J. A. Prescott 115 



volume of solution, and (2) a solution is filtered through a column of 

 soil. The first is by far the more accurate although there are occasions 

 when the second is useful. 



The effect of bringing a soil into equilibrium with a solution of 

 potassium or ammonium salts is that some of the potassium or 

 ammonium disappears from the solution, and other bases, chiefly 

 calcium and magnesium, come in, which together are approximately 

 equivalent to the potassium or ammonium absorbed. This was 

 discovered by Way and has since been abundantly confirmed. 

 E. G. Parker^ states that the total quantities of Ca, Mg and Al in a 

 solution of potassium chloride after treatment with a soil were not quite 

 equivalent to the amount of potassium absorbed but that the difference 

 was practically equivalent to the amount of free acid present in the 

 solution. 



The effect of the presence of other salts on the absorption of any 

 particular base was studied by A. Frank^. A solution of potassium 

 chloride was caused to percolate through a column of soil, the effluent 

 was drawn off at various depths and the amount of potassium absorbed 

 was determined. Sodium chloride was then added to the potassium 

 chloride solution and a fresh percolation carried through; it was 

 found that the presence of sodium salt lowered the absorption of the 

 potassium which was consequently found in high concentrations even 

 in the lower parts of the soil column. 



The absorbed potassium or ammonium was found to be partially 

 removed by a solution of a calcium salt but none of the early workers 

 were able to obtain complete replacement^. 



As pointed out above, Liebig considered that the absorbed bases 

 of a soil were those immediately available to the plant. W. Knop* 

 devised a method for determining the available plant food based 

 on this assumption. He put the matter thus: "Soils of great 

 fertility have a high absorptive power": "Soils of great fertility 

 have a high content of easily replaceable bases" and on this account 

 should show high absorption towards salt solutions. He therefore 

 compared a series of 100 soils by determining the absorption of ammonia 

 from a 0-5 % solution of ammonium chloride. The results obtained 

 were in fair agreement with the agricultural history of the soils. 



1 Journ. Agric. Besmrclu 1913. 1, 179 



2 Landw. Versuchs-Stat. 1866. 8, 45. 



^ Way, Eichhorn, Dietrich, Peters, Schiihmacher. References are given elsewhere. 

 * Bonitirung der Ackererde, 1871. 



8—2 



