n6 Mineral Nutrition of Plants 



terminology, on ion redistribution within intermingling electric double 

 layers (Figures 3 and 4). Accordingly, the contact theory embraces all 

 colloidal systems, or all ionic surfaces, not only soil colloids and plant 

 roots. 



To illustrate the modes of transfer of cations from one negative 

 surface to another let us add radioactive K*-colloid to a sol containing 

 nonradioactive potassium colloid. The two possible types of mechanisms 

 of cation transfer occurring in the mixture may be symbolized as 

 follows: 



Initial state: 



Intermedi 

 ate states: 



J 



\ 



Final state: 



At equilibrium both particles contain one K*. Analysis of the initial 

 and final states will not tell which mechanism was operative in the 

 transfer of K* from one particle to the other. Of course, if the colloidal 

 particles are prevented from coming close together, only the solution 

 exchange can explain the transfer. 



Whenever it is impossible to rule out experimentally one of two 



