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Phosphate and Potassium Decline

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Rethinking P & K Fertility.

IPNI soil fertility

Soil surveys conducted in a number of provinces across Canada are revealing a general decline in soil P and in potash. This has come as a surprise to many growers that have fertilized based on the ‘replacement approach’ of applying amounts of nutrients removed by the crop. Growers and agronomists also have questioned the sufficiency approach recommended by many government extension services. Rightly so, crop yields are much higher than when many of these fertility research trials were done. Then came the ah-ha moment.

Provincial sufficiency approach does not include an adjustment for current or expected yields. The sufficiency approach is based on many years of university research that produced yield response curves for P & K for various soil test levels (figure 1).  Fertilizer recommendations were then based on the most economic rate. The question became if a ‘Build and maintain’ approach which aims to build/drawdown soil test levels to an appropriate level would produce greater returns than other approaches. The ah-ha moment came after research and on-farm field trials revealed that to optimize crop yields one needed both sufficient P & K fertility in the soil AND sufficient amounts of applied fertilizer. There is now a growing body of evidence from research and field trials supporting this build and maintain approach.

In Ontario a series of fertility field trials in corn-soybean-winter wheat rotation has contributed to a paradigm shift in thinking occurred from. These trials showed the highest yields of corn, soybeans, and winter wheat were achieved on ‘built soils’ compared to those with low background fertility even with high P & K rate applied.  On built soils, corn and wheat both responded to starter fertilizer. To read the full report click here: Phosphorus & Potassium Management in Corn, Soys, Winter Wheat (Ontario review)

The new ‘build and maintain’ approach is now being referred to as the long-term sustainability approach that addresses P & K that addresses declining P & K levels, protection of the environment and optimizes crop economic returns. In Manitoba, a review of Phosphorus Fertilization Strategies addressing declining P levels, optimum soil P levels for optimum economic yields, and environmental concerns. Read the full report here: Phosphorus Fertilization Strategies for Long Term Agronomic & Environmental Sustainability (Manitoba review)

The latest survey of soil test levels in North America lead by the International Plant Nutrition Institute (IPNI) was conducted in 2015 and showed different trends, averages, and critical levels in western vs eastern Canada. An interactive tool for displaying soil nutrient levels is available on IPNI website here: STS Soil Test Levels

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