Ammonium fixation and release by clay minerals as influenced by potassium

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.