ABSORPTION OF BASES AND ACIDS 45 



12. Absorption of Bases and Acids. — If a solution 

 containing phosphoric acid, potash, or ammonia 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 soil for phosphoric acid, potash, &c., is of 

 the utmost agricultural importance, as it enables soils 

 to maintain their fertility when washed by rain, and 

 permits of the economic use of many soluble manures. 

 The ingredients of the soil which exercise a retentive 

 power are the hydrates of ferric oxide and alumina, 

 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. When a solution of phosphate of calcium 

 in carbonic acid is placed in contact with an excess of 

 hydrated ferric oxide, the phosphoric acid is gradually 

 absorbed and the calcium left in solution as carbonate. 

 Hydrated alumina acts in the same manner. 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. To the 

 hydrous double silicates the permanent retention of 

 potash and other bases is chiefly due. Humus has a 

 great absorbent power for ammonia ; it also retains 

 other bases with which it can form insoluble com- 

 pounds. 



Magnesia, lime, and soda are retained by 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 



