364 SOILS: PROPERTIES AND MANAGEMENT 



placed it in line with what was known regarding chemical 

 reactions. Zeolites were found to possess absorbent 

 properties of a similar nature toward salts in solution, a 

 characteristic of which is the substitution of bases and 

 the appearance in solution of the released base in com- 

 bination with the acid of the original salt. It was a 

 natural conclusion that true mineral zeolites exist in 

 soil and that the absorptive properties of soil are due to 

 their action. 



Many years later, when the principles of physical 

 chemistry had been applied to the study of colloids, it 

 was show^n that absorptive properties are possessed by 

 certain colloids similar to those characteristic of soils. 

 Zeolites have never actually been isolated from any soil. 

 This fact has always occasioned some doubt as to the 

 hypothesis to which their properties have given rise. 

 Colloids, on the other hand, are well known to occur in soils, 

 but the exact nature of soil colloidal matter is not well 

 understood ; consequently there is considerable indefinite- 

 ness about the extent of their absorptive function, and 

 even Van Bemmelen grants the crystalloids a part in this 

 phenomenon. 



The zeolite hypothesis furnished an explanation for 

 the form in which the available plant-food materials of 

 the soil are held. On it is largely based the idea that 

 the solution of a soil in strong hydrochloric acid repre- 

 sents the nutrients that are available to plants. The 

 silicates that go into solution are held to be the zeolitic 

 silicic acid and the bases with which it is united. The 

 fact that such treatment largely destroys the absorptive 

 properties of a soil is taken as a proof of this. It would, 

 however, answer equally well as an argument in favor 

 of colloidal absorption, as the colloidal condition of the 



