32 PRINCIPLES OF SOIL MICROBIOLOGY 



numbers, due to the fact that a continued dry period brings about a 

 phenomenon similar to partial sterilization of the soil and with the first 

 increase in moisture there is an increase of available organic and in- 

 organic materials, leading to a rapid increase in bacterial numbers. 

 Several investigators found larger bacterial numbers in the soil in sum- 

 mer than in winter, the maximum being reached in July and August. 

 In the spring of the year, with the increase in soil temperature, there is a 

 corresponding increase in bacterial numbers. 62 It was suggested, 38 

 however, that the water content of the soil is the most important factor 

 bearing upon bacterial numbers: cultivation increases the number by 

 increasing the water content. Engberding 38 was unable to demonstrate 

 any influence of temperature upon the numbers of bacteria in mineral 

 soils. This difference in results may be due to the difference in the 

 composition of the soil. The numbers of bacteria (and protozoa) were 

 even found to fluctuate from day to day. 63 Well-marked seasonal 

 changes in the soil population are superimposed on the daily varia- 

 tions in numbers. These changes are not directly influenced by tem- 

 perature or rainfall, but show a similarity to the seasonal fluctuations 

 recorded for many aquatic organisms. 



Conn, 61 after a careful comparison of bacterial numbers in frozen 

 and unfrozen soil, came to the conclusion that the number of bacteria 

 in frozen soil is generally larger than in unfrozen soil, which is true not 

 only of cropped soil, but also of sod and fallow land. This increase in 

 bacterial numbers after freezing was believed not to be due to an increase 

 in moisture content, even though in an unfrozen condition the bacterial 

 numbers seemed to increase and decrease parallel to the moisture con- 

 tent of the soil. The increase in frozen soil seemed to be a result of 

 actual multiplication of the bacteria, rather than of a mere rise of the 

 organisms from lower depths brought about by mechanical forces 

 alone. Conn, therefore, suggested that there are two groups of bacteria 

 in the soil, "summer" and "winter" bacteria, which are active in the 

 respective periods. These results were at first confirmed. 65 The 



62 Remy, 1902 (p. 23). 



83 Cutler, D. W., Crump, L. M., and Sandon, H. A quantitative investigation 

 of the bacterial and protozoan population of the soil, with an account of the 

 protozoan fauna. Phil. Trans. Roy. Soc. London, B, 211: 317-350. 1922. 



64 Conn, H. J. Bacteria in frozen soils. Centrbl. Bakt. II, 28:422-434. 

 1910; 32: 70-97. 1911; 42: 510-519. 1914; N. Y. Agr. Exp. Sta. Tech. Bui. 35. 

 1914. 



66 Brown, P. E., and Smith, R. E. Bacteria at different depths of some typical 

 Iowa soils. Iowa Agr. Exp. Sta., Res. Bui. 8: 281-321. 1912. 



