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Phosphate Sorption-Desorption Characteristics by Magnetically Separated Soil Fractions

Dep. of Soil Science, Univ. of Çukurova, Adana, Turkey

A. R. Mermut^* and H. Tiessen

Univ. of Saskatchewan, Saskatchewan Inst. of Pedology, Dep. of Soil Science, Saskatoon, SK S7N 0W0, Canada

*Corresponding author.

ABSTRACT

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 (Fe_d) 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 Fe_d 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 Fe_d 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% Fe_d 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.

Contribution no. R654 from the Saskatchewan Inst. of Pedology, Univ. of Saskatchewan.

Received for publication October 13, 1989.