424 NATURE AND PROPERTIES OF SOILS 



logical problems of the soil, no definite relationships have 

 been established between any given transformation and the 

 productivity of the soil. General correlations have been re- 

 peatedly observed x but specific relationships, when recorded, 

 are difficult to ascribe to other than chance concordance. Of 

 all of the biological transformations, nitrification seems most 

 likely to correlate with productivity, since most plants use 

 large amounts of nitrate nitrogen. 



Available data seem to show that there is a general correla- 

 tion between the nitrifying capacity of soils and their crop- 

 producing power. 2 Such a statement, however, does not imply 

 that the productivity of soils, insofar as nitrogen is a limiting 

 factor, is especially controlled by nitrification. Arable soils 

 usually contain abundant nitrifying organisms, which seem 

 to oxidize ammonia to the nitrate form as fast as it is pro- 

 duced. It would appear that nitrification is only one of the 

 many factors that govern productivity, a high nitrate content 

 of a soil accompanying, rather than controlling, high crop 

 production. 



233. Reduction of nitrates and allied compounds. — Ni- 

 trates may be removed from the soil in three ways: (1) by 

 drainage, (2) by plant absorption, and (3) by reduction to 

 free nitrogen. The loss of nitrogen by leaching and by crop- 

 ping has already been adequately treated. It has been shown, 

 for example (see par. 163), that as high a loss as 77 pounds of 

 nitrogen to the acre a year may be expected from a heavy 



1 Ashby, S. F., The Comparative Nitrifying Power of Soils; Jour. 

 Chem. Soc, London, Vol. 85, pp. 1158-1170, 1904. 



Russell, E. J., and Hutchinson, H. B., The Effect of Partial Sterili- 

 zation of Soils on the Production of Plant Food; Jour. Act. Sci., Vol. 

 Ill, pp. 111-144, 1909. * 



Kellerman, K. F., and Allen, E. R., Bacteriological Studies of the 

 Truckee-Carson Irrigation Project; U. S. Dept. Act., Bur. Plant Ind., 

 Bui. 211, 1911. * ' ' 



Brown, P. E., Relation Between Certain Bacterial Activities in Soils 

 and Their Crop Producing Power; Jour. Act. Res., Vol. V, pp. 855- 

 869, 1916. ' ^ F 



a Gainey, P. L., The Significance of Nitrification as a Factor in Soil 

 Fertility; Soil Sci., Vol. Ill, No. 5, pp. 399-416, 1917. 



