Although low solubility and slow cycling control P circulation in a wide range of ecosystems, most studies that evaluate soil P bioavailability use only indices of short-term supply. The objective here is to quantify changes in P fractions in an Ultisol during the growth of an old-field pine forest from 1957-2005, specifically changes with fractions of organic P (Po) and inorganic P (Pi) associated with Fe- and Al-oxides and Ca compounds. Changes in soil P were estimated from archived mineral-soil samples collected in 1962 shortly after pine seedlings were planted and on six subsequent occasions (1968, 1977, 1982, 1990, 1997, and 2005) from eight permanent plots and four mineral-soil layers (0-7.5, 7.5-15, 15-35, and 35-60-cm). Despite the net transfer of 82.5 kg ha-1 of P from mineral soil into tree biomass and O horizons, relatively labile P in mineral soils was not diminished, as indexed by anion-exchange resins, and NaHCO3 and Mehlich-III extractants. An absence of depletion in labile P fractions masks the major restructuring of soil P chemistry driven by ecosystem development. During 28 years of forest growth, Po and Pi significantly and substantially decreased in fractions associated with Fe and Al oxides and Ca compounds, and these decreases accounted for much of the P supplied to biomass and O horizons, and apparently for buffering labile soil fractions as well. Changes in soil P are attributed to the P-sink strength of the aggrading forest (at 2.9 kg ha-1 y-1 over 28 yr), legacies of fertilization which enriched slow cycling fractions of Po and Pi, and the changing biogeochemistry of the soil itself.