IV. THE REACTIONS OCCURRING IN SOILS 53 



was noticed by Bronner in 1836. It was investigated in 1850 by 

 Way, 1 , by Peters in 1860, by Knop in 1868, by Armsby in 1877, 2 and 

 by Van Bemmelen in 1878. 3 



It is found that when salts are applied to the soil, there is a re- 

 placement of one base by another (lime or soda from the hydrated 

 double silicate), and that the absorbed oxide is only slightly soluble in 

 w r ater, more soluble in water containing carbon dioxide, and easily 

 soluble in hydrochloric acid. The absorbed oxide can be re-ex- 

 changed by treating the soil with a solution of a salt of another 

 metal. Alkaline hydroxides, carbonates and phosphates are absorbed 

 without any replacement. That the zeolitic silicates are the main 

 agents in effecting this absorption is said to be proved by 



1. Soils richest in silicates soluble in hydrochloric acid, have the 

 greatest absorptive power. 



2. Soils treated with strong hydrochloric acid, which decomposes 

 the basic zeolites, possess practically no absorptive power for salts, 

 though they will still absorb, without exchange of metals, alkaline car- 

 bonates and hydrates. 



3. Powdered natural zeolites, e.g., chabazite, a hydrated silicate 

 of alumina, lime and potash, have been shown to possess the power 

 of readily exchanging their lime for other bases when treated with 

 saline solutions. 



The exact nature of the reactions by which the bases are retained 

 has not been determined, especially as the composition of the zeolitic 

 silicates in the soil is unknown. Assuming these substances to be 

 analogous in composition to a mineral, stilbite, the change with 

 potassium sulphate would be thus represented : 

 Al 8 8 .Ca0.6Si0 2 .5H 2 + K 2 SO 4 = Al 2 O 3 .K 2 O.6SiO 2 + 5H 2 O + CaS0 4 . 



This reaction would in no case go to completion as represented, but 

 equilibrium would be attained when the solution contained a certain 

 ratio of calcium sulphate to potassium sulphate. 4 



Hall and Gimingham 5 in 1907, investigated the retention of am- 

 monium salts by clay and humus of soils. They found, too, that the 

 reaction was one of double decomposition and that an equivalent to 

 the ammonium withdrawn, of calcium, magnesium, potassium, or 

 sodium from the zeolitic double silicates of the clay, or of calcium 

 from calcium humate, went into solution. In no case was there 

 any evidence of the formation of free acid. 



Ferric hydrate and aluminium hydrate or hydrated basic carbon- 

 ates of these metals are also present in some soils and have the power 

 of retaining potash, lime and ammonia, and, to a still greater extent, 

 phosphoric acid (see footnote on page 52). The bases, however, are 

 absorbed without replacement and can be removed by prolonged wash- 

 ing with water. The absorption may possibly be due to the weak 



1 Jour. Roy. Agric. Soc., 1850, 313. 



2 Amer. Jour. Sci., 14, 25. 



;; Landw. Versuchs. Stat., 21, 135 ; Jour. Cheni. Soc., 1878, Abstracts, 598. 

 4 See App3ndix to this chapter for further explanation of such reactions. 



3 Jour. Chem. Soc., 1907, Trans. G77. ' 



