420 NATURE AND PROPERTIES OF SOILS 



isms. Recent experimentation, 1 however, indicates that car- 

 bon dioxide has little or no effect on nitrification and am- 

 monification as long as appreciable quantities of oxygen are 

 present. Aeration, insofar as most biological activities are 

 concerned, has to do more with the presence of oxygen than 

 the elimination of the carbon dioxide which is always form- 

 ing. 



Since aeration is such a factor in nitrification, the trans- 

 formation is very largely confined to the surface layers of 

 soil, except in the rich and porous subsoils of arid and semi- 

 arid regions. The lack of nitrate formation in the lower 

 depths is probably influenced by temperature as well as by 

 lack of oxygen and organic matter. At 5° C. nitrification is 

 very feeble. The optimum temperature seems to range from 

 25° to 30° C. The drainage of a soil probably promotes nitri- 

 fication quite as much by facilitating a rise of temperature 

 as by promoting the entrance of oxygen, especially in the 

 spring. 



The speed with which nitrification proceeds in a soil is 

 governed to a marked extent by water content, 2 the process 

 being retarded by both low and high moisture conditions. 

 In practice, it is safe to assume that the optimum moisture 

 as recognized for higher plants is optimum for nitrification 



Glummer, J. K., Some Effects of Oxygen and Carbon Dioxide on 

 Nitrification and Ammonification in Soils; Cornell Agr. Exp. Sta., Bui. 

 384, 1916. 



2 Coleman, L. C, Untersuchungen iiber Nitrification; Centbl. f. Bakt., 

 II, Bd. 20, Seite 401-420 and 484-513, 1908. 



Fraps, G. S., The Production of Active Nitrogen in the Soil; Tex. Agr. 

 Exp. Sta., Bui. 106, 1908. 



Patterson, J. W., and Scott, P. K., The Influence of Soil Moisture 

 upon Nitrification; Jour. Dept. Agr., Victoria, Vol. 10, pp. 275-282, 

 1912. ** 



Stewart, B., and Greaves, J. E., The Production and Movement of 

 Nitric Nitrogen in Soil; Centbl. f. Bakt., II, Bd. 34, Seite 115-147, 

 1912. ' 



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

 Wheat Seed-bed Upon Bacterial Activity in the Soil; Soil Sci., Vol. 

 II, No. 2, pp. 193-204, 1916. 



