DECOMPOSITION OF ORGANIC MATTER IN THE SOIL. 263 



land contained only 26 kg. (57.56 pounds) of nitric nitrogen per acre 

 to the same depth. Stewart and Greaves found in limestone soil in 

 Utah 64 kg. (142 pounds) of nitric nitrogen per acre, under corn; 

 98 pounds under potatoes, and only 12 kg. (27 pounds) under alfalfa. 

 Under the same conditions fallow land contained 74 kg. (165 pounds) 

 of nitric nitrogen per acre. The smaller amount of nitric nitrogen found 

 under alfalfa bears out the observations already made by a number of 

 other investigators that the accumulation of nitrates under legumes is 

 smaller than it is under non-legumes. While several explanations have 

 been offered to account for this fact, it is generally agreed that legumes 

 assimilate nitrate nitrogen more rapidly than non-legumes. Unusual 

 circumstances may favor, at times, the accumulation of quantities of 

 nitrate large enough to destroy all vegetation. It is reported, for 

 instance, by Headden that he has found in limited areas in Colorado as 

 much as 9,0718.5 kg. (100 tons) of nitrate per acre foot of soil. 



The amount of nitrate nitrogen in the soil is influenced by the growing 

 crop not alone because of the nitrogen absorbed by the latter, but because 

 of the moisture relations as affected by growing plants. It is quite 

 apparent that a large crop dries out the soil more rapidly than a small 

 crop. When the soil moisture is sufficiently depleted, nitrification stops 

 and the further accumulation of nitrates becomes impossible, while their 

 disappearance is hastened by the constant demands of the crop. The 

 disappearance of soil nitrates is, likewise, hastened by the leaching action 

 of rain and by certain species of bacteria that transform them into other 

 nitrogen compounds. 



DENITRIFICATION. Experimental Study. Denitrification may be de- 

 fined as the reduction of nitrates by bacteria, involving the evolution of 

 nitrogen gas or of nitrogen oxides. In a more general way, denitrification 

 has been defined as the partial or complete reduction of nitrates by bacteria. 

 The term direct denitrification has been suggested for complete reduction, 

 and indirect for the partial reduction to nitrites or ammonia. The term 

 denitrification should not be employed to designate losses of nitrogen gas 

 due to the oxidation of ammonia, or to the disappearance of nitrates 

 following their conversion into proteins by microorganisms. 



The reduction of nitrates in the presence of fermenting organic matter 

 was noted by Kuhlmann as early as 1846. The same fact was recorded 

 many years later by Froehde and by Angus Smith. In 1868 Schoenbein 

 expressed the belief that nitrates may be reduced to nitrites by fungi. 



