*I’m grateful to Dr. Richard Ferguson, former Extension Soil Fertility Specialist, for reviewing this article; Dr. Charlie Wortmann, Extension Soil Fertility Specialist, for sharing research-based studies with me; and Glen Slater, SCAL Research Technician, for sharing details of the SCAL study mentioned below.
Nitrification Inhibitors: Two weeks ago, my column included a general overview regarding nitrogen losses in addition to general summary statements based on research. Nitrification inhibitors are best thought of as an insurance policy against loss of applied ammonium-based fertilizer due to excess rain in the first month or so after fertilization. Dr. Richard Ferguson, former Extension Soil Fertility Specialist, and his team have conducted much research regarding the use of nitrification inhibitors over time. Much effort has occurred in sandier soils within the Central Platte NRD. However, an ongoing study at UNL’s South Central Ag Lab (SCAL) near Clay Center is conducted on silt loam soils which are more common in the UBBNRD. Twenty-eight years of this study are summarized here. The majority of the study at SCAL compared Spring pre-plant anhydrous vs. side-dress application with and without the use of nitrification inhibitor N-Serve (nitrapyrin). This product was the product available back in the 90’s. The study actually began by comparing 2 different in-season side-dress applications before switching to pre-plant vs. side-dress in the early 2000’s. Soil samples for nitrate and ammonium content were taken when funding was available. In talking with Dr. Charlie Wortmann, UNL Extension Soil Fertility Specialist, he mentioned that they found yield to be correlated to amount of nitrogen in the soil. A yield increase due to nitrapyrin applied pre-plant was observed in 6 of 28 years with a mean yield change of 2 bu/ac/year. Only 1 of 28 years was a yield increase observed when nitrapyrin was applied in season during side-dress application with a mean yield change of 0 bu/ac/year. In this study, they found that delayed side-dress N with nitrapyrin could reduce plant N uptake. A detailed analysis of nitrapyrin effects on N uptake and soil accumulation from early years of this study can be found in Ferguson et. al (1991). The duration of inhibitor efficacy was measured in other studies where it was found a nitrification inhibitor such as nitrapyrin to last around two weeks with Spring applications. They’ve seen it last as little as 1 week to as long as 6 weeks depending on soil temperatures and moisture. The use of inhibitors is not advised in season as research showed they can release N too slowly for the crop demand resulting in yield loss and/or resulting in increased leaching of nitrogen when it was released too late in the growing season for crop uptake. In general, the research is more supportive for nitrification inhibitor use in sandy soils vs. heavier textured soils; yield benefit to a nitrification inhibitor may be none to a few bushels/acre for silt loam or silty clay loam soils.
In-Season Applications of Nitrogen: In-season application means the nitrogen is applied when the crop is actively growing and utilizing it, greatly reducing potential for loss. Recent on-farm research studies compared side-dress applications using either the UNL equation/Maize N model or industry models such as Climate Field View. In all these studies, the recommended rate was compared to rates that were at least 30 pounds over and under the recommended rate. Some of the studies went as high as +/- 50 lbs/acre compared to recommended rate. I’ve compiled these results in a table at https://jenreesources.com/2019/04/14/jenrees-4-14-19/. Take homes: In none of the studies did the addition of 30-50 lbs N/ac above the recommended rate increase the yield statistically. A few of these studies also compared side-dress applications vs. pre-plant alone. One situation resulted in a statistically lower yield with pre-plant alone while the other two resulted in no yield differences. Many of you have most likely heard of Project Sense in which a grower’s nitrogen application was compared to 75 lbs pre-plant + variable rate in-season application using sensing technology. This project was an on-farm research partnership with Nebraska Extension, Nebraska Corn Board and Corn Growers, and numerous NRDs including UBBNRD. From 2015-2017, 48 site-comparisons were conducted. The results showed 28.7 lbs less nitrogen was applied using the Project Sense method with a loss of 1.4 bu/ac in yield, and greater profitability of $7.24/ac. All of these results were statistically significant at the 95% level.
To reduce the risk of leaching or denitrification, consider applying the majority of nitrogen close to when crop demand is high with more nitrogen applied during the growing season vs. pre-plant. Also for consideration is the fact that farm operations differ in equipment and labor and growing seasons like 2019 present additional challenges, including for nutrient management.