kohlendioxyds im Boden. Centrbl. Bakt. II, 14: 723-736. 1905. 



