ABSORPTION BY SOIL PARTICLES 297 



of wood is desired. An acre of apple trees in bearing 

 removes as much plant-food from the soil in one season 

 as does an acre of wheat. 



Farm manure and a complete fertilizer may be used, 

 of which the constituents should be in a fairly available 

 form, as a constant supply is necessary. 



V. ABSORPTION BY THE SOIL OF SUBSTANCES 

 IN SOLUTION 



If the brown water extract from manure be filtered 

 through a clay soil not containing soluble alkalies, the 

 filtrate will be nearly colorless. Many solutions of dye 

 stuffs are affected in the same way. Solution of alkali or 

 alkaline earth salts are more or less modified by this 

 operation, the bases being retained by the soil. Thus 

 when a solution of the nitrate, sulfate, or chloride of any 

 one of these bases is filtered through the soil, a part of 

 the base is absorbed by the soil, while the acid comes 

 through in the filtrate. If these bases are in the form 

 of phosphates or silicates, not only the base is absorbed 

 but the acid as well. 



158. Substitution of bases. Associated with the 

 absorption of the base from solution, there is liberation 

 of some other base from the soil, which combines with 

 the acid in the solution and appears in the filtrate as a 

 salt of that acid. 



When absorption takes place from solution, the base 

 is never entirely removed, no matter how dilute the 

 solution may be. A dilute solution of potassium chloride 

 filtered through a soil will produce a filtrate containing 



