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Phosphorus is a nonrenewable resource, raising concerns that agricultural practices may deplete reserves. Organic farming with low P inputs can result in deficient levels of plant‐available phosphorus (available‐P). The purpose of this study was to determine if common organically managed rotations are depleting P reserves or if large reserves still exist in unavailable forms. The research was performed in the 13th year of the Glenlea Long‐term Crop Rotation and Management study in southern Manitoba. The site has three 4‐yr rotations under Organic and Conventional management: spring wheat (Triticum aestivum L.)‐alfalfa (Medicago sativa L.)‐alfalfa‐flax (Linum usitatissimum L.) (forage grain) with and without manure compost, and spring wheat‐field pea (Pisum sativum L.)‐spring wheat‐flax (grain only), as well as a restored prairie grass planting (Prairie). Conventional treatments received synthetic fertilizers …