American Society of Agronomy

Soil phosphorus in an organic cropping system


Source: American Society of Agronomy

Phosphorus is a nonrenewable resource, raising concerns that agricultural practices may deplete reserves. (For one overview discussion of phosphorus, see Phosphorus Famine: The Threat to Our Food Supply in the June 2009 Scientific American.)

Organic farming with low phosphorus  inputs can result in deficient levels of plant-available phosphorus (available-P).A group of researchers from Canada set out to determine if common organically managed rotations are depleting phosphorus reserves or if large reserves still exist in unavailable forms.

The difficulty of replacing phosphorus is one of the major concerns in organic systems. Without added phosphorus from external sources, phosphorus reserves will eventually be depleted. However, the onset of phosphorus limitation in organic systems depends on four major factors: (i) the initial size of phosphorus reserves, (ii) the rate of phosphorus export through crop harvests, (iii) the ability of plants to access unavailable soil phosphorus reserves, if present, and (iv) the rate of replacement of phosphorus exported from the system.

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 phosphorus extraction procedure revealed organic management to have lower concentrations of readily available phosphorus than conventional but recalcitrant forms were similar between systems. The Prairie had phosphorus concentrations similar to conventional in all forms.

Estimated cumulative phosphorus balance indicated that organic grain-only rotations compared to conventional had low phosphorus removal resulting in slightly lower concentration of available-P forms. The high yielding and phosphorus removal rotation of forage-grain decreased available- phosphorus forms to below an agronomic response threshold.

The researchers state that only high yielding, high phosphorus export organic rotations are a concern for developing phosphorus deficiency depending on initial reserves and the length of time without additional inputs.

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