University of Delaware Scientists Participate in a National Effort to Improve Fertilizer Recommendations

Amy Shober, Professor and Extension Specialist, Nutrient Management and Environmental Quality,; Jarrod Miller, Assistant Professor and Extension Specialist, Agronomy,

It has been decades since soil test correlation and calibration research was a priority in the United States, but there is a national effort underway to change this. University of Delaware is participating in a national effort among land-grant and government scientists to develop a Fertilizer Recommendation Support Tool (FRST). The group envisions that the FRST online decision support tool will help farmers and crop consultants make science-based nutrient management decisions. The group’s mission is “to provide soil-testing transparency as the basis of fertilizer rate recommendations, removing political and institutional (public and private) bias from soil test interpretation, and fostering the best possible science in nutrient management decisions.” Historical data from hundreds of soil test correlation and calibration studies have already been added to the FRST database. Unfortunately, there is also a lot of soil test correlation and calibration data that was lost or destroyed when scientists moved on. As such, there is a renewed effort to generate new soil fertility and crop response data to “fill in the gaps” in the database.

We have teamed up with researchers at University of Maryland College Park and Virginia Tech to evaluate corn response to potassium fertilizer rates. Why did we choose to evaluate potassium? Well, in 2015 a local CCA reported that soil test K concentrations were declining at a rate of 10 to 18 mg/kg per year under a standard cropping rotation (e.g., corn, full season soybean or corn, winter wheat, double-crop soybean) even when K fertilizers were applied at land-grant recommendations. Soil test K declines of this magnitude provide evidence that our existing K fertilizer rates were too low for today’s high yielding corn crops, especially when corn is produced on our sandy, low-CEC Delmarva soils. While we did increase K fertilizer recommendations recently, we did so based on best scientific judgement. These current fertilizer rate trials will provide evidence that provide further information to help us fine tune K recommendations for corn.

In 2021, we established field trials to evaluate five rates of potassium fertilization on the Delmarva Peninsula (Figure 1). There are two sites in each state: Delaware, Maryland, and Virginia. In addition, we selected two sites in the following soil test categories: Low (high probability of crop response), Medium (moderate probability of crop response), and Optimum/High (low probability of crop response). Potassium fertilizer rate treatments were 0, 54, 108, 161, and 214 lb K2O/ac and were replicated five times at each site. We are collecting soil (both Mehlich 3 and Mehlich 1) and crop response and plant tissue data following a rigorous minimum data set, which was developed by the FRST team to ensure essential information for development of fertilizer recommendations was included. Once completed, data from our regional field trials will be added to the FRST database. Stay tuned for more information once we harvest the trials.


Figure 1. Aerial image of UD potassium rate response trials planted on a field with medium soil test potassium (Mehlich 3 K = 30 FIV or 52 ppm). While we see no visual response to K fertilizers from the air, we will evaluate plant tissue concentrations and yields to determine plant response.