CHAPTER XIII 

 THE ABSORPTIVE PROPERTIES OF SOILS 1 



It has been known from very early times that soils were 

 able to take up and tenaciously hold such materials as salts 

 and dyes. Aristotle, for example, noticed that sea water was 

 purified when passed through sand. This capacity of soil 

 to absorb and fix, more or less completely, materials added 

 to it is called adsorption. The earliest quantitative experi- 

 ments were made by H. S. Thompson in England. He found 

 that the soil was able to absorb considerable quantities of 

 ammonia from ammonium sulfate, the acid radical being 

 liberated. The importance of absorption phenomena has since 

 attracted much attention, both from the practical and the 

 theoretical standpoint. 2 



138. Types of absorption. — Two general types of absorp- 

 tion are usually recognized, physical 3 and chemical. In the 

 former case the absorbed material is supposed to be concen- 

 trated on the surfaces of the absorbing substance, no chemical 

 reaction taking place. The absorptive capacity of charcoal 

 and cotton for dyes is a good example of such a phenomenon. 

 In many cases, however, absorption is due to chemical reac- 



1 The literature on absorption by soils is so complicated and contra- 

 dictory that only those concepts which are more or less definitely estab- 

 lished and which have a practical bearing on soil management will be 

 considered. 



2 A good review of literature will be found as follows : 



Patten, H. E., and Waggaman, W. H., Absorption by Soils; U. S. 

 Dept. Agr., Bur. Soils, Bui. 52, 1908. 



Prescott, J. A., The Phenomenon of Absorption in its Relation to 

 Soils; Jour. Agr. Sci., Vol. VIII, No. 1, pp. 111-130, Sept., 1916. 



'Physical absorption is sometimes spoken of as adsorption. The ten- 

 dency at present is toward the elimination of this term. 



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