PHOSPHATE SORPTION IN RELATION TO ALUMINUM AND IRON-OXIDES OF OXISOLSFROM 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 adeq uate for half of the soils as suggested by the Bray P1 test and determ ination of the standard P requirement (SPR), i.e., the amount of P sor bed 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 alumin um (Al) (Ale) and iron (Fe) (Fee) and related (not significantly) to t he difference between dithionite-citrate-bicarbonate-extractable Fe (F e-d) and oxalate-extractable Fe. Accordingly, P-max is fairly well pre dicted 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, ma inly goethite. This goethite was found by X-ray diffraction analysis t o consist of crystals larger than normally found for pedogenic Fe oxid es. The difference between P-max and P-calc for this soil could, there fore, be attributed to the occurrence of these large Fe oxide crystals , because P sorption will decrease with increasing crystal size (decre asing specific surface area).