60 THE REACTIONS OCCURRING IN SOILS. IV. 



particles of the soil, an action similar to that exerted by char- 

 coal towards salts in solution. 



With the potassium and ammonium salts and with phos- 

 phoric acid, however, the case is very different. In addition 

 to the physical absorption or adhesion alluded to (adsorption), 

 there is a chemical retention exerted by certain constituents 

 of soils for these substances. It can be shown by direct 

 experiment that dilute solutions of potassium or ammonium 

 salts or of phosphates, if filtered through a sufficiently thick 

 layer of soil, are robbed of some of their constituents. In 

 most cases the acids of the ammonium or potassium salts are 

 found in the filtrate in combination with calcium. 



This retentive power is apparently to be attributed mainly 

 to the presence of hydrated silicates analogous in composition 

 to the minerals known as zeolites (so called from their frothing, 

 due to evolution of steam, when heated on platinum wire in the 

 blow-pipe flame). It is, however, extremely unlikely that frag- 

 ments of such minerals should exist in the soil, as they are, as 

 a rule, easily decomposable and not very abundant in rocks. 

 It seems more probable that the retentive substances are tran- 

 sition bodies produced in the weathering of such silicates as 

 felspar, mica, &c., and consequently that they are not perma- 

 nent ingredients in the soil and are present in an amorphous 

 state. 



The phenomenon of the retention of certain substances by 

 soils was noticed by Bronner in 1836. It was investigated in 

 1850 by Way,- by Peters in 1860, by Knop in 1868, by Armsby 

 in 1877,1 and by van Bemmelen in 1878.]: 



It is found that when salts are applied to the soil there 

 is a replacement of one base by another (lime or soda from the 

 hydrated double silicate), and that the absorbed oxide is only 

 slightly soluble in water, more soluble in water containing 

 carbon dioxide, and easily soluble in hydrochloric acid. The 

 absorbed oxide can be re-exchanged by treating the soil with a 

 solution of a salt of another metal. Alkaline hydroxides, 

 carbonates, and phosphates are absorbed without any replace - 



*J.R.A.S. 1850, 313. 

 t Amer. Jour. Sci. 14, 25. J Landw. Versuch. Stat. 21, 135; J.C.S. 1878, abst. "><)X. 



