CHEMISTRY OF SOILS 113 



in quantity sufficient to form a cementing matrix, and the 

 mass becomes consolidated into a hard sandstone. It is 

 in this way also that the calcareous pans are formed in 

 subsoils. 



Carbonic acid is, the feeblest of all acids, in the sense that 

 it is displaced from its compounds with bases by every other 

 acid. Calcium carbonate is no exception t!o this rule ; it 

 reacts with acids forming the corresponding calcium salts, 

 and carbonic acid is given off. 



CaC0 3 + 2 HN0 3 = Ca(N0 3 ) 2 + H 2 + C0 2 



Calcium Nitric acid. Calcium nitrate. Carbonic 



carbonate. acid. 



It is to this property and its insoluble, neutral character 

 that lime owes its greatest value as a constituent of the 

 soil. It is important that a neutral condition should be 

 maintained in the soil. Both acids and alkalis interfere 

 with the growth of plants and with the activity of lower 

 organisms concerned with fertility. Acid substances are 

 constantly Jbeing formed by the decay of organic matter, 

 and sometimes also as a result of chemical changes in 

 certain minerals, and, if allowed to accumulate, produce 

 that " sour " condition which is inimical to fertility. The 

 acids cannot be neutralised by addition of alkalis, for the 

 excess of alkali would produce a disastrous effect on the 

 physical condition of clay, and is probably as deleterious 

 to the plants as acids. In short, the cure would be worse 

 than the disease. 



Calcium carbonate, however, for the reasons given above, 

 serves the purpose equally well. The acids unite with 

 the lime, forming neutral compounds, and the carbonic 

 acid is expelled. Being itself an insoluble substance, of 

 neutral reaction, a large excess of it can be stored in the 

 soil without detriment to the growing plants or lower 

 organisms, and, as a rule, with great benefit to the physical 



S.M. x 



