Cation fixation ammonium

Cations that are not fixed in clay lattices also tend to compete for active sites and thereby depress ammonium fixation. N5mmik (1965). states that the depressive effect is as foUows Na" < Mg < Ca < Ba < H. , ... 

Ammonium-N is a plant-available form in addition to NOs , but in normal upland soils it is converted fairly rapidly to NOa" by a series of microbial reactions collectively called nitrification (Figure 2.7). Most NH4+ fertilizers are converted to NO3" within a few weeks, but this period can vary with soil temperature, moisture, O2 availability, soil pH, and the manner in which NH4 or NH3 was applied. Soils of pH of less than 5.0 nitrify more slowly than more neutral soils. Concentrated zones of NHi" can also reduce the rate of its conversion to NO3". As a cation, NH4+ can be absorbed onto cation exchange sites and is therefore normally not subject to leaching as is NO3. In addition, NH4" can move into the interior of clay lattice structures and be protected from nitrification or plant uptake this is called ammonium fixation. While not significant on most soils, fixation can be appreciable on certain clayey soils. Since NH4+ is absorbed on exchange sites, it also does not move with water to plant roots like NOa" therefore, it is normally not as important a form of available N for upland crops as NOa". 

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