THE FORMATION OF SOIL 21 



of water can absorb and retain a large pro- 

 portion of this gas. The water in the soil 

 obtains its carbonic acid gas in various ways. 

 In the first place, in falling through the air, 

 rain-water dissolves a certain amount of 

 this gas, which it carries in solution to the soil, 

 where it comes into contact with the stones 

 and rocks. In proportion to the amount 

 of oxygen and nitrogen present in the atmo- 

 sphere, the quantity of carbonic acid gas is 

 very small, but, as the affinity of water for 

 carbonic acid gas is very much greater than 

 for the other constituents of the atmosphere, 

 it follows that the gaseous substances dis- 

 solved from the atmosphere by the falling 

 rain are relatively much richer in carbonic 

 acid gas than the atmosphere from which 

 they have been derived. 



The water in the soil, however, derives the 

 bulk of its carbonic acid gas from the supplies 

 which are liberated during the oxidation of 

 vegetable matter in the soil. Soil always 

 contains more or less vegetable matter, and 

 in many cases a large amount is present, as, 

 for instance, where soil is rich in humus, 

 derived, it may be, from peat, or from decom- 

 posing turf, or from the residues of previous 

 crops, or from farmyard manure. These 

 organic substances, calculated dry, generally 



