Dissolution of phosphate rocks in soils. 1. Evaluation of extraction methods for the measurement of phosphate rock dissolution

The dissolution of phosphate rock (PR) in soil was determined by measuring the amount of either the dissolved inorganic phosphate (Pi) and Ca or the residual P and Ca remaining as undissolved PR in the soil. The dissolved Pi was extracted by either 0.5M NaOH or 0.5M NaHCO₃ and the dissolved Ca was extracted using either 0.5M BaCl₂/triethanolamine (TEA) or 1M NH₄OAc. Undissolved PR was extracted by 1M HCl following the extraction with either 0.5M NaOH or 0.5M BaCl₂/TEA. From the mixture of PR with dry soil, the 0.5M NaOH extracted between <0.1 and 4.7% of the undissolved Pi and the 0.5M BaCl₂/TEA extracted between 2.8 to 7.8% of the undissolved Ca. From PR incubated with moist soil, these reagents extracted most of the dissolved Pi or Ca (95 to 99%). The 0.5M NaHCO₃ extracted between < 0.1 to 4.8% of the undissolved Pi and the 1M NH₄OAc extracted between 5.2 to 10.5% of the undissolved Ca from the PR/dry soil mixture. However the extraction of dissolved Pi and Ca from PR incubated with moist soil by these reagents was incomplete (30 to 80%). The 0.5M NaHCO₃ and 1M NH₄OAc extractants tended to underestimate the extent of PR dissolution when the levels of dissolved Pi and Ca were high. The difference between the amounts of 0.5M NaOH extractable Pi (ΔP) and 0.5M BaCl₂/TEA extractable Ca (ΔCa) in the PR-treated soil and in the soil alone gave a more accurate estimate of the extent of PR dissolution in laboratory incubated soils. However, under field conditions where the Pi and Ca released during the dissolution of PR can be removed by plant uptake and leaching, these two methods, which measure the amount of the dissolution products, may underestimate the extent of dissolution. Under these conditions, the amount of residual P or Ca extracted by 1M HCl is recommended as the best estimate of the amount of undissolved PR remaining in soil.