Phosphorus Adsorption and Fractionation in a Two-phase Olive Mill Waste Amended Soil

Two-phase olive mill waste (TPOMW) is a by-product of olive oil extraction, contains up to 90% organic matter, and may be used as soil amendment. To investigate the impact of TPOMW amendments to soil on the fractionation, adsorption, and buffering capacity of P, experiments were conducted on soils collected from a typical olive grove amended for 5 yr with 0, 30, and 60 Mg ha-1 of TPOMW. A modified Hedley fractionation scheme evaluated the change in soil P forms while surface models were used to fit the data. The P fractionation results suggested and increase in HCl-P fraction. Other P pools, such as residual-P, NaHCO3-Pi, NaOH-Pi, NaOH-Po, soluble-P, and NaHCO3-Po increased to lesser degrees. A two-surface Langmuir model fit the adsorption data better than a uniform layer model and underestimation of the P adsorption was observed when the traditional Langmuir equation was used. The TPOMW amendments significantly decreased the P adsorption maxima ( = 0.05) and most of the P was adsorbed on low energy sites, independent of the amendment applications received. The TPOMW soil application decreased indices of P-binding intensity at both high and low affinity sites, decreased the P equilibrium buffering capacity, and increased the equilibrium P concentration (EPC). Coupled with the low energy of P binding, results implied an increase in the readily soluble P, and thus runoff P losses could potentially increase in TPOMW amended soil.