Ionic Size in Relation to Fixation of Cations by Colloidal Clay

M papers have appeared in recent years on the fixation of soluble soil potassium into an unavailable form. Much doubt still exists, however, concerning the actual mechanics of the reaction by which the potassium is rendered unavailable. The present paper presents a mechanism for this fixation process, by which it is felt many of the apparently conflicting facts can be reconciled, and which appears to have possibilities beyond the problem of potassium fixation. Most of the early work on potassium fixation in the soil was concerned with the effect of calcium additions upon the soluble potassium level of the soil; widely differing results were reported. Little attempt was made to explain the process until Maclntire (7) proposed that lime affected the hydrolysis of insoluble potassium minerals in the soil. Many investigators have considered the potassium to be released from the minerals or soil complex, notably Magistad (8), and more recently Bray and DeTurk ( I ) , who studied the equilibrium between the soluble and unavailable forms. Peech and Bradfield (9), as well as Jenny and Shade (4), showed that the action between calcium and potassium followed the usual base exchange relationships. The latter investigators suggested a fixation into organic combinations and observed a masking effect of organic colloids over exchange positions on the clay minerals which should contribute to exchange. Volk (n) showed that drying could cause fixation, as was later confirmed by Chaminade and Drouneau (2) and Gorbunov (3). Volk also showed that fixation was confined to the colloidal fraction and proposed that the potassium was fixed into muscovite. Chaminade and Drouneau suggested that the potassium must go into the crystalline nucleus as fixed potassium, since it was liberated by grinding. Gorbunov could not find the seat of fixation by X-rays and proposed that drying destroyed the diffusion layer of cations, resulting in selective retention of potassium. Joffe and Kolodny believed that potassium fixation was correlated with soluble phosphate. Volk, Gorbunov, Shaw and Maclntire (10) all showed that lateritic colloids did not fix potassium. The following working hypothesis was visualized as a possible means of explaining these apparently divergent opinion: The potassium fixation caused by drying should be related to the ionic size of potassium and the contraction of the expanding lattice of the montmorillonite type of minerals. The expanding lattice type of minerals is prevalent in the humid graybrown and podsolic soils which fix potassium readily, whereas the predominating mineral type in the lateritic soils which do not fix potassium is the nonexpanding kaolinite type. As is well known, the montmorillonite type minerals have variable ooi spacings depending _ upon their water content, the sheets being far apart when fully hydrated and almost in contact when dehydrated; the distance between the sheets depends upon the degree of hydration. It is thought that the greater part of the exchangeable ions are held between these sheets by charges arising from substitutions within the lattice. Under conditions of maximum hydration the sheets are far apart and should have little effect upon free exchange, but upon dehydration the free space between the sheets is greatly reduced and should hinder exchange of the adsorbed ions. The dehydration process is usually thought of as being quite reversible so that the sheets respread when water is added, thus again leaving the adsorbed ions free to exchange. However, because of the ionic size of potassium and the structure of the minerals it appears likely that K-saturated systems should form a particularly stable structure upon dehydration, which would not swell on remoistening, thus leaving the potassium effectively trapped or fixed. It is generally accepted that the exposed surface between the sheets of the expanding lattice type mineral consists of a layer of oxygen ions, arranged hexagonally. The empty spaces in the center of the hexagons are the size of oxygen ions—2.8 A. The diameter of the potassium ion is 2.66 A so that it should fit quite snugly in this space. As a potassium system is dehydrated the sheets will contract and the potassium will lose its hull of oriented water molecules thus becoming effectively smaller. As the process goes to completion it appears probable that the contracting sheet might force the ion into the free space in the exposed surface. Once in this position it should be held very tightly because (a) the potassium ion is