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DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Extractable Iron and Aluminum Effects on Phosphate Sorption in a Savanna Alfisol

Dep. of Soil Sci., Faculty of Agriculture, Ahmadu Bello Univ., PMB 1044, Zaria, Kaduna State, Nigeria

^* Corresponding author (joagbenin{at}yahoo.com)

Phosphate management problems in weathered tropical soils are mostly associated with the abundance of free Fe and Al oxides. The objectives of this study were to determine the effect of dithionite- and oxalate-extractable Fe and Al on P sorption in a savanna Alfisol. Twelve pedons from four cultivated fields and three pedons from an uncultivated natural site were examined for profile distribution of dithionite- and oxalate-extractable Fe and Al. Sixty samples, consisting of surface and subsurface samples, were chosen from the pedons to determine P sorption. Dithionite-extractable Fe and Al (Fe_d and Al_d) and oxalate-extractable Al (Al_o) increased with soil depth except for soils from one field, while oxalate-extractable Fe (Fe_o) decreased from the surface up to 20 cm depth, and thereafter remained constant with depth. The soils had low P sorption capacity. Phosphate sorbed ranged from 103 mg kg^-1 in the surface soils to 460 mg kg^-1 in the subsurface soils representing between 6 and 29% of applied P. Phosphate sorbed was linearly related to Fe_d (r² = 0.71), Al_d (r² = 0.69) and Al_o (r² = 0.52), and was unrelated to Fe_o. Stepwise regression indicated that 73% of the variability of P sorbed by the soils was explained by Fe_d and Al_d. The coefficients of the multiple regression model indicated that a unit change in Al_d concentration changed P sorbed by 74 mg kg^-1 as compared with 21 mg kg^-1 by Fe_d. Since Al_d is Al³⁺ substituted isomorphically for Fe³⁺ in crystalline Fe oxides in soils, the degree of this substitution appeared to have a profound effect on P sorption and fertility of savanna Alfisols.

Abbreviations: Al_d, dithionite-extractable Al • Fe_d, dithionite-extractable Fe • Al_o, oxalate-extractable Al • Fe_o, oxalate-extractable Fe

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