112 SOILS AND MANUBES 



contain practically none at all. Average fertile soils may 

 contain from 2 to 10 per cent, of lime. Those which 

 contain less than 1 per cent, require careful management, 

 especially if they contain much clay or organic matter. In 

 sandy soils lime is not so necessary for fertility, and it is 

 usually present in smaller quantity in soils of that class. 



The calcareous ,matter of soils is derived from shells, 

 chalk, limestone, marls, and the lime artificially applied 

 in case of need. Some limestones are hard and crystalline, 

 but chalk is amorphous, soft, friable and very porous. 

 Chalky soils are usually very poor, but, owing to their great 

 porosity, retain moisture and yield a short sweet herbage 

 well adapted for pasturing sheep. The downs are almost 

 entirely devoted to this kind of cultivation. 



Calcium carbonate is practically insoluble in pure water, 

 but combines with carbonic acid to form a more soluble 

 bicarbonate, 



CaCOs + C0 2 + H 2 = CaH 2 (C0 3 ) 2 



That is why it dissolves in water charged with carbonic 

 acid. The bicarbonate, however, is quickly decomposed 

 on boiling the solution and more slowly at ordinary tem- 

 peratures ; the carbonic acid is evolved, and the normal 

 carbonate is reprecipitated, 



CaH 2 (C0 3 ) 2 = CaC0 3 + H 2 + C0 2 



The stalactites seen in the caves at Cheddar and other 

 limestone districts, and in grotto structures, have been 

 formed in this way, and the formation of limestones from 

 deposits of shells is attributed partly to the same cause. 

 The precipitation of calcium carbonate consequent upon 

 the evaporation of carbonic acid or its absorption by other 

 substances from bicarbonate solutions produces a cement- 

 ing effect on sandy soils. Carbonate of lime sometimes 

 becomes infiltrated in this way through a mass of loose sand 



