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 NH₄⁺ ions may be fixed by 2:1 clay minerals, thus delaying the effect of the N fertilizer. Because NH₄⁺ 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 NH₄⁺. Fixation of NH₄⁺ ions was higher when K⁺ was applied after NH₄⁺, while the influence of the K⁺ application rate was less pronounced. Mobilization of non-exchangeable NH₄⁺ was retarded when K⁺ was applied at the high rate after NH₄⁺. 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 NH₄⁺. After the second harvest the blocking effect of K⁺ on the release of non-exchangeable NH₄⁺ was attenuated and the highest yields of the third cut were reached in the treatments with the high K⁺ application rate after NH₄⁺. Total dry matter yield was highest when K⁺ was applied at the low rate after NH₄⁺. Our results show that K⁺ governs fixation and release of non-exchangeable NH₄⁺, 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 NH₄⁺ in both the short and long term.