Long-Term Impact of Nitrogen Fertilization on Corn Production, Soils and Nitrogen Cycling Processes in Minnesota

Nitrogen (N) is one of the most vital inputs for high corn yields, but managing it effectively is challenging. Long-term applications of fertilizer change physical, chemical, and biological soil properties over time, which directly impacts organic matter, soil health, and overall soil productivity. Because these soil changes take many years to develop, long-term research sites are necessary to evaluate how soil health including physical, chemical, and biological properties evolve and how fertilizer recommendations should adapt.

Why the Study Was Needed

Short-term studies cannot fully capture how repeated fertilizer application alters soil organic carbon, nutrient cycling, and water holding capacity. Establishing long-term research sites allows scientists to create a gradient of soil conditions to evaluate nutrient management practices over time. Additionally, in the shorter term, these trials generate real-world response data to update Minnesota’s Maximum Return to N (MRTN) calculator and evaluate whether newer fertilizer products (like ESN blends or Agrotain inhibitors) offer yield or efficiency advantages over standard urea fertilizer.

Key Results from the 2025 Growing Season

• Yield Response: Corn grain yields responded strongly to nitrogen fertilizer across all study locations.
• Fertilizer Source Performance: Switching between standard urea, ESN blends, or Agrotain-treated urea generally had no significant effect on corn yield or grain nitrogen removal across most sites. The only exception was at the irrigated sandy site (Becker), where compared to the high-proportion ESN blend, Agrotain alone yielded higher in continuous corn and the low-proportion ESN blend performed better in corn after soybean.
• Economic Optimum Rates (EONR): Optimal nitrogen application rates varied depending on location and crop rotation.
• Corn-after-Soybean: averaged across N sources the EONR ranged from 102 to 115 lb N/acre on fine-textured soils, which was lower than current MRTN guidelines (150 lb N/acre). On irrigated sand, EONR averaged 226 lb N/acre.
• Continuous Corn:  averaged across N sources the  EONR averaged 127 lb N/acre at Lamberton, 179 lb N/acre at Morris, 200 lb N/acre at Waseca, and 330 lb N/acre on irrigated sand at Becker. The 1/3 ESN-2/3 Urea blend fairly consistently (6 out of 7 times) reduced the EONR while producing similar yields to the standard urea treatment (less nitrogen to obtain similar yields).
• Residual Nitrogen & Soil Health: Over-applying fertilizer above the calculated EONR sharply increased residual inorganic nitrogen left in the top 36 inches of soil post-harvest. Higher historical nitrogen rates on corn also left higher residual soil nitrogen during the following soybean year.

Evaluation of Goals and Objectives

The long-term and short-term goals for this trial were met:

• Long-term objectives: Research plots are successfully established across six diverse regions in Minnesota to measure over the long-term changes in soil properties.
• Short-term objectives: The team successfully gathered response data to feed into the regional MRTN database and evaluated corn response to urea versus ESN/Agrotain blends across various soil types and weather conditions.

Practical Implementation for Farmers

• Stick to Optimal Application Rates: Apply nitrogen close to the Economic Optimum Rate (EONR) recommended for your region and rotation. Applying rates higher than necessary does not boost yields and dramatically increases leftover inorganic nitrogen in the soil profile, raising the risk of nutrient loss.
• Standard Urea Remains Cost-Effective: In most situations, standard urea produced cost-effective benefits compared with ESN/urea blends. Unless managing high-N loss conditions, paying extra for enhanced-efficiency nitrogen sources may not yield higher returns.
• Credit Previous Rotations: Corn following soybeans required significantly lower nitrogen rates to reach maximum yield compared to continuous corn. Ensure you are taking full credit for the rotational benefit when calculating spring fertilizer passes.