THE COLLOIDAL PROPERTIES OF SOIL 157 



action the amount of base brought out is equivalent to the 

 acid set free, i.e. to the amount of base adsorbed by the soil. 



Thus the modern position is essentially that of Knop, but 

 the idea has been expanded, and, above all, the phenomena have 

 been connected up with a wide range of others. 



Miyake's experiments (202«) indicate that the rate of ab- 

 sorption of the ammonium ion by soil follows the ordinary time 

 rate for the diffusion of liquids into absorbing substances, viz. 

 X = K/"* where x = the amount absorbed, / = time and 

 K and m are constants. The rate, however, is affected by the 

 presence of other ions. 



The absorption of potassium by soil has been studied by 

 Schreiner and Failyer,^ by Patten and Waggamann - and re- 

 cently by McCall, Hildebrandt and Johnston.^ 



The Action of Dilute Acids on Soils. 



It will be shown later that the reaction between dilute 

 acids and soil is of very great importance to the soil chemist 

 in enabling him to form some estimate of the amounts of 

 the mineral plant nutrients present. The reaction ought in 

 principle to be simple, but numerous investigations by Hall 

 and Amos (120^), de Sigmund and others, have shown that 

 it is not, and that it falls quite out of line with the ordinary 

 chemical reactions. In particular different acids, even at 

 equivalent concentrations, do not dissolve out the same amount 

 of material, nor can any connection be traced between the 

 "strength" of the acid and the amount of its action. 



Russell and Prescott (241^) have studied the reaction 

 between dilute acids and the phosphates in the soil and find 

 that it can be interpreted as a simple solvent action followed 

 by an adsorption. 



When a soil is acted upon by a dilute acid the amount of 

 P2O5 dissolved is found to increase continuously with the 

 concentration of the acid. The curves obtained for different 



^ Bull. 32, U.S. Bureau of Soils, 1906. 

 "^Bull. 52, U.S. Bureau of Soils, 1908. 

 ^Journ. Phys. Chem., 1916, 20, 51-63. 



