ABSORPTION AND RETENTION I J .Y SOILS. 61 



merit. That the zealitic 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 decom- 



poses the basic zeolites, possess practically no absorptive 

 power for salts, though they will still absorb, without 

 exchange of metals, alkaline carbonates and hydrates. 



(3) Powdered natural zeolites, e.g., chabazitc, a hydrated 



silicate of alumina, lime, and potash, have been shown 

 to possess considerable power of 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 composi- 

 tion of the zeolitic silicates in the soil is unknown. Assuming 

 these substances to be analogous in composition to a mineral 

 stilbitc, the change with potassium sulphate would be thus 

 represented : 



+ CaS0 4 . 



This reaction would in no case go to completion as repre- 

 sented, but equilibrium would be attained when the solution 

 contained a certain ratio of calcium sulphate to potassium 

 sulphate/ 1 ' 



Ferric hydrate and aluminium hydrate or hydrated basic 

 carbonates 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 foot-note on 

 pa^e 59). The bases, however, are absorbed without replace- 

 ment and can be removed by prolonged washing with water. 

 The absorption may possibly be due to the weak acidic 

 properties possessed by ferric oxide and alumina, leading to 

 the formation of salt-like compounds analogous to spinel, 

 MgO.AlA. 



It must be remembered that complete absorption never 

 occurs, but that a small proportion of the substance always 

 remains in the solution. 



See Appendix to this Chapter for further explanation of such reactions. 



