UNL Nitrogen Equation: Soil tests are a tool to determine residual soil nitrate and other nutrients. Various equations are used depending on the specific soil test lab, advising company, or university. None are perfect as none can predict the weather. As soil test recommendations come in, people ask me questions about the UNL Nitrogen Equation or sometimes make misinformed statements. Sharing more about it in this column. The information in quotations is from a revised Extension Circular on nitrogen management that will be published shortly.

“The accuracy of the Nebraska nitrogen algorithm for corn has been generally validated through many demonstration projects, on-farm research, and detailed research studies in Nebraska from 2004 through 2023. On average the algorithm produces a nitrogen fertilizer recommendation approaching the economic optimum fertilizer N rate (EONR), though the accuracy will vary with location and year. For the majority of fields in Nebraska, the Nebraska algorithm will be within ± 30 lb N/acre of EONR.” This shows the inability for any of us to predict the correct N rate due to the inability to predict weather the upcoming growing season. Thus, why soil tests are a tool to be used with a reactive approach like in-season nitrogen sensing technologies like Sentinel Fertigation.

The UNL N recommendation for corn grain (lb/ac) = [35 + (1.2 × EY) – (8 × NO3-N ppm) – (0.14 × EY × OM) – other N credits] × Priceadj × Timingadj where:

EY = expected yield (bu/ac) which should be about 105% of the five-year yield average
NO3-N ppm = average nitrate-N concentration in the root zone (2–4 foot depth) in parts per million
OM = percent soil organic matter (with a minimum of 0.5 and a 3 percent maximum)
Priceadj = adjustment factor for prices of corn
N Timingadj = adjustment factor for fall, spring, and split applications

I often hear, “UNL says it’s 1.2 lb of N/bu of corn produced”. But that’s an incorrect statement. The UNL nitrogen equation does have a 1.2 factor in it. But the equation never stopped there. UNL was one of the few universities that took the soil into account by subtracting off additional factors. “The value of 1.2 lb N uptake/bu of corn is often mistakenly viewed as a fertilizer N requirement, which is inaccurate. The current UNL N rate algorithm for corn starts with corn uptake of 1.2 lb N/bu, then credits N from various sources. These N credits include soil residual inorganic N, N mineralized from soil organic matter, N contributed from irrigation water, N released from decomposing legume plant materials, and N from manure. Once N credits from these various sources are accounted for, the remaining N requirement is filled with inorganic fertilizer.”

The UNL equation credits soybean as the previous crop for 45 lb of N due to decomposition of the plant material due to the low carbon to nitrogen ratio. On-farm research studies in this part of the State found, in irrigated soybean, we could credit more: 1 lb of nitrogen for every bushel of soybean raised up to 60 bushels (so up to 60 lb N ‘credit’ from soybean).

Irrigation water often contains a significant amount of nitrate-N (and other nutrients including sulfur) that is readily available to corn. In the past, Dr. Charlie Wortmann shared, for every 10 acre-inches of water applied, one can take ppm in the sample X 2.265 to determine pounds of the nutrient applied via irrigation. Because yearly irrigation varies, one could take a 3-year average.

I use the equation without the price and timing adjustment. Higher corn prices will show that one can apply more nitrogen economically. However, the research curve will show little to no yield increase for that additional nitrogen. The timing adjustment assumes nitrogen loss in the fall, building in N for that, and assigns a small reduction if applying nitrogen in-season. Hopefully this helps as you receive soil test recommendations this fall and consider adjusting them to the UNL N equation. Also, a reminder to consider a base rate of 70-100 lb N applied pre-plant.