152 CHkMlCAL COMPOSITION OF THE SOIL [chap. 



point to realise is, that plants feed on the solutions in 

 the soil water, and when the compounds in the soil can 

 renew this solution quickly and maintain it at a compara- 

 tively high concentration, they may be described as 

 "available" plant foods, whereas the "dormant" plant 

 foods gbjQ rise to soil solutions of low concentration, 

 which are not speedily repaired when the plant has 

 extracted the constituent in question. In the case of 

 nitrogen, the renewal of the solution depends upon the 

 attack of bacteria upon the nitrogen compounds in the 

 soil ; as regards phosphoric acid and potash, more 

 purely physical and chemical actions regulate the rate 

 of solution. The formation of the soil solutions has 

 already been discussed, and it was stated that the 

 carbon dioxide excreted by the roots of the plant has a 

 considerable effect in aiding the attack of the soil water 

 upon the mineral constituents of the soil. It is easy to 

 show in the laboratory that a solution of carbon dioxide 

 in water is a better solvent of such phosphates and 

 potash compounds as occur in the soil than pure water 

 is — almost in proportion to the amount of carbon dioxide 

 dissolved — and that such a solution must exist in the 

 soil is evident from the composition of the gases in the 

 soil. In most soils the actual solid particles only occupy 

 about 60 per cent, of the total space, the rest being 

 filled up by water and air. When the soil is saturated 

 with water the air is expelled, but complete expulsion is 

 only possible when the soil is saturated very carefully 

 from below, so as gradually to displace all bubbles. 

 Under ordinary conditions some air is always entangled 

 even when the soil is soaking, and as soon as drainage 

 gets established there will be 20 per cent, or more of 

 air, even though the soil is apparently very wet. The 

 air in the soil, however, does not long retain its original 

 composition; by the respiration of the roots and the 



