Abstract: Soil vermiculite clays of varying tetrahedral and octahedral composition and cation exchange capacity (CEC) were examined for their ability to fix K+ in both the wet and dry states. Fixation capacity, expressed as per cent of the CEC, in the wet state was fairly high for most samples but it was enhanced greatly upon drying the K saturated samples. This enhancement indicated that each sample contained a number of vermiculite species with different CECs.
The vermiculite clays, as a group, exhibited a much higher fixation capacity at a much lower CEC than those of the coarse grained vermiculites. This enhanced fixation is believed due to the dioctahedral nature of the coarse grained vermiculites. In samples of nearly equal CECs only those containing Al3+ in tetrahedral positions exhibited an enhanced fixation capacity in the dry state but not in the wet state.
In was remarkable to find that the state of oxidation of crystal structure iron strongly affected the fixation and the CEC. Reduction of Fe3+ to Fe2+ caused a decrease in fixation even though the CEC increased as a result of this change. Conversely these reactions and their effects were found to be reversible.
The variation in the orientation of the dipole of the hydroxyl ion in the octahedral layer with respect to the cleavage plane of the crystal is believed to be responsible for some of the noted differences.