68 Mineral Nutrition of Plants 



a common cation (12, /]). The analogy with weak acids and bases 

 therefore breaks down. These colloidal electrolytes do not readily adapt 

 themselves to classifications based ©n the behavior of small molecules 

 in true solution. 



3. Kaolinite stands somewhat apart from the other three clays in its 

 ionizing properties. The acid clay is much less dissociated than hydro- 



TABLE III 

 Cationic Fractions Active for Four Clays at Definite Stages of Neutralization* 



*Data from references 12, 13, 15 



gen montmorillonite, hydrogen beidellite, or hydrogen illite, yet the 

 salts with monovalent cations are more dissociated. A structural reason 

 can be suggested for this difference. Kaolinite has an electrostatically 

 neutral framework, with ionization probably restricted to broken edges 

 of the sheets. The other clays have negatively charged framework struc- 

 tures, the charges being distributed over the cleavage surfaces of the 

 sheets. These charges are neutralized by mobile cations whose activities 

 we measure. 



