404 



ABSORPTION OF MINERAL SALTS 



of alumino-silicates and although of colloidal dimensions possess a definite 

 crystalline structure. 



Although it is known that certain uni- and bivalent cations, especially 

 K+ and Mg++, may occur within the crystalline lattice of the micelles of 

 colloidal clay, from the standpoint of plants a much more important role in 

 the ionic relations of soils is played by cations located at the surface of the 

 clay particles. The term "surface" as here used includes the surfaces of 

 the minute capillaries within the micelles as well as their external boundary 

 layer. The micelles of colloidal clay are almost invariably negatively 

 charged, and the cations associated with them may be regarded as occupy- 

 ing a position analogous to the ions in the outer layer of an electrical double 

 layer (Chap. V). The cations most commonly associated with the clay 

 particles of natural soils in this manner are Ca++, Mg++, K+, Na + , and 

 H + . Cations are also similarly associated with the soil colloidal particles 

 of organic origin. 



Under certain conditions some of the cations of one kind can be dis- 

 placed from the micelles by another kind of cation. For example, if a 

 neutral soil is treated with a potassium chloride solution, some of the added 

 K+ ions replace Ca++ ions associated with the clay micelles, an equivalent 

 quantity of Ca++ ions being displaced into the solution, pairing off with 

 the residual chloride ions. This reaction is depicted diagrammatically by 

 the following equation: 



Ca++-f 2K+ + 2CI- 



-f Ca++ + 2 Cl- 



Actually each clay micelle may have many cations associated with it, but in 

 the interests of simplicity the reaction has been written in terms of only 

 one adsorbed Ca++ ion. Other ions associated with micelles will be dis- 

 placed in a similar way, although the proportion of any kind of adsorbed ion 

 entering into such exchange reactions will vary greatly depending on the 

 conditions under which the exchange of ions takes place. 



The phenomenon which has just been described is called base exchange 

 or catioti exchange. Such interchanges of cations take place very rapidly 

 and are reversible. In neutral and slightly alkaline soils Ca++ is the prin- 

 cipal replaceable cation, although appreciable quantities of Mg++ may 

 sometimes also be present. The H+ ion is the principal replaceable cation 

 in acid soils, and the Na+ ion occupies a similar position in alkali soils. 

 All of the exchangeable cations are not retained by the micelles with equal 



