Phosphate sorption in relation to aluminum and iron oxides of oxisols from Ghana

Phosphate (P) sorption characteristics of six natural Ghanaian Oxisols, selected because of their hydrological and topographical suitability for agriculture, were evaluated. Availability of P appears to be adequate for half of the soils as suggested by the Bray P1 test and determination of the standard P requirement (SPR), i.e., the amount of P sorbed at a concentration of 0.2 ppm P (6.46 mu M). The SPR was found to be very closely related to P-max (Langmuir P sorption capacity), which in turn, was significantly correlated with oxalate-extractable aluminum (Al) (Ale) and iron (Fe) (Fee) and related (not significantly) to the difference between dithionite-citrate-bicarbonate-extractable Fe (Fe-d) and oxalate-extractable Fe. Accordingly, P-max is fairly well predicted by the model of Borggaard: P-calc=0.211.Al-o+0.115.Fe-o+ 0.05.(Fe-d-Fe-o)+0.3, except for one soil strongly enriched in Fe brides, mainly goethite. This goethite was found by X-ray diffraction analysis to consist of crystals larger than normally found for pedogenic Fe oxides. The difference between P-max and P-calc for this soil could, therefore, be attributed to the occurrence of these large Fe oxide crystals, because P sorption will decrease with increasing crystal size (decreasing specific surface area).