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.