NUMBERS OF MICROORGANISMS 



35 



numbers of bacteria usually increase from the surface to a depth of 4 to 

 6 inches; 77 this is followed by a decrease to a depth of 24 inches m humid 

 soils. The maximum numbers were found at six inches of depth; 78 in 

 some cases 79 the maximum was found at a depth of 4 inches, where the 

 greatest numbers of organisms are found followed by a more or less 

 regular drop. Soils under shade (orchard, forest, meadow) have the 

 highest numbers of bacteria at a depth of 1 inch, then the numbers 



Numb era 



10,000,000fy 

 9,000,00 

 8,OOO,OO0fX 

 7,000,000 

 6,000,000 

 5,000,000 

 4,000,000 

 3,000,000 

 2,000,000 

 1,000,000 



Inches ° 

 deplh- 



.„_.._ Soil C. 

 Soil D. 



Fig. 2. Numbers of bacteria at different depths of soil, on the average of 

 several determinations throughout the year: A, rich garden soil; B, cultivated 

 orchard soil; C, clay soil, under timothy; D, acid forest soil (from Waksman). 



decrease with depth, while soils exposed to the sun have the highest 

 numbers at a depth of 4 inches. 80 



77 Chester, F. D. The bacteriological analysis of soils. Del. Agr. Exp. Sta. 

 Bui. 65. 1904. 



78 King, W. E., and Doryland, C. J. T. The influence of depth of cultivation 

 upon soil bacteria and their activities. Kans. Agr. Exp. Sta. Bui. 161: 211-242. 

 1909. 



79 Brown and Smith, 1912 (p. 32). 



80 Waksman, Soil Sci., 1 : 363-380. 1916. 



