It is well accepted that non-exchangeable NH₄-N plays an important role in the N dynamics of arable soils. However the widely-used Silva/Bremner method for determining this N fraction is very time consuming and the use of the hazardous hydrofluoric acid (HF) is indispensable. In the modification the use of HF is avoided and the quantity of analysed samples per unit of time could be increased by a factor of 2.5. After pretreating soil samples with KOBr to destroy soil organic matter soil samples are dried and the content of non-exchangeable NH₄-N is measured using a CNS analyzer. The results of the analysis of 3 out of 4 different soils show no significant differences between both methods.  

No abstract

It is well established that non-exchangeable NH 4+ is an important N source for plants.

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.