272 University of California Publications in Agricultural Sciences [Vol.3 



accepted and has been taught and is still very largely taught in the 

 agricultural colleges today under the subject of "fixation" or "ex- 

 change" of bases in soils. The presence in the soil of double silicates 

 or zeolites was thus assumed by soil investigators, and Hilgard gave 

 the idea much prominence in connection with his methods and 

 hypotheses on soil analysis. Further, the idea served as a basis for 

 the use, by Lawes and Gilbert, of sodium sulphate and of magnesium 

 sulphate in connection with the application of fertilizers to their 

 experimental plots with the end in view of setting free potassium, from 

 its silicate combinations in the soil, for use by the plant. 



All of this has led to the statement, universally employed by authors 

 of texts on soils, that the application of lime and gypsum to soils 

 results, among other changes wrought by them, in the making "avail- 

 able" of potassium and other ions* of a similar nature. As recently 

 as 1907, Hall and Giminghanr adduced experimental evidence on the 

 interaction between clay and ammonium sulphate, which appeared to 

 mark that reaction as one obeying the mass law, thus seemingly lend- 

 ing support to the validity of Way's hypothesis. Hall and Giming- 

 ham 's evidence was soon shown by Cameron and Patten 4 to be in- 

 complete, however. They demonstrated that the mass law does not hold 

 when a wider range of concentrations than that employed by the former 

 investigators is tested and a new hypothesis was necessary to explain 

 it. This was furnished by Van Bemmelen, who proved that absorption 

 by soils was closely parallel to that by colloids which he had studied, 

 and which may be explained by the formula ?//m=/fc 1/n in which 

 y is the amount absorbed by a quantity m of the absorbent, c the con- 

 centration of dissolved substance when equilibrium is attained, and 

 K and n are constants depending on the nature of the solution and the 

 adsorbent. Such a formula has since been shown to hold for absorp- 

 tion of phosphates by Prescott 3 and for absorption of ammonium salts 

 by Wiegner. 6 In accordance with this conception of the soil as a 

 colloid-containing body, the colloidal particles possess the power of 

 holding ions which are adsorbed from salt solutions and give such ions 

 up with relative facility to new solutions containing other ions for 

 which they are substituted. 



* In this case, and throughout this paper, the term "ion" is not used in the literal sense. 

 It is not intended to convey the idea that the authors believe that a given ion by itself is 

 absorbed or set free, for we are actually inclined to the belief that in these cases, as in absorp- 

 tion by plants, not ions, but compounds, are absorbed as units. 



3 Hall, A. D., and Gimingham, C. T., The interaction of ammonium salts and 

 the constituents of the soil, Trans. Chem. Soc, vol. 91, p. 677, 1907. 



4 Cameron, F. K., and Patten, H. E., The distribution of solute between water 

 and soil, Jour. Phys. Chem., vol. 11, p. 581, 1907. 



