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Research Explores Best Fertilizer Practices for Low-Yielding Zones for Corn and Soybeans

Portions of farm fields in Minnesota have under-performing areas that cost farmers money due to consistently below average yields. In 2020, AFREC partnered with retailers and farmers to conduct the following research:

  • Identify the management zones of up to 25 Minnesota corn and soybean fields
  • Establish an on-farm research approach to evaluate a range of fertilizer rates in these low-yielding areas

The purpose of this work was two-fold. The first goal was to evaluate the capabilities of modern precision agriculture fertilizer spreaders for conducting on-farm trials. The second goal was to assess current fertilizer recommendations—particularly for low-yielding areas.

Management zones (low, medium and high performing areas) were identified using previous yield monitor data, although other methods can be used to create zone designations such as soil type and remote sensing (see Managing a Fertilization Program in Variable Soils).

Each trial had three rates of fertilizer replicated at least three times for a total of nine plots per trial. Across the established trials, more than 200 individual plots (25 trials x 9 plots per trial) support the analysis in the tables, charts and graphs. Low-yielding areas are designated as “Zone C,” with medium- and high-yielding areas designated as “Zone B” and “Zone A,” respectively.

In Chart 1, each bar represents a separate trial with the color indicating the nutrient that was tested in the trial. This chart suggests that when growers increased fertilizer rates above their normal practices, yields increased by slightly more than 3 bushels per acre. This is similar to increases observed in the prior AFREC on-farm research data sets from 2019 and 2018. The largest yield increases were from nitrogen trials placed in the lowest yielding areas (Zone C).

Charts 2 and 3 above illustrates the yield averages using the farmer’s typical fertilizer rate for each trial. In 2020 trials, corn yields averaged 230 bushels per acre and soybean yield averaged 68 bushels per acre. The state of Minnesota average yields for 2020 were 191 bu/ac for corn and 50 bu/ac for soybeans. The fields selected for the AFREC research project are representative of high-yield systems, achieving yields above state averages.

Across all trials, when growers reduced fertilizer rates below their typical practices, yields decreased on average by more than 4 bushels per acre. Of particular note, four nitrogen rate trials had yield reductions in excess of 18 bushels per acre.


Although these results are from a single year of research, there are trends in the results that can help guide growers and consultants when developing a fertilizer management program for fields with variable soils that include low-performing areas:

  1. The technology exists today for growers and advisors to conduct on-farm trials. The results from these trials can be used to reinforce or challenge current university guidelines used on your farm.
  2. From the limited number of trials, there is a general tendency for the yields to be more impacted in low yielding areas of fields by selecting lower fertilizer rates.
  3. Increasing fertilizer rates in C-zones (the low-yielding areas) increased yields by a greater margin than A or B Zones (high and medium yielding areas).
  4. Decreasing fertilizer rates shows a negative yield response in several of the B and C Zone nitrogen trials. This suggests that higher rates of nitrogen fertilizer may be needed in low-yielding areas.

It should be noted that within zones, the highest level of variability is seen in the lowest yielding zones. This “noise” in the data creates difficulty in using statistical analysis to prove that yield increases are due only to fertilizer rates (and not drainage, for example). The practical implication of this is that growers will likely need multiple years of observations in the same areas of the field to boost their confidence in the results. The creation of reliable and repeatable management zones is likely to be one of the most important aspects in adopting this approach.

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