5.4 Accumulation of phosphate in the soil compartment For agricultural purposes, the buffering capacity and the total plant available P pool are important. The plant available phosphate fraction consists of the P in soil solution (i.e. < 0,5 kg phosphate/ha), the reversible bound P on hydroxides of aluminium and iron (i.e. ~ 600 kg phosphate/ha) and the quasi irreversible P pool ( i.e. 2000+ kg phosphate/ha). Phosphate is taken up by the plant from soil solution, the reversible bound phosphorus acts as a buffer and replenish immediately phosphorus taken up by the plant. Quasi irreversible bound phosphate replenish phosphate at a lower rate (years). Next mineralisation of organic phosphate forms in soil enters inorganic phosphate that redistribute over soil solution and pools of sorbed phosphates. Because phosphate has a tendency to co-precipitate with various minerals and to adsorb on mineral surfaces, most of the phosphate is contained in mineral precipitate, partly decayed organic matter and adsorbed onto mineral surfaces, and will not be readily available (see e.g. FAO, 2008 or Schoumans et al., 2008). Figure 5.2 shows the changes of the phosphate pool in the soil compartment over time. The total timescale covers several thousand years, but the rate at which changes occur depends on climatic conditions. Apatite is the primary phosphate mineral, which is slowly broken down by weathering, and transformed into a fixed and occluded phosphate pool (precipitates with iron and aluminium oxyhydroxides and precipitation of calcium phosphates), an absorbed phosphate pool (absorption by different clay minerals) and an organic phosphate pool. In particular, the phosphate in the fixed and occluded pool is hardly available for plants, whereas the phosphate in the absorbed pool, and to a lesser extent the organic pool, is more readily available for plants. Figure 5.2: Overview of changes of the natural phosphate pool in the soil compartment as a function of time. The total timescale covers several thousand years (after Walker and Syers, 1976). 39 Pagina 48

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