SOIL GASES. 71 



air." The authors just alluded to ascribe denitrification to a 

 reaction which they thus formulate : 



5CH la O fl + 24KN0 3 - 24KHC0 3 + 6CO, + 18H,0 + 12N 2 . 



Sugar. 



This reaction is quite possible and would be attended with 

 the evolution of heat. 



Deherain |- found that the addition of starch to a soil resulted 

 in the almost complete destruction of nitrates, but that when 

 straw was added, even to the extent of 1 % of the soil, only 

 about ^ of the nitric nitrogen was lost. He suggests that the 

 injurious effects of large additions of farm-yard manure to a soil 

 may be largely due to the nitrification being checked rather 

 than to actual destruction of nitrates. 



Soil Gases. The interstices between the particles composing 

 a soil are usually occupied by air, except when heavy rain or 

 other cause fills them with water. This enclosed air must not 

 be regarded as confined, but is constantly undergoing renewal 

 by diffusion from the air above. Inasmuch as the processes 

 going on in a soil are accompanied by, and indeed largely 

 consist of, oxidation, it is obvious that the air within a soil 

 will be poorer in oxygen than that of the atmosphere above. 

 Schloesing:[: in 1890 published the results of a number of 

 analyses of air sucked out from soil from various depths. 

 This was always found to contain only the gases of the atmo- 

 sphere, no measurable amount of marsh gas or other com- 

 bustible gas being detected. The general results of these 

 determinations were : 



1. The sum of the percentage amounts of carbon dioxide and 



oxygen is equal to 21. 



2. The amount of carbon dioxide varies very much, from 



about 1 % to as high as 8 or 10 %, the oxygen from 10 

 to 20%. 



3. In general the amount of carbon dioxide increases with the 



depth (up to 50 or 60cm.) from which the sample is 

 collected. This is due not to diminished production 

 near the surface, but to more rapid diffusion there. 



* Ainpola & Ulpiani, Gazzetta, 1898, i. 410 t Ann. Agron. 1898, 130. 



t Compt. Rend. 109, 173. 



