22 AGRICULTURE 



contain about fifty per cent, of carbon, and 

 in the process of decay this element unites 

 with oxygen to form carbonic acid gas. 

 The air present in the interstices of the soil, 

 therefore, is specially rich in this gas, and 

 water in the soil has full opportunity of 

 taking up such gas and of thereby adding 

 to its powers of solution. Other things 

 being equal, there is little doubt that the 

 more carbonic acid gas there is present in the 

 soil the greater is the amount of plant food 

 available for the use of crops, such liberation 

 of plant food being effected through the 

 agency of water charged with this gas. Not 

 only are such substances as potash and 

 phosphates dissolved from the stones and 

 rocks, and rendered available for the use of 

 plants, through the agency of carbonic acid, 

 but even when applied as manures these 

 same substances are more effective in the 

 presence of considerable supplies of carbonic 

 acid. 



An agency that must be mentioned in 

 connection with the formation of soil is 

 plant roots, which break down stones and 

 rocks in two ways. In the first place they 

 push their way into the cracks and minute 

 fissures of stones and rocks, and, having 

 gained an entrance, they grow, and exert 



