The Effect of Humus on Cationic Interactions in a Beidellite Clay

Previous studies of cationic interactions have been made for the most part using relatively pure clay minerals. This has been a logical place to start, but to be of maximum practical benefit in solving agronomic problems sooner or later the resulting principles must be projected back to the natural soils. Too little is known about the relationship between ionic activities in plant media and nutrient uptake. But certainly one would not expect close correlation between these activities in pure clay and nutrient uptake from a soil high in this particular clay mineral unless there were no other substances present which possess exchange properties capable of interfering with activities and interactions caused by the clay mineral. In this connection it is obvious that the effect of humus on the cationic activities and their inter-relationships is of utmost importance. As a follow-up to a similar study made previously with relatively pure beidellite clay, the present study has dealt with mixtures of 10 parts of the same clay to 1 part of crude humic acid. The activities of K, Na, and Ca in homoionic and in biionic systems were determined using the clay membrane electrode technique. The results obtained are summarized as follows: The exchange capacity of the clay-humic acid systems was only 73% of that of the sum of the component parts. Since the base saturations of the biionic systems were made up on the basis of the sum of the exchange capacities, the actual exchange capacities were exceeded somewhat. This relatively higher base saturation in the clay-humus systems resulted in higher activities of the cations in all cases than in the clay alone. In spite of the higher activities in the clay-humic acid systems the interactions of the cations were very similar to those in the clay alone. If the clay-humic acid systems studied can be considered as somewhat analogous to the situation in the field, this relatively small interference by the humic acid may possibly indicate that information gotten from similar studies with pure clay could be projected with some degree of assurance to a whole soil which contains a predominance of that particular clay mineral. Evidence is presented in support of the contention that the loss in cation exchange capacity as a result of interaction of the component materials was due to a loss of exchange capacity by the humic acid rather than the obstruction of exchange spots on the clay.