. INORGANIC EEACTIONS IN SOILS. 59 



AlA-(CaO:MgO)2Si0 2 + 2C0 2 + 3H 2 



Anorthite. 



= (Ca:Mg)(HC0 8 ) a + Al a 8 :2SiO a .2H a O 



Bicarbonate of lime or magnesia. Kaolin. 



The calcium carbonate present in the soil also becomes 

 soluble 



CaC0 8 + H a O + CO, = Ca(HC0 8 ) a 



and is either absorbed by the plant or carried away in the 

 drainage water. 



Exactly how the phosphoric acid exists in the rocks and 

 minerals forming a soil it is difficult to determine. A portion 

 probably occurs as calcium phosphate, Ca H P 2 8 , and undergoes 

 a change thus : 



Ca 8 P A + 2CO a + 2H a O = Ca 2 H 2 P A + Ca(HC0 8 ) a 



or 2CaHP0 4 . 



The monohydrogen calcium phosphate, CaHP0 4 , being 

 slightly soluble in water, is available to plants."' It is pos- 

 sible, too, that the solution of calcium bicarbonate thus formed, 

 together with that furnished by the calcium carbonate in the 

 soil, may react upon such silicates as orthoclase and liberate 

 their potash as carbonate, the lime uniting with the other 

 constituents of the mineral : 



The soluble potassium salt is then either absorbed by the 

 roots of the crop growing on the soil or held in a weak state 

 of combination by the hydrated silicates present. 



The substances which become soluble owing to the action of 

 water, carbon dioxide, and other reagents in a soil are not 

 necessarily washed out of the soil by the drainage. The 

 chlorides, sulphates, carbonates, and to a less extent, perhaps, 

 the silicates, especially of lime and soda, are in great measure 

 thus removed, the only action interfering with their complete 

 removal being apparently the surface attraction exerted by the 



* In the presence of ferric hydrate, often present in soils, the phosphoric acid of 

 tricalcium phosphate is, by prolonged action of carbon dioxide and water, converted 

 into ferric phosphate, the lime being removed as carbonate. G. v. Georgievics, J. S.C.I. . 

 abst. 1892, 254. 



