Agricultural soils under long-term organic management can become deficient in plant-available phosphorus (P) over time.  In an attempt to off-set P deficiencies, application of phosphate rock (PR) has been proposed; however evidence of their effectiveness is required before PR use can be widely recommended in Canada.  This field study had two objectives: to evaluate the effects of five PR of varying mineralogy and origin (Ontario, Canada; S.E. USA) on yield and above-ground tissue P (%) of a buckwheat (Fagopyrum esculentum) crop; and to determine if mulching and incorporating the residues from buckwheat grown with PR influenced subsequent soil test P (STP) and soil P flux (anion exchange) in the year following buckwheat planting, mulching and PR application.  Buckwheat was planted on four organically managed fields that were amended with PR at 100, 400, and 800 kg P ha^-1 and a control (0 P).  Two conventional field sites were included with additional soluble P fertilizer treatments (MAP and TSP).  Calphos PR significantly increased buckwheat tissue P concentrations over the control at application rates of 400 and 800 kg P ha^-1.  Pebbled PR and Calphos PR applied at 800 kg P ha^-1 resulted in tissue P concentrations equivalent to those of TSP and MAP at 100 kg P ha^-1.  At harvest, the buckwheat residue (2.7 to 3.0 t DM ha^-1) on each plot was returned to half of the plot from which it originated.  Differences in GM tissue P (%) explained 74% of the variation in cumulative soil P flux.  STP increased with Calphos applications of 400 and 800 kg P ha^-1 compared to the control (7.2, 7.7, and 6.2 mg P kg soil^‑1, respectively).  These results suggest soil P availability is improved with P-rich GM, however the effects may be too low to achieve agronomic benefits.