Absorption by Soils of Potassium. 169 



From a solution which is less than saturated, an absorbent can remove 

 a quantity of the solute which is less than its maximum absorbent 

 capacity, but which is, nevertheless, a definite amount for any particular 

 concentration. They use this quantity as a measure of the specific 

 absorptive capacity of the medium with respect to that particular 

 solution. A solution of potassium chloride brought into contact with 

 the soil will lose some of its potassium at a rate which gradually de- 

 creases until the salt reaches an equilibrium between the soil and the 

 solution. The weight of potassium chloride absorbed by 1 gram of 

 soil represents the specific absorptive capacity of the soil for that 

 concentration of the salt at that particular temperature. If conditions 

 in the soil are altered, more salt may be absorbed, or a part of the salt 

 already absorbed may go back into solution. Thus it is seen that the 

 absorption process is a reversible phenomenon. 



In summarizing his recent work at the Bureau of Soils, Parker 1 states : 



"The rate of adsorption of the chlorine ions from a solution by soils is much 

 less than of potassium ions. The selective adsorption of potassium from a 

 potassium chloride solution by a soil increases in amount with the concentra- 

 tion up to a certain point, and then remains practically constant. In general, 

 the smaller the soil particles the greater the selective adsorption of the potas- 

 sium from a potassium chloride solution by the soil." 



Williams 2 has recently called attention to some special cases of selec- 

 tive adsorption to which he has given the name " negative adsorption." 

 He cites the work of Gore 3 as the first case of negative adsorption. 

 He also calls attention to the work of Lagergren, 4 who observed that, 

 upon shaking solutions of electrolytes with charcoal or silica, the con- 

 centration of the salt solution increased instead of decreasing. Using 

 blood charcoal, Williams found negative adsorption with potassium 

 chloride at certain concentrations. Up to a concentration of 0.0563 

 gram of salt per gram of solution, the absorption was positive, becom- 

 ing negative with further increase in concentration. Gore's work, 

 cited above, gives instances of negative adsorption at low concentra- 

 tions, becoming positive in less dilute solutions of the same salt. In all 

 of the work cited the investigators have had their interest centered upon 

 the condition of the solution after equilibrium had been reached ; conse- 

 quently, the solutions were left in contact with the absorbing medium 

 for from 24 hours to several days. In the extensive studies made in 

 the Bureau of Soils, 5 United States Department of Agriculture, the 

 contact time was usually a 24-hour period. 



Throughout the work in which soils have been the absorbing medium, 

 there exists a very great deal of uncertainty concerning the fundamental 

 character of the phenomenon. The evidence from the use of chemi- 



Uournal of Agricultural Research, U. S. Dept. Agriculture, vol. i, No. 3 (1913). 



2 Trans. Faraday Society, vol. x, part 1, Aug. 1914. 



3 Chemical News, 69, pp. 23, 33, and 44 (1894). 



4 Bihang till K. Svenska Vet-Akad. Handlinger, 24, n, 4 (1898). 



5 Bul. 32 (1906) and Bui. 52 (1908). 



