Highlights from Minnesota’s Updated Corn Fertilizer Guidelines

By Dan Kaise

An update was made to our corn fertilizer guidelines this spring. Here are a few things to note regarding changes to the guidelines:

1. Nitrogen: The U of M guidelines use the Maximum Return to Nitrogen (MRTN) approach, which can be updated yearly. The guidelines presented in the 2020 update represent data collected through the 2018 growing season. We will be looking to add new data to the calculator following the 2020 growing season.

2. Phosphorus: The only change made to the phosphorus guidelines was the inclusion of a recommended strategy for use in a build-and-maintain approach. This approach aims to reduce the risk of soil test values increasing to a point where the risk of P loss increases. We realize that many growers are using a maintenance-based approach, and for owned ground there can be some positives for a maintenance-based approach. We recommend that you use a soil test range near the optimal soil test category and use a low input, starter approach when soils are above critical soil test levels if phosphorus is going to be applied.

3. Potassium: Changes were previously made to the corn K guidelines which adjusted the soil test ranges for the current guidelines. We are in the process of further evaluating the guidelines, which may result in some changes. One ongoing study is taking a closer look at soil chemistry to determine whether changes to the K guidelines need to be made.

4. Sulfur: The corn sulfur guidelines were further updated to reflect situations in poorly drained soils where responses have been more common. We currently have a number of trials in place studying different sources of sulfur, and the results are intriguing. New data will be included in future posts, so stay tuned.

5. Micronutrients: Zinc is still the micronutrient that is most likely to be deficient for corn production. We have looked at other micronutrients like boron but have not found widespread deficiencies. The boron data was summarized in a previous blog post. One thing I stress to many growers is to not neglect N, P, K or S in lieu of micronutrients. Shorting the crop on macronutrients can have a greater impact on yield than micronutrients.