Solubility Characteristics of Residual Phosphate in a Fertilized and Limed Ultisol

Except at very low P rates, residual soil P (i.e., that measured by most extraction methods) continues to increase with annual additions of P fertilizer. Phosphorus retention mechanisms, however, are still much debated and difficult to prove or disprove. In an effort to determine whether the solubility of phosphate minerals controlled soil‐solution P concentration in the field under some conditions, samples from the Ap of a Benndale sandy loam (Typic Paleudults) were collected from plots of a field experiment with different pH and P levels and analyzed in the laboratory. Several soil pH levels were first established (5.0–8.0); this was followed by annual additions for 7 yr of concentrated superphosphate at five rates varying from 0 to 392 kg P ha −1 yr −1 and cropping annually. Soil‐solution composition was determined on samples taken 1, 3, and 5 yr after discontinuing P fertilization. Except for the “no P” treatment, solution P increased with increasing pH up to pH 5.8, then it decreased as pH increased, suggesting the accumulation of a basic phosphate mineral such as hydroxyapatite at about pH 5.8 and above. However, this was not matched by the ion activity product (IAP) for Ca 5 OH(PO 4 ) 3 or any other P mineral. For instance, at each P level, pCa 5 OH(PO 4 ) 3 decreased almost linearly with increasing pH so that each P level had one and only one pH at which pCa 5 OH(PO 4 ) 3 equalled 57.5, the accepted solubility product for hydroxyapatite. Thus, IAP's could not be used to predict, even indirectly, that solid‐phase minerals were or were not controlling solution P. Sequential equilibration of selected soil samples with 0.01 M MgCl 2 , however, provided data that strongly supports the hypothesis that hydroxyapatite was present in a “high P, high pH” soil but not in a “low P, high pH” or a “high P, low pH” soil.