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.

Minnesota Long-Term Phosphorus Trial

A team of nutrient and fertilizer specialists within the University of Minnesota collaborated in long term phosphorus trials located at six locations across the main agronomic growing areas of Minnesota. Experimental sites were located at Crookston, Morris, Lamberton, Waseca, Rochester, and Becker where University of Minnesota Research and Outreach Centers were located. Initiated in the fall 2010, replicated trials were established to develop over four growing season four soil test phosphorus (STP) treatments classified as Low, Medium, High, and Very High.

In the fall 2014, each of these treatments were split into four subplots and subplots were either fertilized with phosphorus fertilizer or not over the next three growing seasons. Corn was used as the test crop in 2015 and 2016 and soybean used as the test crop in 2017. Yields comparisons were made between subplots fertilized and those not fertilizer to test crop response to applied fertilizer within each pre-established STP class. Maximum yields with each pre-established STP class were compared across classes to determine if there is a yield potential difference.

At this time, data is being fully analyzed and interpreted. Initial findings suggest response to applied phosphorus fertilizer varies somewhat among the various experimental sites, but the most consistent response to applied P fertilizer was when the established STP was Low. In some site-years, response to applied phosphorus also occurred at the Medium class and in a couple of situations in the High class. We have found no evidence that building and maintaining a high STP class, as would be the case with a Build and Maintain phosphorus management approach, will increase the yield potential compared to Sufficiency management approach.

This trial was funded by the Minnesota Agricultural Fertilizer Research and Education Council. It is 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).