CHEMISTEY OF SOILS 107 



soluble in excess of the reagent, but when freshly pre- 

 cipitated retains both potash and ammonia. 



The retention of bases by clay, free from colloidal 

 hydrates of iron and alumina, is probably due to the forma- 

 tion of compounds with hydrated silicates, such as zeolites 

 (p. 22) or other secondary products, derived from the 

 decomposition, or partial decomposition, of minerals, e.g., 

 felspars. 



It has been shown that the natural zeolites possess 

 much the same power of absorption as artificially prepared 

 hydrated silicates, and that soils in which silicates soluble 

 in acids are most abundant have the highest absorptive 

 power. The absorptive power of clay is, therefore, gener- 

 ally attributed to the presence of these substances. 



If the absorption of potash by aluminium hydrate be 

 due to the formation of aluminates, it is to be expected that 

 kaolin, which is a partial hydrate of alumina, 



f SiO OA1 (OH) 2 

 1 SiO OA1 (OH) 2 



and other similar compounds would exhibit perhaps in 

 modified degree a similar power of absorption. This view 

 is consistent with the fact that the removal of soluble sili- 

 cates from the soil by treatment with hydrochloric acid 

 does not destroy the power of absorbing alkaline hydrates 

 and carbonates, but, as it causes the simultaneous removal 

 of the lime, it necessarily destroys the power of absorbing 

 neutral salts. 



Humus also exhibits considerable power of absorbing 

 both potash and ammonia, but, as the humates of these 

 bases are soluble in water, it is probable that the 

 phenomena are due to purely physical and not to chemical 

 causes. Whitney has called attention to the importance 

 of this aspect of the question. He points out that clays, 

 calcareous soils and humus present an enormous extent 



