56 Agricultural Chemistry. 



retained by its absorptive power the depletion of fertility would 

 go on at a much more rapid rate than it actually does. Most 

 soils contain substances, which have the power of uniting with 

 potassium, ammonium, and to a less extent with calcium com- 

 pounds and with phosphates, converting them into insoluble 

 forms. If a solution containing phosphoric acid, potash or am- 

 monia is poured upon a sufficiently large quantity of fertile soil, 

 the water which filters through will be found nearly destitute of 

 these substances. This retentive power of a soil is of the great- 

 est agricultural value as it enables it to maintain its fertility 

 when washed by rain and permits of the economic use of many 

 soluble manures. Ferric oxide, a common ingredient of soil and 

 one to which the red color of many soils is due, will retain and 

 fix any soluble phosphate. When a solution of phosphate of cal- 

 cium in carbon dioxide is placed in contact with an excess of 

 hydrated ferric oxide, the phosphoric acid is gradually absorbed 

 and the calcium left in solution as a carbonate. Hydrated 

 alumina acts in the same way. Ferric oxide and alumina have 

 also a retentive power for ammonia, potash and other bases, but 

 the compounds formed are more or less decomposed by water. 



The permanent retentive power of soils for potash and other 

 bases is chiefly due to the hydrous double silicates. 



Humus has a great absorptive power for ammonia. It also re- 

 tains other bases with which it can form insoluble compounds. 



Magnesia, lime and soda are retained by the soil, but in a 

 less powerful manner than are potash and ammonia. When a 

 solution of a salt of potassium or ammonium is placed in contact 

 with a fertile soil, lime will come into solution and take the place 

 of the potash or ammonia, which is by preference, absorbed. 



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 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 drain- 



