Hydrochloric Fractions in Hedley Fractionation May Contain Inorganic and Organic Phosphates

Accurately characterizing phosphorus (P) forms is a prerequisite to developing effective remediation strategies to minimize the adverse environmental impact of agricultural expansion. Modified or unmodified Hedley sequential fractionation procedures have been widely used for characterizing P forms in soil, animal manure, and biosolids. Hydrochloric acid (HCl) fractions in these procedures have often been assumed to contain no organic P. As a result, many researchers for the last two decades have measured concentrations of inorganic P in HCl fractions without measuring organic P. In this study, we measured colorimetrically concentrations of inorganic P in untreated (control) HCl fractions after 3‐phytase incubation or oxidative autoclaving of these fractions. Phosphorus concentrations in some samples were also determined by inductively coupled plasma optical emission spectroscopy. The increases in measured P concentrations, compared with the controls, were attributed to organic P in these fractions. Data for 15 soil and eight manure samples indicated that, although HCl fractions of some samples contained negligible amounts of organic P, others contained significant amounts of organic P. The concentrations of organic P were greater than inorganic P in one soil sample and two manure samples. We recommend that organic P in HCl fractions be measured experimentally to determine whether the HCl fraction contains inorganic and organic P. Identification of possible organic P species contained in HCl fractions of the Hedley sequential fractionation schemes would provide more accurate and comprehensive knowledge of the fates and interchanges of P forms in soil and animal manure under various environmental conditions.