CHEMISTEY OF SOILS 135 



gases to undergo a process of densification ; water vapour 

 is condensed, ammonia is absorbed and the oxygen rendered 

 much more active. The gases are not, however, perma- 

 nently occluded ; a constant exchange takes place between 

 the air of the soil and that of the atmosphere by the process 

 of diffusion ; those which are absorbed are replenished, and 

 those which are produced by chemical changes in the soil- 

 chiefly carbon dioxide are diffused. 



Soils of finer texture present a greater internal surface 

 and have, therefore, a greater power of absorbing atmos- 

 pheric gases, but the same conditions tend to retard diffu- 

 sion and cause the soil to retain more water. Under natural 

 conditions, therefore, the air finds much readier access 

 to soils of coarser texture. 



On marshy lands, in which the pores are permanently 

 blocked with water, carburetted hydrogen, carbon mon- 

 oxide, etc., are produced by the decomposition of organic 

 matter, but under the conditions of rapid oxidation, which 

 obtain in cultivated land, these gases are not formed, or, 

 if formed, are immediately oxidised, and are never found 

 in the soil gases. 



The air of the soil, therefore, contains the same gases 

 that are found in the atmosphere, but they are often present 

 in very different proportions. In general, the air of the 

 soil contains less oxygen and more carbon dioxide. The 

 oxygen is withdrawn by the oxidation of minerals and 

 organic matter, and carbon dioxide is produced. The pro- 

 portion of nitrogen by volume is not greatly affected by 

 these changes because carbon dioxide occupies the same 

 volume as the oxygen consumed. For example, the air of 

 a forest soil was found to contain 0'93 per cent, of carbon 

 dioxide and only 19 '5 per cent, of oxygen. The oxidation 

 of organic matter is greatly accelerated by the action of 

 micro-organisms and under conditions favourable to their 

 growth chiefly moisture, warmth and a plentiful supply 



