Blocking the release of potassium from clay interlayers by small concentrations of
NH4+ and Cs+ |
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Authors: | G Springob |
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Institution: | Institut für Bodenkunde der Universität Hannover, Herrenhäuser Strasse 2, 30419 Hannover, Germany |
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Abstract: | 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 NH4+ and Cs+, can reduce the release rates of K+ which is exchanged by Ca2+, even if these monovalent cations are present in concentrations of only a few μm . Percolation experiments were carried out with different illitic soil materials, some containing vermiculite, with 5 m m CaCl2 at pH 5.8 and 20°C, in some cases for over 7000 h. NH4+ and Cs+ both caused a large decrease in the rate at which K+ was released, Cs+ especially. Suppression began at 5 μm NH4+ Blocking by 20 μm NH4+ was easily reversible: the release rates readily increased when NH4+ was omitted from the exchange solution. Blocking by 2 μm Cs+ was equal to approximately 90% of that at 10 μm Cs+. Larger concentrations of Cs+ than 10 μm 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 CaCl2. The blocking effect was reproduced in several different soil materials using 10 μm Cs+ but was most pronounced in vermiculite-rich samples. As NH4+ is present in most arable soils, at least in concentrations of a few μm , 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 Ca2+ 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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