ABSORPTIVE-POWER OF SOIL. 27 



due, an insoluble basic phosphate of iron being- produced. 

 Alumina acts in the same manner. Ferric oxide and 

 alumina have also a retentive power for ammonia and 

 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 retain very 

 little potash or ammonia when these are applied as salts 

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

 or 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-water united with calcium. The addition of 

 marl, chalk or lime may thus greatly increase the reten- 

 tive power of a soil for bases. 



The 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 dependent on immediate 

 supplies of manure. 



There can be little doubt that the active plant food con- 

 tained 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, hy- 

 drous silicates, and humus. Different crops have very diff- 

 erent powers of attacking these various forms of plant food. 



