72 Mineral Nutrition of Plants 



divalent cation in a mixture (7). This was regarded as particularly 

 important because of the vast literature dealing with potassium-calcium 

 relationship in plant nutrition. To clarify the potassium-calcium situa- 

 tion in soil colloids naturally seemed the first step. 



If each of the cations concerned were held with a single bonding 

 energy, then in a clay colloid saturated with a mixture of two cations 

 we should anticipate that the fraction active for each should be constant. 

 The activity for each cation would be directly proportional to the total 

 amount present. On the other hand, with a range of bonding energies 

 for each of the cations concerned, a variety of situations may arise, 

 depending upon their quantitative relationship. Thus the fraction 

 active of the one cation will become dependent upon the amount of 

 the second ion present, as well as on its nature. In this sense one some- 

 times refers to the effect of one ion upon the activity or the fraction 

 active of another. Actually, no direct effect of the nature of a steric 

 hindrance, etc., need be present. The mutual relationships found are 

 consequences of the energy differences of reactive spots on the silicate 

 surfaces. 



The importance of determining one cation in presence of another 

 was realized as soon as membranes sensitive only to monovalent cations 

 had been perfected. Measurements of potassium ion activities were made 

 in a series of Putnam clay systems with differing proportions of calcium 

 and potassium (12). It was concluded that in the middle range of 

 base saturation (pH 5.5-57), there was little effect of calcium on potas- 

 sium. We now know that this result is a special case and that it was 

 largely due to the particular proportions employed. In general, as we 

 shall see below, the potassium ion activity and the fraction active are 

 greatly influenced by the proportion of calcium. 



The total information on these relationships is now considerable, 

 the following list of clay minerals having been investigated; two ben- 

 tonites, Putnam clay (beidellite), two illites, halloysite, kaolinite, and 

 attapulgite. One of the bentonites was reported on by McLean and 

 Marshall (7), the other systems have been investigated here by S. A. 

 Barber (Ph.D Thesis, 1949). The complete results, including studies of 

 magnesium-potassium relationships by E. O. McLean, will be pub- 

 lished elsewhere. 



