It is postulated that stabilized ammonium fertilizers improve fertilizer-N utilization by crops, leading thus to higher yields with the same fertilizer rate, especially on sandy soils. However, it must be taken into consideration that in clayey soil at least a part of the NH 4 + ions may be fixed by 2:1 clay minerals, thus delaying the effect of the N fertilizer. Because NH 4 + and K + have similar size and valence properties and therefore compete for the same non-exchangeable sites of 2:1 clay minerals, we investigated the influence of time and K + application rate on both fixation and release of NH 4 + . Fixation of NH 4 + ions was higher when K + was applied after NH 4 + , while the influence of the K + application rate was less pronounced. Mobilization of non-exchangeable NH 4 + was retarded when K + was applied at the high rate after NH 4 + . At the first harvest yield formation of ryegrass was neither influenced by the amount as well as the application time of K + , because plant available N was not growth limiting, while yield of the second harvest was significantly higher with the low K + application rate after NH 4 + . After the second harvest the blocking effect of K + on the release of non-exchangeable NH 4 + was attenuated and the highest yields of the third cut were reached in the treatments with the high K + application rate after NH 4 + . Total dry matter yield was highest when K + was applied at the low rate after NH 4 + . Our results show that K + governs fixation and release of non-exchangeable NH 4 + , which should be taken into consideration when applying ammonium containing N fertilizers like ammonium sulfate, ammonium sulfate nitrate and ENTEC. Thus K + can affect N availability when N is applied as NH 4 + in both the short and long term.