362 SOILS: PROPERTIES AND MANAGEMENT 



meter solution of 50 molecular equivalent KC1 — that 

 is, 2\ times the concentration of the former solution - 

 absorbs 2\ times as much, or 2.1 to 2.5 molecular equiva- 

 lent. This applies also to concentrations five times 

 stronger than the first mentioned above, but beyond that 

 the relation is not so simple. It serves, however, to 

 illustrate the manner in which the absorption takes place 

 from dilute solutions. 



264. Selective absorption. — A selective absorption is 

 very common, especially from solutions of salts having 

 weak acids, a greater fixation of the bases taking place 

 than of the acids. Dissociation of the salts takes place 

 in the solution, the bases being absorbed, in consequence 

 of which further dissociation occurs; and this proceeds 

 until an equilibrium is established between the absorbing 

 and combining power of the colloidal material and the 

 reverse action of the water and resulting acids. In this 

 way the absorptive power decreases as the amount ab- 

 sorbed becomes greater. 



The colloidal silicates possess the property of absorbing 

 a certain base when presented to it in solution, and con- 

 tributing in return a chemically equivalent quantity of 

 some other base. Potassium is most firmly combined 

 in the soil and most strongly withdrawn from solution, 

 with an exchange of a chemically equivalent quantity of 

 calcium, sodium, and magnesium, which passes into the 

 solution. If a soil is treated with a solution of potassium, 

 magnesium, sodium, or calcium salts of equal concentra- 

 tion, the concentration of the solution in the end is less 

 for the potassium than for the magnesium, and less for 

 the magnesium than for the sodium and the calcium, 

 because the potassium is most strongly bound in the 

 colloidal material, while the calcium and sodium are least 



