18 THE SOIL OF THE FAEM. 



work as plant food. Its use in the form of sand consists 

 in its influence on the texture of the sand. 



Alumina is a valuable constituent of soils as giving sub- 

 stance and stiffness of texture to the soil. In combina- 

 tion with silica (as the silicate of alumina) it is clay. It 

 is contained in greatest proportion in the stiffer clays, but 

 it rarely exceeds ten per cent, of the whole mineral con- 

 stituents of a soil. Clay soils contain on an average from 

 six to ten per cent, of alumina. In sandy soils it varies 

 from one to four per cent. ; and in marls, calcareous soils, 

 and vegetable moulds from one to six per cent. 



The larger the percentage of alumina m the soil, the 

 more difficult is its cultivation — the adhesive character of 

 the earth offering a stubborn resistance to the passage of 

 the plow and other implements through it. 



Calcic carbonate, a combination of lime and carbonic 

 acid, varies from about ninety per cent, in some marls 

 and limestone soils to mere traces in some other soils. 

 Clays and loams generally contain one to three per 

 cent, of this substance. Less than one per cent, may 

 be regarded as a defective proportion. AAliere a soil is 

 deficient in lime, the lime present exists in it mostly in 

 combination with the organic acids, and is more abun- 

 dant in the surface than m the subsoil. 



Phosphoric acid is contained in all good soils, but only 

 in small quantities, when compared with their other con- 

 stituents. It exists in combination with lime, iron, and 

 alumina. Phosphate of lime is its most common combi- 

 nation. It is generally found in very minute quantities, 

 but in clays its percentage is sometimes more than one 

 per cent. In general, even very fertile land contains less 

 than this proportion, and the average amount is probably 

 about a half per cent. The supply of phospliates is 

 shown by analysis to be derived in the main from the 

 rocks themselves. The fossilii'erous rocks yield it most 

 abundantly. 



