158 SOIL BACTERIOLOGY 



changes; in others it seems to improve the crop-producing 

 power of the soil. 



For a discussion of this problem see the publications of 

 Richter, Pickering, Seaver, and others. 



Richter, Landw. Vers. Stat, Bd. 47, p. 269, 1896. 



Pickering, Jour. Agr. Sci., vol. ii, p. 411, 1908; vol. iii, p. 32, 1908. 



Seaver, Mycologia, vol. i, p. 131, 1909. 



Seaver and Clark, Biochemical Bui. No. 9, p. 413, 1912. 



Lyon and Bizzell, Bui. 275, Cornell Exp. Sta., 1910. 



Lathrop, Bui. 89, Bur. of Soils, U. S. Dept. Agr., 1912. 



Johnson, J., Science, vol. xliii, pp. 434, 435, 1916. 



Method for Sterilizing Soil: 



1. Small test-tubes of soil may be sterilized by heating 

 in the autoclave for two hours on two successive days at 

 15 pounds' pressure. 



2. When it is desirable to sterilize much larger amounts 

 of soil, the time of heating should be increased. 



3. If earthenware jars are used, the cold air in the bottom 

 is removed very slowly, therefore it is necessary to heat for 

 several hours. 



4. A 4-liter earthenware jar of soil requires at least six 

 hours or longer at 15 pounds' pressure to kill all forms of 

 bacteria. 



Growing Plants Free of Microorganisms. The method 

 to follow in growing plants free of bacteria depends largely 

 upon two factors: the size of the plant and the time of 

 growing period. If small plants are used clover, alfalfa, 

 etc. and it is not necessary to grow to maturity, large 

 test-tubes or glass cylinders may be used. 



Kellerman, K. F., Cir. 120, U. S. Dept. Agr. Bur. Plant Ind., 1913. 



In order to grow plants for a long period of time in a 

 medium free from infection a vessel of special design is 



