46 THE CHEMISTRY OF THE FARM 



ammonia, which is, by preference, absorbed. The 

 relative mass of the acting bodies has, however, a 

 great influence, and it is possible to liberate potash 

 from a soil by the application of a large quantity of a 

 sodium salt, although the sodium is the weaker base. 



Soils destitute of lime retain very little potash or 

 ammonia when these are applied as salts of powerful 

 acids, as, for instance, as chlorides, nitrates, or sul- 

 phates. 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 water united with calcium. The addition of 

 marl, chalk, or lime may thus greatly increase the 

 retentive power of a soil for bases. 



Phosphoric acid is the most firmly retained of all 

 the substances absorbed by soil. The bases absorbed 

 by soil are slowly removed by the action of water. 

 The action of the water is least in a soil which has 

 absorbed little, or has been already washed, and 

 greatest when the soil has been heavily manured. 

 A soil always contains some plant food in solution. 



The permanent fertility of a soil is nearly connected 

 with its power of retaining plant food. In the case 

 of a soil containing clay, only traces of phosphoric acid, 

 ammonia, or potash are ever found in the drainage 

 waters. Sandy soils, from their smaller chemical 

 retentive power and free drainage, are of less natural 

 fertility, and much more dependent on immediate 

 supplies of manure. 



There can be little doubt that the active 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, hydrous silicates, or humus. Different 



