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Influence of Phosphorus and Potassium Applications in a Multi-Year Spring Wheat-Soybean Crop Rotation

Study author(s): Dr. Dave Grafstrom, UMN, and Melissa Carlson, MN Wheat On-Farm Research Network
Years of study: 2017-2022
Location(s): Northwestern MN, Roseau, MN

Important: for the complete report, including all tables and figures, please download using the links to the right.

Read Full 2022 Report
Read Full 2023 Report


In 2022, no differences were detected from any of the fertility treatments compared to the untreated in wheat or soybeans. These results were contrary to those obtained in 2021 which suggested that the combination of P and K produced more wheat yield than the single products alone, especially at 40, 60, 80 and 100 units of each product. A possible explanation for the differential response of wheat to the fertility treatments between 2021 and 2022 can be found in the North Dakota Fertilizer Handbook. This publication gives a probability of a response to applied fertilizer based on soil test levels in the field. As an example, with soil P levels of 4-7 ppm (Olsen) applied phosphorus would have a 60-80% chance of a positive yield response. Based on this data, the probability of little or no response to applied P would be 20-40%. The 2022 season probably was one of the times with little to no response to applied fertilizer. Another possible explanation would be the average crop yields in 2022 due to delayed planting of wheat and soybeans. With a higher yield potential, the probability of a positive yield response to applied fertilizer would be higher than in years with average-to-below crop yield production.

Due to the cold and wet spring two on-farm sites were harvested in 2022. A wheat site in Elbow Lake experienced good growing conditions with wheat yields in the 80 bu/ac range. At the 95% confidence level the plus 50 treatment of P&K produced 3.8 bu/ac more wheat than the farmer practice. No differences were observed between treatments in the wheat quality analysis. At the soybean site near Baudette, no differences were detected in soybean yield or quality between the plus 50 of P&K and the farmer practice. However, harvested soybean yields were in the mid-30 bu/ac range which indicated that environmental conditions were not favorable for soybean growth and development at this site in 2022.

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