Sequential fractionation, originally developed by Hedley et al. (1982, New Phytol. 91:45-56), has been widely used for investigating the forms and transformations of soil P under various environmental conditions. In this approach, P is classified as H2O- or resin-, 0.5 M NaHCO3-, 0.1 M NaOH-, 1 M HCl-extracted P, and residual P. Where this approach considers both inorganic and organic P (difference between total P and inorganic P) to exist in water, NaHCO3 and NaOH fractions, inorganic P is assumed to only exist in the HCl fraction. This assumption has been widely accepted, leading researchers to measure only inorganic P in the HCl fraction and disregarding the possibility that organic P might also be present in HCl extracts. In this study, we determined the concentrations of inorganic P, enzymatically hydrolyzable organic P, and total P of sequentially-extracted 1 M HCl fractions in 15 soils and 8 animal manures to evaluate the validity to categorically excluding organic P components in 1 M HCl fractions in the Hedley sequential fractionation method. Our data indicate that whereas some samples did contain zero to negligible amounts of organic P in HCl fractions, other samples contained significant quantities of organic P. The concentrations of organic P were even greater than those of inorganic P in one soil and two manures. This indicates that excluding organic P determination in HCl fractions may lead to erroneous interpretation. Elemental analysis and enzymatic hydrolysis suggested that some of the organic P species in sequentially extracted HCl fractions were Ca-bound hydrolyzable organic P, implying their potential bioavailability in the long term. Determination and monitoring of HCl extractable organic P species with other sequentially extracted P forms may provide insight into the fates and interchange of P species in soil and animal manure.