412 THE HOME, FARM AND BUSINESS CYCLOPEDIA. 



also be constantly taken up by the roots, and employed as plant food. 

 The loss of nitrates by drainage is thus far less when the land is under 

 crop than in the case of a bare fallow. 



Phosphoric acid, potash, and ammonia are very rarely found in drain- 

 age water. If a solution containing phosphoric acid, potash, or ammonia 

 is poured on a sufficiently large quantity of fertile soil, the water which 

 filters through will be found destitute of these substances. This retentive 

 power of soil for phosphoric acid, potash, &c., is of the utmost importance 

 in agriculture. The action is a complex one. All salts are doubtless re- 

 tained to some extent by soil through mere mechanical adhesion ; salts, 

 thus feebly retained, as nitrates and chlorides, can be easily removed by 

 washing with water. Other substances are, on the contrary, retained by 

 chemical affinity ; these are not removed by washing, or but to a small 

 extent. The ingredients of the soil which exercise a chemical retention 

 power are the hydrates of ferric oxide and aluminia, the hydrous silicates 

 of aluminium, and humus. 



Ferric oxide is a common ingredient of soils ; to it the red colour of 

 many soils is owing. To the presence of ferric oxide the retention of 

 phosphoric acid is chiefly due, an insoluble basic phosphate of iron being 

 produced. Aluminia acts in the same manner. Ferric oxide and aluminia 

 have also a retentive power for ammonia and potash, but the compounds 

 formed are more or less decomposed by water. To the hydrous silicates 

 the permanent 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 man- 

 ner 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 pre- 

 sent the potassium or ammonium salt is decomposed, the base is retained 

 by the soil, while the acid escapes into the drainage-water united with 

 calcium. The addition of carbonate of calcium may thus greatly increase 

 the retentive 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 referred to that is in union with ferric oxide, 



