716 PRINCIPLES OF SOIL MICROBIOLOGY 



determined at the end of 15 and 30 days incubation at 25° to 28°C. The nitrate 

 content of the original soil should always be determined. 11 



At first the solution method, with or without soil extract, was used 12 for meas- 

 uring the nitrifying power of the soil. This method was later found 13 entirely 

 unreliable and the use of the soil method was suggested. Lohnis and Green 14 

 called attention to the fact that many of the known critical factors in solution 

 studies on nitrification were ignored by those who have criticized them severely. 



A definite correlation between the nitrifying power of a soil and its 

 crop productivity has been observed by various investigators. 15 In 

 some cases a correlation was found to exist between the nitrogen con- 

 tent of the soils and their nitrifying powers, 16 as shown in figures 61 

 and 62. 



Some investigators reported that the nitrifying power of a soil may 

 or may not correlate with its crop producing power and that continuous 

 cropping, especially without fertilization, was found to reduce the 

 nitrifying power of the soil. 17 While nitrification is a valuable and 

 essential asset in fertility, it probably does not, under normal conditions, 

 become a limiting factor in productivity. 18 This is suggested on the 



11 Waksman, S. A. Microbiological analysis of soils as an index of soil fer- 

 tility. V. Methods for the study of nitrification. Soil Sci., 15: 241-260. 1923. 



"Remy, 1902 (p. 710); Lohnis, 1904 (p. 687); Gutzeit, E. Einwirkung des 

 Hederichs auf die Nitrifikation der Ackererde. Centrbl. Bakt. II, 16: 358- 

 381. 1906; Buhlert and Fickendey. Zur Methodik der bakteriologischen Bo- 

 denuntersuchung. Centrbl. Bakt. II, 16: 399-405. 1906. 



11 Stevens, F. L., and Withers, W. A. Studies in soil bacteriology. I. Nitri- 

 fication in soils and in solutions. Centrbl. Bakt. II, 23: 355-373. 1909. 



l * Lohnis and Green, 1914 (p. 6S8). 



16 Gainey, P. L. The significance of nitrification as a factor in soil fertility. 

 Soil Sci., 3: 399^116. 1917; Lipman, C. B. The nitrifying power of soils as 

 indices to their fertility. Proc. Soc. Prom. Agr. Sci. 35th Ann. Med. 1914, 

 33-39; Cal. Agr. Exp. Sta. Bui. 260, 107-127; Given, G. B. Bacteriology of the 

 general fertilizer plots. Penn. Agr. Exp. Sta. Rpt. 1912-13, 204-206; Brown, 

 P. E. Bacteriological studies of field soils. Centrbl. Bakt. II, 35: 234-272. 

 1912; Jour. Agr. Res., 5: 855-869. 1916; Burgess, P. S. Can we predict probable 

 fertility from soil biological data? Soil Sci., 6: 449-462. 1918; Ashby, S. F. 

 The comparative nitrifying power of soils. Trans. Chem. Soc, 85: 1158-1170. 

 1904; Kellerman, K. F., and Allen, E. R. Bacteriological studies of the soil 

 of the Truckee-Carson Irrigation Project. U. S. Dept. Agr., Bur. PI. Industry, 

 Bui. 211. 1911; Waksman, S. A. Microbiological analysis of soil as an index 

 of soil fertility. VI. Nitrification. Soil Sci., 16: 55-67. 1923. 



16 Fraps, G. S. Nitrification studies. Science, 42: 68. 1916. 



17 Allen, E. R., and Bonazzi, A. On nitrification. Ohio Agr. Exp. Sta. Tech. 

 Bui. 7. 1915. 



18 Gainey, 1917 (p. 534). 



