CHEMICAL CHANGES 24! 



The conclusion is reached that the absorptive power of soils 

 for bases is due to the presence of silicates which react with the 

 substance which it will absorb. The reaction is reversible. The 

 lime, soda, magnesia, and potash of simple or complex silicates 

 enters into reaction with the substance absorbed by the soil, until 

 equilibrium is reached between the solution and the reactive soil 

 particles. We can hardly expect the law of mass action to be 

 followed, for the reason that the absorbing silicates are probably 

 mixtures with different reactivity. 



Attempts have been made to explain absorption as physical ad- 

 hesion to the soil particles. While a portion of the absorbed sub- 

 stance may be held in this way in soils composed of fine particles, 

 this theory does not account for the replacement of the absorbed 

 base by other bases, or for the varying absorptive power of differ- 

 ent soils of the same physical composition, or for the loss of the 

 absorptive power of a soil by treatment with an acid, and partial 

 restoration of it by addition of calcium carbonate. 



Absorption of Phosphoric Acid. When a phosphate is brought 

 in contact with a soil, both base and acid will disappear partly 

 from solution. The base follows the laws of absorption as out- 

 lined above. The phosphoric acid follows about the same laws, 

 but the cause of the absorption is different, and is due to reaction 

 with basic substances, such as hydrated oxides of iron and alum- 

 inium, and carbonate of lime, with production of the much less 

 soluble phosphates of calcium, aluminium, and iron. It is also 

 possible that the phosphates have power to decompose some of 

 the weak silicates. Phosphates are absorbed more slowly than 

 bases. 



Changes of Phosphoric Acid in the Soil. Calcium phosphate, 

 in the presence of aluminium and iron hydroxides, has a tendency 

 to change into phosphates of these bases, which are much 

 less available to plants. The calcium phosphate dissolves, 

 and the solution reacts with the hydroxides in question, forming 

 phosphates which are much less soluble. The presence of calcium 

 carbonate will hinder the reaction, since it also will react with the 

 phosphoric acid in solution. 



