Maintaining phosphorus (P) fertility represents a challenge for organic producers in the northern Great Plains because of high pH, calcareous soils that limit P availability. Green manure (GM) crops and organic P fertilizers represent two approaches to improve soil P availability.  Our objective was to determine the effect of GM crops fertilized with rock phosphate (RP) and bone meal (BM) on P availability to a subsequent wheat crop.  During the first phase of a greenhouse study, pots (15 cm diam. X 12.5 cm) were filled with a low soil test P soil (Olsen P = 4 ug P g^-1). Buckwheat (Fagopyrum esculentum cv. Mancan), spring pea (Pisum sativum L.), spring wheat (Triticum aestivum cv. Hank) and a non-crop control (or fallow) were fertilized with RP, BM and monocalcium phosphate (MCP) at rates equivalent to 0, 10 and 25 kg ha^-1 available P. During the second phase, spring wheat was seeded 3 weeks after GM residue was incorporated. Buckwheat P uptake was enhanced (p=0.05) by all P sources and was able to use the less soluble BM equally as well as the more soluble MCP.  In contrast, spring pea P uptake was significantly higher for MCP than RP and BM, indicating it was less effective at utilizing the less soluble organic P sources.  P uptake of the wheat crop in the second phase of the study followed the order, fallow>wheat=buckwheat>spring pea.  The results of this study indicate that buckwheat has the capacity to increase P availability of less soluble organic P fertilizers and that inclusion of buckwheat may be preferential to spring pea for enhancing P availability in organic farming rotations in the northern Great Plains.