Blocking the release of potassium from clay interlayers by small concentrations of NH 4 + and Cs +

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Summary: To understand the process and the kinetics of potassium release from the clay interlayer in natural and arable soils in more detail, I tested the hypotheses that large, monovalent cations, especially NH 4 + and Cs + , can reduce the release rates of K + which is exchanged by Ca 2+ , even if these monovalent cations are present in concentrations of only a few μ m . Percolation experiments were carried out with different illitic soil materials, some containing vermiculite, with 5 m m CaCl 2 at pH 5.8 and 20°C, in some cases for over 7000 h. NH 4 + and Cs + both caused a large decrease in the rate at which K + was released, Cs + especially. Suppression began at 5 μ m NH 4 + Blocking by 20 μ m NH 4 + was easily reversible: the release rates readily increased when NH 4 + was omitted from the exchange solution. Blocking by 2 μ m Cs + was equal to approximately 90% of that at 10 μ m Cs +. Larger concentrations of Cs + than 10 μ m did not further reduce release but rather caused a slight increase, probably because of enhanced exchange of K + by Cs + without exfoliation of the interlayer space. Blocking by Cs + was not reversible within > 7000 h of percolation by 5 m m CaCl 2 . The blocking effect was reproduced in several different soil materials using 10 μ m Cs + but was most pronounced in vermiculite‐rich samples. As NH 4 + is present in most arable soils, at least in concentrations of a few μ m , I conclude that the observed effects are of significance in the K dynamics processes in soils, for example near the roots of plants. Further, very small concentrations of Cs + in exchange solutions containing a large background of Ca 2+ appear to be useful for suppressing K + release from the interlayer in laboratory studies, probably without significantly altering the exchange at outer mineral surfaces.
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