EFFECT OF SEQUENTIAL ADDITION OF SELECTED ORGANIC ACIDS ON PHOSPHATE SORPTION IN AN ALFISOL IN NIGER STATE, NIGERIA

Abstract

Sorption of phosphate (PO4) in the presence of low molecular weight organic ligands on tropical soils may be the rule and not the exception in rhizopheric soils. We studied the sorption of phosphate as influenced by sequential additions of selected organic acids(acetate, citrate and tatrate) in an Alfisol in Minna in Southern Guinea Savanna of Nigeria. Five levels of phosphorus (P) (0, 50, 100, 200 and 400 µg g-1 soil) with or without addition of 0.05 M concentration of acetate, citrate and tartrate were introduced into the soil in sequential order. The P was either introduced before organic acids or organic acids were introduced before P and/or were added simultaneously in a completely randomized design at three replicates. The concentration of PO4sorbed by the treatment without the introduction of organic acid (control) was significantly (P ˂ 0.05) higher than the concentration of the PO4sorbed when organic acids were added to the soils. The reduction in the amount of phosphate sorbed when organic acids were introduced to the soil was found to be dependent on the sequence of the additions and the type of organic acids used. Adding organic acids before P to the soil significantly(P ˂ 0.05) reduced PO4 sorption much more than either adding P before organic acid or adding both organic acid and P simultaneously. These findings suggest that the organic acids were probably interposed between PO4 ionsand sorption sites in the soils thus shielding PO4 ions from direct interaction or bonding with soil surface. Organic acids were possibly sorbed to sites not only specific to ligands but also to sites common to both PO4 ions and ligands in the soil. Reduction on P adsorption and increased in P availability appears to followed the order of citrate > tartrate ˃ acetate. In practice, application of these organic inputs should be done prior to planting and P fertilization and it suggested that sufficient time be allowed for the sorption of the applied organic materials before the application of the P fertilizer.