Fixation and defixation of ammonium in soils: a review

Fixed NH 4 + (NH 4 + f) and fixation and defixation of NH 4 + in soils have been the subject of a number of investigations with conflicting results. The results vary because of differences in methodology, soil type, mineralogical composition, and agro-climatic conditions. Most investigators have determined NH 4 + f using strong oxidizing agents (KOBr or KOH) to remove organic N and the remaining NH 4 + f does not necessarily reflect the fraction that is truly available to plants. The content of native NH 4 + f in different soils is related to parent material, texture, clay content, clay mineral composition, potassium status of the soil and K saturation of the interlayers of 2:1 clay minerals, and moisture conditions. Evaluation of the literature shows that the NH 4 + f-N content amounts to 10–90 mg kg−1 in coarse-textured soils (e.g., diluvial sand, red sandstone, granite), 60–270 mg kg−1 in medium-textured soils (loess, marsh, alluvial sediment, basalt) and 90–460 mg kg−1 in fine-textured soils (limestone, clay stone). Variable results on plant availability of NH 4 + f are mainly due to the fact that some investigators distinguished between native and recently fixed NH 4 + while others did not. Recently fixed NH 4 + is available to plants to a greater degree than the native NH 4 + f, and soil microflora play an important role in the defixation process. The temporal changes in the content of recently fixed NH 4 + suggest that it is actively involved in N dynamics during a crop growth season. The amounts of NH 4 + defixed during a growing season varied greatly within the groups of silty (20–200 kg NH 4 + -N ha−1 30 cm−1) as well as clayey (40–188 kg NH 4 + -N ha−1 30 cm−1) soils. The pool of recently fixed NH 4 + may therefore be considered in fertilizer management programs for increasing N use efficiency and reducing N losses from soils.