DECOMPOSITION OF ORGANIC MATTER IN THE SOIL 333 



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 90,718.5 kg. (100 tons) of nitrate per acre foot of soil. 



The amount of nitrate nitrogen in the soil is influenced by the grow- 

 ing 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, nitrifica- 

 tion 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 

 defined as the reduction of nitrates by bacteria, involving the evolu- 

 tion 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 sug- 

 gested for complete reduction, and indirect for the partial reduction 

 to nitrites or ammonia. The term denitrification should not be em- 

 ployed to designate losses of nitrogen gas due to the oxidation of 

 aromonia, 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. For more than a decade after that, data were rapidly accu- 

 mulating in support of Schoenbein's contention, until in 1882 Gayon 



