Effect of soil–dung manure incubation on the solubility and retention of applied phosphate by a weathered tropical semi-arid soil

The combined application of organic residue and inorganic phosphate fertilizer has been suggested as a means of increasing the availability and solubility of applied phosphate (PO4) in weathered soil. We studied the impact of dung manure on the solubility and retention of applied PO4 in a tropical semi-arid soil under laboratory conditions. Surface and subsurface samples, collected from a profile pit, were incubated with varying rates of dung manure (0–10%) for 14 days. Increasing dung rate increased resin, bicarbonate (–HCO3) and hydroxide (–OH) extractable PO4 in all soils. But PO4 sorption, determined by a single point sorption method, decreased with increasing dung rate. Desorption studies showed strong recovery of sorbed PO4 by ion exchange resin, which was significantly greater than PO4 recovered in –HCO3 and –OH extracts. In a bid to determine the mechanism by which dung manure inhibited PO4 retention and increased its solubility, a separate sorption experiment in which humic substances (HS) extracted from the dung manure were added to the soils in sequence with PO4 was conducted. Adding HS before PO4 to the soil reduced PO4 sorption maxima and binding strength more strongly than either adding PO4 before HS or adding HS and PO4 simultaneously. We, thus, concluded that HS was probably interposed between PO4 ions and sorption sites in the soils, thus shielding PO4 ions from direct interaction or bonding with soil surface, hence it was possible to recover sorbed PO4 in soils incubated with dung manure largely by ion exchange resin.