392 NATURE AND PROPERTIES OF SOILS 



since many organisms cannot grow in the artificial media 

 commonly used. Moreover, it is almost impossible to break 

 up the clump of colonies in such a way as to determine the 

 number of individuals present. It is fairly certain, however, 

 that the numbers of bacteria in soil are very large, possibly 

 ranging from 500,000 to 100,000,000 to a gram of dry soil. 

 Good soils seem, in general, to carry the greatest numbers. 

 The bacterial flora, as well as the other soil organisms, fluctu- 

 ate markedly with season, the numbers usually being great- 

 est in the summer months. 



215. Conditions affecting bacterial growth. 1 — Many con- 

 is then well shaken in order to produce a suspension containing the 

 bacteria originally present in the soil. Dilutions of 1 to 20,000, 1 to 

 100,000 and 1 to 200,000 based on the original soil sample are made. 

 Gelatin or agar plates are then inoculated, three from each dilution. 

 After adequate incubation the colonies on the plates are counted, each 

 colony supposedly representing one original organism. The numbers of 

 bacteria that were present in the original soil are then calculated. The 

 agar or gelatin of the plates generally receive a sterile extract from the 

 soil together with certain added materials, organic or inorganic, in order 

 that the growth of the bacteria may be hastened. 



Such a count does not represent by any means all of the bacteria of 

 the soil, as some groups will not develop at all, while others require 

 special media. Slowly growing groups of organisms, that would prob- 

 ably appear if time were given, escape the count, since the plates are so 

 quickly covered by more abundant growths. The suspension from the 

 soil, used to inoculate the plates, does not contain all of the organisms 

 as single individuals, since it is impossible completely to break down the 

 clump formation. This tends to make the counts too low. Special 

 media and technique are of course necessary in studying fungi, algse 

 and actinomyces. 



1 Rahn, Otto, The Bacterial Activity in Soil as a Function of Grain-size 

 and Moisture Content; Mich. Agr. Exp. Sta., Tech. Bui. 16, 1912. 



Plummer, J. K., Some Effects of Oxygen and Carbon Dioxide on Nitri- 

 fication and Ammonification in Soils; Cornell Agr. Exjy Sta., Bui. 384, 

 1916. 



Greaves, J. E., and Carter, E. G., Influence of Barnyard Manure 

 and Water Upon the Bacterial Activities of the Soil; Jour. Agr. Res., 

 Vol. VI, No. 23, pp. 889-926, 1916. 



Brown, P. E. The Influence of Some Common Humus-forming Mate- 

 rials of Narrow and of Wide Nitrogen-carbon Ratio on Bacterial Num- 

 bers; Soil Sci., Vol. 1, No. 1, pp. 49-75, 1916. 



Waksman, S. A., Bacterial Numbers in Soils, at Different Depths and 

 in Different Seasons of the Year; Soil Sci., Vol. I, No. 4, pp. 363-380, 

 1916. 



Gainey, P. L., The Effect of Time and Depth of Cultivating a Wheat 



