204 Report of the Department of Bacteriology of the 



general technique 



The technique used by various soil bacteriologists has differed 

 not only in the kinds of culture media used but also in the length 

 of incubation and the temperature employed. The two latter 

 factors are fully as important as the composition of the medium in 

 determining the final count. Fischer incubated his plates at 16-20° C. 

 and counted during the second week. Temple used a temperature 

 of 25° C. for six days. Lipman and Brown held their plates three 

 days at " about 25° C." This last method is open to criticism, for 

 it has been found in the studies reported upon in this bulletin that 

 so many new colonies continue to appear on Lipman's and Brown's 

 media after three days that the count may be as much as three 

 times as high on the tenth day as on the third. The counts pub- 

 lished by Lipman and by Brown, indeed, are considerably lower than 

 those obtained upon their media in the course of the present investi- 

 gation, in which the longer incubation time has been used. 



In the present work gelatin plates have been incubated for seven 

 days, agar plates for fourteen. It would undoubtedly have been 

 more satisfactory to hold gelatin plates a few days longer; but as 

 liquefaction often prevents a count under these circumstances, 

 seven days has been chosen for the routine incubation time. In the 

 case of agar plates, on the other hand, very few new colonies develop 

 after the tenth day, and the longer period of incubation seems to be 

 unnecessary. The use of the fourteen-day period was begun before 

 this fact was known, and it was continued throughout the work 

 in order to make all the results comparable. 



The temperature used for incubation has been 18° C. The incu- 

 bator employed 19 is one that can be kept at a very constant tem- 

 perature; and it has never reached a temperature as high as 19° 

 except on the hottest summer days. In the case of gelatin, the 

 use of this low temperature is very important, because it prevents 

 rapid liquefaction. 



The soils chosen for making these tests have been of as great 

 a variety as could be obtained in this locality. They vary in texture 

 from muck to sand. They are of the following various origins: 

 glacial lake deposit (Dunkirk series), glacial till of the New York 

 drumlin area (Ontario series), glacial till from Devonian shales 

 and sandstones (Volusia silt loam), alluvial (Genesee soils) and 

 a limestone residual soil mixed somewhat with glacial materials 

 (Honeoye stony loam). The nomenclature used is that adopted 

 by the Bureau of Soils of the United States Department of Agri- 

 culture. 20 



19 Conn, H. J., and Harding, H. A. An Efficient Electrical Incubator. N. Y. Agr. 

 Exp. Sta., Tech. Bui. 29:1-10, 1913. 



20 U. S. Dept. Agr. Bureau of Soils, Bui. 96, pp. 1-791, 1913. See also Soil 

 Survey of Ontario County, New York, published by this Bureau, 1912; pp. 1-55. 



