C. Edmund Marshall 



7i 



bonding energies. For very small exchanges against hydrogen or other 

 cations these would behave alike, but for moderate or large exchanges 

 considerable differences would be apparent. Thus, in order to interpret 

 the behavior of soils with regard to plant growth in the field, we should 

 employ both the activities and the fraction active at various moisture 

 contents. In addition we need control experiments designed to show 



TABLE IV 



Cationic Fractions Active or Four Clays at Definite Stages of Neutralization 



Using Divalent Cations 



Note: The values for the equivalence have been arbitrarily chosen to correspond with those 

 of Table III. Actually the inflexions on the pH curves are somewhat different for 

 divalent cations than for monovalent. Data from reference j. 



how far the living root changes its external ionic environment as 

 growth proceeds. 



Evidence obtained with one monovalent and one divalent cation 



Since we have perfected membranes of two types, those sensitive 

 only to monovalent cations and those sensitive to all cations, it has 

 proved possible, within limits, to determine one monovalent and one 



