The University of Minnesota has actively engaged in developing phosphorus fertilizer recommendations for crop production systems for several decades. Two different approaches are used to develop recommendations:

  • Sufficiency approach - Soil testing results and research data indicate the likelihood the crop will respond to phosphorus fertilizer and how much to apply for optimum yield provide. This data is used to develop recommendations for fertilizing the crop to provide sufficient phosphorus.
  • Build and maintain approach – Soil is fertilized to achieve a target soil test level and then fertilized to maintain that soil test level after it is reached. Generally, there is little likelihood the crop actually responds to the phosphorus fertilizer application at the target soil test level.

In today's high-yield environment, many universities and private entities are carefully evaluating both approaches. Maintaining a high soil test phosphorus level reduces the possibility of a phosphorus-deficient crop under abnormal growing conditions. However, high soil test levels increase environmental risks, and there is little economic return from the crop for the fertilizer applied. The sufficiency approach increases the likelihood of an economic return for the fertilizer applied, but the risk of a phosphorus deficiency is increased.

Overall, greater yield potential exists with the build and maintain approach than the sufficiency approach in the current environment. Some argue the build and maintain approach provides for a greater yield potential, but data to support this hypothesis is thin or nonexistent. We have initiated long-term research to test this hypothesis.

Minnesota Long-Term Phosphorus Trial

We initiated research trials at six locations across Minnesota in the fall of 2010 to establish a range of soil test phosphorus levels for side-by-side comparisons. The probability of a crop responding to the application of phosphorus fertilizer decreases as the soil test level increases. Over four growing seasons, we established soil test levels of low, medium, high, and very high at each site.

In 2015, each soil test level was split into subplots and planted to corn. One subplot received no phosphorus fertilizer for the 2015 crop—the crop must rely on the phosphorus in the soil from the previous four years of management. Another split-plot received 150, 90, 30, and 30 pounds phosphate (P2O5Ac-1) per acre on the low, medium, high, and very high test plot, respectively.

These trials are currently in the field and will be repeated in 2016. Specifically, we are looking for:

  • What is the crop response to phosphorus fertilizer within each soil test level?
  • Whether there was a response to phosphorus fertilizer, was there a yield potential difference among the soil test levels? In other words, does a low soil test with a high rate of phosphorus fertilizer produce the same yield as a very high soil test with some phosphorus fertilizer?

The Minnesota Agricultural Fertilizer Research and Education Council funds this trial. It is a collaboration of Albert Sims, NWROC site; Carl Rosen, Becker site; Dan Kaiser, WCROC site; Jeff Strock, SWROC site; and Jeff Vetsch, SROC and Rochester sites.

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