THE SOIL 47 



the silicic acid and potassium carbonate being dissolved and 

 either carried away in the drainage, or the latter may be 

 absorbed by the roots of a plant or by some of the ab- 

 sorptive constituents of the soil. The calcium carbonate is 

 dissolved by the carbon dioxide solution — 



OaCOj + H,0 + CO, = Oa(HC03), 



— and is either carried away or absorbed, or perhaps acts upon 

 felspar or other silicate : 



A],0,.K,0.6SiO, + CaCHCOg), + OH^O = K^CO, + CaCO, 

 + Al,03.2SiOa.2H,0 + 4H,SiO,. 



The phosphoric acid in minerals probably exists largely as 

 tricalcium phosphate, CajPgOg. This substance is nearly 

 insoluble in water, but by the action of carbon dioxide may be 

 changed thus : 



Ca,P,08 + 200, + 2H,0 - Ca^H^PPg + Ca(HC03),. 



The dihydrogen dicalcium phosphate, ctt*, as it may be called, 

 the monocalcium monohydrogen phosphate, CallPO^, is 

 slightly soluble in water, and therefore available for plants. 

 In contact with ferric hydrate or aluminium hydrate it is 

 converted into ferric phosphate, FePO^, or aluminium 

 phosphate, AlPO^, and held back in the soil in a very finely 

 divided state, and, though then insoluble in water, is capable 

 of being dissolved by the acid juices of plants' roots. In fact, 

 many of the substances which become soluble owing to the 

 action of carbon dioxide, water, or other reagents do not 

 necessarily remain in a soluble form. If they did they would 

 be to a great extent washed out in the drainage water. 



Most soils contain substances which have the power of uniting 

 with potassium, ammonium, and, to a less extent, calcium com- 

 pounds, and with phosphates, converting them into insoluble 

 compounds. If solutions of potassium or ammonium sulphate 

 or of sodium phosphate be filtered through a thick layer 

 S)f soil, the filtrate will be found to be almost free from 



