E. J. R1T8SELL AND A. Applkyard 5 



methane or any other combustible gas. We may therefoi'e safely 

 assume that the residual gas is practically all nitrogen. 



This then represents the ordinary composition of the air filling the 

 pores of the soil at a depth of G inches, the layer within which most 

 of the important soil changes go on. As already pointed out it is very 

 similar to ordinary atmospheric air but there are three important 

 differences which may have much greater effects than would at first 

 be expected : 



1. The amount of carbon dio.xide though low in the absolute, is 

 nevertheless about ten or more times as high as in atmospheric air. 



2. The amount of moistuie pi'esent in the soil air is greater than 

 in atmospheric air and is usually nearer the saturation point. 



3. The soil air is still, there being much less opportunity for actual 

 movement than in the atmosphere. 



It is outside our present subject to discuss the effects of these 

 characteristics and we need only indicate a few ways in which they 

 may be expected to act. 



There is considerable evidence that microorganisms are very 

 sensitive to the medium in which they are placed, and the relatively 

 high proportion of carbon dioxide in the soil atmosphere is likely to 

 affect their activity. It is therefore necessary to take this factor into 

 account before applying to the soil any deductions from bacteriological 

 investigations made in the laboratory under ordinary atmospheric 

 conditions. 



In consequence of its stillness and its intimate contact with the 

 moist soil particles, the soil air is likely to be saturated or nearly 

 saturated with water vapour, and this condition is known to be 

 favourable for organisms and to reduce the need for free liquid water. 



The effect of the extreme stillness of the air, however, cannot be 

 gauged ; physiologists recognise that movement in the air is necessary 

 for the comfort and well being of humans, and we should no doubt 

 find the soil atmosphere intolerable from this cause alone, but it is 

 difficult to form any estimate of its effect on microorganisms. 



But this free air filling the pore spaces is not the only air in the 

 soil. During the course of other experiments we had occasion to 

 evacuate flasks containing soil, and we found that the vacuum per- 

 sistently began to fall soon after exhaustion appeared to be complete. 

 Gas was being evolved from the soil, but it came out only very slowly 

 even when a good mercury pump was kept at work for several days. 



