Phosphate Sorption-Desorption Characteristics by Magnetically Separated Soil Fractions
- M. Saying,
- A. R. Mermut and
- H. Tiessen
A soil sample from the surface horizon of a Paleustollic Torrert from southeastern Turkey was size fractionated, then subfractionated further by magnetic separation to study P sorption and desorption behavior of these separates. Citrate-dithionite (CD) extractable Fe constituents (Fed) were found effective P sorbers in silt-size particles, especially in the coarse-silt fraction (50–20 µm). A five-fold increase in P sorption was noted with an 8.5-times increase in Fed content, suggesting that, in this fraction, P sorption is largely controlled by Fe oxyhydroxides. Carbonates in sand and coarse-silt fractions as well as CD-extractable Al and Si, and oxalate-soluble Al and Si in all fractions had no significant effect on P sorption. In the coarse-clay fraction, besides the Fed content, clay minerals also affected P sorption. Desorption of P was, in general, lower in the highly magnetic fractions, and decreased with decreasing particle size. Fluctuations in P desorption among different concentrations of added P in each particular fraction cannot be explained clearly. This may suggest the possibility of P precipitation in a 24-hr experimental time period. Removal of about 2.9% Fed from the fine clay (<0.2 µm) resulted in a 22% decrease in sorption, and an increase in desorption, from 32 to 44% to 62 to 80%. The results suggest that Fe oxyhydroxides and other Fe-bearing constituents are important P sorbents not only in tropical soils, but also in soils of the temperate semiarid regions.
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