Blocking the release of potassium from clay interlayers by small concentrations of NH4+ and Cs+

To understand the process and the kinetics of potassium release from the cl ay interlayer in natural and arable soils in more detail, I tested the hypo theses that large, monovalent cations, especially NH4+ and Cs+, can reduce the release rates of K+ which is exchanged by Ca2+, even if these monovalen t cations are present in concentrations of only a few mu m. Percolation exp eriments were carried out with different illitic soil materials, some conta ining vermiculite, with 5 mm CaCl2 at pH 5.8 and 20 degrees C, in some case s 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 mu m NH4+. B locking by 20 mu m NH4+ was easily reversible: the release rates readily in creased when NH4+ was omitted from the exchange solution. Blocking by 2 mu m Cs+ was equal to approximately 90% of that at 10 mu m Cs+. Larger concent rations of Cs+ than 10 mu m did not further reduce release but rather cause d a slight increase, probably because of enhanced exchange of K+ by Cs+ wit hout exfoliation of the interlayer space. Blocking by Cs+ was not reversibl e within > 7000 h of percolation by 5 mm CaCl2. The blocking effect was rep roduced in several different soil materials using 10 mu m Cs+ but was most pronounced in vermiculite-rich samples. As NH4+ is present in most arable s oils, at least in concentrations of a few mu m, I conclude that the observe d effects are of significance in the K dynamics processes in soils, for exa mple near the roots of plants. Further, very small concentrations of Cs+ in exchange solutions containing a large background of Ca2+ appear to be usef ul for suppressing K+ release from the interlayer in laboratory studies, pr obably without significantly altering the exchange at outer mineral surface s.