COLLOID CHEMISTRY OF THE SOIL IN RELATION TO PLANT NUTRITION 725 



between this idealized molecular unit and the actual kinetic units pres- 

 ent. If the latter are treated as immobile, then we are back at the simple 

 Donnan situation in which the soluble colloidal system forms the in- 

 ternal phase and a dilute true solution containing no colloid provides 

 the external phase in equilibrium with it. The effect of the colloid on 

 the chemical potentials of the water and of a salt can be measured, but 

 the converse problem of assessing the effects of the water and of the 

 salt on the chemical potential of the colloidal phase remains intractable. 



If no complete thermodynamic solution of this problem is avail- 

 able, can our progress be advanced by partial solutions? At what points 

 can extra-thermodynamic concepts and procedures usefully be intro- 

 duced? Fortunately, if the exchangeable cations are treated as inde- 

 pendent species, several approaches are possible. Their consistency 

 one with another and with the thermodynamic results on complete 

 molecules can be experimentally tested. 



This is not the place to go into details of the involved contro- 

 versies that raged a few years ago regarding the significance of differ- 

 ent types of measurement (Coleman et al., 1951; Peech ei al., 1953). 

 The problem has now been solved for dilute bentonite and exchange- 

 resin systems by careful comparison of three electrometric methods: 

 ( 1 ) determination of salt activities as outlined above, ( 2 ) conventional 

 potentiometric measurements with potassium chloride liquid junctions, 

 and (3) conductivity determinations, including variation with fre- 

 quency and cataphoresis of colloidal particles (Deshpande and Mar- 

 shall, 1959). It is clear that the older classical interpretation of the 

 second method as affording a valid measure of ionic chemical potential 

 still stands. This is of particular importance in considering the soil as a 

 source of cationic nutrients for plants. It means that we can obtain a 

 solution as regards that part of the problem which most concerns us, 

 since plants do not take up soil colloids congruently in solution. 



Ionic bonding and its effects 



As the author has shown, the situation of a given cation in relation 

 to the colloidal exchanger can usefully be expressed in free-energy 

 terms (Barber and Marshall, 1951, 1952; Marshall, 1951), since soil 

 colloids are commonly only partly dissociated. They can thus be con- 

 sidered as exerting a bonding energy toward the cation. Numerically 

 this is the difference in free-energy status between the cation as it exists 

 in the soil or suspension and the corresponding value for a completely 

 dissociated ion. If c is the total concentration of exchangeable cation 

 and a is the activity, then the mean free bonding energy is RT In c/a. 

 This quantity is a variable property of the colloidal system as a whole. 

 It reflects changes in concentration, in the degree of saturation with a 



