High Yielding Organic Crop Management Decreases Plant‐Available but Not Recalcitrant Soil Phosphorus

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 and herbicides whereas the Organic received no inputs other than a one‐time application of manure compost. The modified Hedley sequential P extraction procedure revealed organic management to have lower concentrations of readily available P than conventional but recalcitrant forms were similar between systems. The Prairie had P concentrations similar to conventional in all forms. Estimated cumulative P balance indicated that organic grain‐only rotations compared to conventional had low P removal resulting in slightly lower concentration of available‐P forms. The high yielding and P removal rotation of forage‐grain decreased available‐P forms to below an agronomic response threshold. Only high yielding, high P export organic rotations are a concern for developing P deficiency depending on initial reserves and the length of time without additional inputs.