724 PLANT GROWTH AND PLANT COMMUNITIES 



cations and water can be simulated by systems comprised of ( 1 ) an 

 inert skeleton of sand and silt grains, (2) a colloidal cation exchanger 

 such as a clay, (3) electrolytes in true solution, (4) water. We shall 

 simplify this further for the moment by ignoring the skeleton, by tak- 

 ing a homionic clay, and by considering the chloride of the exchange- 

 able cation to be the sole salt in true solution: for example, potassium 

 clay, potassium chloride, and water. 



Taking the last first, the chemical potential of the water is affected 

 by both the salt and the clay, and it is accurately defined in terms of 

 its vapor pressure at a given temperature and external pressure. The 

 chemical potential of the potassium chloride can be thermodynami- 

 cally defined through the electrical potential of a cell, one electrode of 

 which is reversible to the chloride ion and the other to the potassium 

 ion. It is affected by the chemical potential of the clay and that of the 

 water. The chemical potential of the clay cannot so easily be defined. 

 To treat it as a conventional solid phase in thermodynamic terms— 

 that is, to say that it is in its standard state with chemical potential 

 zero— solves no problems and affords no insights. Ionic exchangers bear 

 little resemblance to sparingly soluble salts such as barium sulphate. 

 The equilibrium with an external true solution is only partial and op- 

 erates through diffuse ionic atmospheres and Donnan effects. The solu- 

 tion is at no point congruent in composition with the solid phase. 



Yet exchangers can exert very strong effects on the chemical poten- 

 tial of molecular species in solution. If potassium clay is added to a 

 dilute solution of potassium chloride, then the chemical potential and 

 activity of the salt can be greatly changed, as indicated in Table I. It is 

 almost as though another salt were added, and indeed it is easy to cal- 

 culate how much potassium nitrate would produce the same change. 

 Would it not be possible therefore to treat the exchanger as homo- 

 geneously distributed in solution? The molecule would then be the 

 amount of exchanger associated with one monovalent exchange site. 

 The difficulty with this approach is that there is no unique relationship 



TABLE I 



Effect of Potassium Bentonite Clay on the Activity of 

 Potassium Chloride in Dilute Solutions 



