Addressing imbalances in phosphorus accumulation in Canadian agricultural soils


Reid, K., Schneider, K., Joosse, P. (2019). Addressing imbalances in phosphorus accumulation in Canadian agricultural soils. Journal of Environmental Quality, [online] 48(5), 1156-1166.

Plain language summary

Phosphorus is an essential element for all life, and an adequate supply is needed in the soil for optimum crop production. Too much phosphorus, however, can lead to algae blooms in ponds and lakes if it runs off into surface water. Ideally, phosphorus additions to the soil would match pretty closely with the amount removed with crop harvest, so the soil reserves of phosphorus stay in an adequate range. About 40% of the agricultural land in Canada appears to be meeting this target, but half is falling short of this target while 10% has far more phosphorus in the soil than crops need. Unfortunately, the high phosphorus areas continue to build while the low phosphorus areas are further depleted. These areas are not evenly spread across the country, with most of the deficient areas in the prairies while the excessive areas are in pockets scattered across the country but most concentrated in BC, southern Ontario, southern Quebec, Nova Scotia and Prince Edward Island. Most of the surplus phosphorus in the areas of high accumulation comes from livestock manure. Much of the imbalance can be corrected with better nutrient management, but in some cases redistributing the livestock to different areas will be the most effective technique.


Adequate phosphorus (P) is needed for crop production, but excessive P poses a potential risk to water quality. Results from the cumulative P balance calculations within the indicator of risk of water contamination by phosphorus (IROWC-P) developed in Canada were assessed to determine the spatial and temporal trends in P accumulation at a regional scale and to consider the implications of these trends. Regional cumulative P balances were calculated from census data as a proxy for soil test P (STP) values, including the contribution of fertilizer or manure P to these balances. Ideally, over time we would see a convergence of soil test values at the low end of the critical response range for crop growth, where agronomic and environmental considerations are balanced, but this does not appear to be the case for many regions in Canada. Nationally, about 61% of agricultural land was predicted to be low in STP, and over half of this land is failing to replace the P that is removed each year. While only about 10% of the agricultural land has accumulated significantly more P than is needed for crop growth, almost all of this land is continuing to accumulate P rather than drawing it down. Manure is the dominant P source for continuing accumulation in regions with high or very high estimated STP; reducing this input will be difficult because of the nature of manure and the investment in buildings and infrastructure tied to specific locations, but it is clear that current Canadian policies need strengthened.

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