20 THE CHEMISTRY OF THE FARM. 



potash, but the compounds formed are more or less de- 

 composed by water. To the hydrous silicates the perma- 

 nent retention of potash and other bases is probably chiefly 

 due. Humus has a great absorbent power for ammonia. 

 Other bases, as magnesia and lime, are also retained by 

 soil, but in a less powerful manner than are potash and 

 ammonia. 



Soils destitute of carbonate of calcium take up very 

 little potash or ammonia when these are applied as salts 

 of powerful acids, as for instance, the chlorides, nitrates, 

 and sulphates. When carbonate of calcium is present the 

 potassium or ammonium salt is decomposed, the base is 

 retained by the soil, while the acid escapes into the 

 drainage-Avater united with calcium. The addition of 

 carbonate of calcium may thus greatly increase the re- 

 tentive power of a soil for bases. 



The fertility of a soil is nearly connected with its power 

 of retaining plant food. Sandy soils, from their small 

 chemical retentive power, and free drainage, are of small 

 natural fertility, and dependent on immediate supplies of 

 manure. 



There can be little doubt that the plant food contained 

 in soil which is capable of being taken up by roots, exists 

 either in solution, or in the states of combination just re- 

 ferred to — that is in union with ferric oxide, hydrous sili- 

 cates, and humus. Different crops have very different 

 powers of attacking these various forms of plant food. 



The operations of tillage and drainage serve in several 

 ways to increase the amount of plant food which is at the 

 disposal of a crop. 



By tillage the surface soil is kept in an open porous 

 condition, favourable for the distribution of roots. By 



