Practical Implications for Farmers and Growers
Sidedressing nitrogen fertilizer on corn on a Centre County farm involved in the Thriving Ag project. Credit: Charlie White
Authors: Charlie White, Kathleen Arrington, Andrew Lefever, Zoelie Rivera-Ocasio
What is the issue?
Nitrogen (N) recommendations for corn have always been challenging to develop because of the dynamic nature of the many different N sources that contribute to meeting crop needs. The amount of N made available from organic N sources, such as soil organic matter, cover crop residues, and manure, is also mediated through microbial activity. Properties of the microbial community such as their composition (bacteria, fungi, protozoa, etc.) and physiology (carbon use efficiency, biomass carbon to nitrogen [C:N] ratio) can affect N availability, but these properties are rarely measured and taken into account when making N fertilizer recommendations.
Yield goal-based N recommendations, which use the expected crop yield multiplied by an N requirement coefficient (i.e., 1 lb N/bushel corn yield), have served well for many years as a starting point for making N fertilizer recommendations for corn. But there are many methods available to improve upon these recommendations. More accurately determining N fertilizer requirements can help farmers save money, increase production efficiency, and improve environmental quality.
What did we find and why does it matter?
In the Thriving Ag project, we developed a new method to determine N recommendations for corn that accounts for N supplied by soil organic matter and cover crop residues based on the microbial processes that control N mineralization (the breakdown and release of available N from organic N sources). We also updated the calibration of the previously developed Pre-sidedress Soil Nitrate Test (PSNT) to account for modern production practices, such as higher-yielding crops, no-till, cover cropping, and manure injection.
In the new N recommendation tool, measurements of total soil carbon and nitrogen, soil texture, cover crop N content, and carbon to nitrogen ratio are used to adjust N fertilizer recommendations by using a biogeochemical model that accounts for N mineralization and the corn yield response to N fertilization. The model, which was calibrated based on local field experiments, uses field-specific characteristics that affect N mineralization, including the carbon use efficiency of the soil microbial community and the soil C:N ratio. These inputs are relatively accessible to farmers and agronomists through small adaptations to current soil sampling and analysis methods. A simple-to-use graphical interface was developed (Figure 1) that calculates the additional amount of recommended N fertilizer needed to economically reach the yield goal for a site.
Figure 1. The interface for a new nitrogen fertilizer recommendation tool that accounts for soil organic matter and cover crop residues.
We also found that the PSNT was an accurate tool to determine an N fertilizer sidedress rate recommendation during the early growth period of corn. While this soil test was developed and initially calibrated with data collected in the 1990s, we found that the formula used to make the N rate recommendations needed to be recalibrated to accurately predict the N rates needed in modern production systems. With the new calibration, the PSNT can be used in fields with a long-term manure history under a variety of management practices including cover cropping, no-till, and manure injection.
Through on-farm validations of these recommendation tools, we have observed that many farmers in PA have an opportunity to reduce N fertilizer applications from their standard practices, especially in areas with a legacy of manure application. Across seven on-farm validation sites conducted in Lancaster County, PA, the average N fertilizer reduction suggested by these tools and validated by experimental results was 48 lbs N/ac less than the grower standard practice. However, this average masks the farm-by-farm results, where some farms were already using the correct rate of N as their standard practice and did not need to reduce their rate, while some farms could have reduced their rates by 60 to 85 lbs N/ac.
Overall, these results indicate that many farmers have a large opportunity to reduce N fertilizer application to corn, which can save money and help to protect against N losses to the environment. These newly developed and updated N recommendation tools can help farmers more confidently determine N requirements and reduce application rates when called for.
What did we do?
The new N recommendation tool that credits cover crops and soil organic matter (N Tool) is based on a different concept of calculating crop N fertilizer requirements than the yield goal-based system. Instead, the recommendations are calculated based on a metric called delta yield, which is the difference between the potential corn yield at a site given no N limitations, and the corn yield that could be produced with no fertilizer added. The corn yield without fertilizer added reflects the amount of N supplied by soil organic matter and cover crop residues, in cases where cover crops are used. Delta yield is a better predictor of economically optimal N rates for corn production than the yield goal alone, because it accounts for the N supplied by soil, and determines the fertilizer requirement based not on the entire yield goal, but the additional yield beyond what the soil is capable of producing without fertilizer. We compiled a large database of corn yield response trials to calibrate an equation to predict the economically optimum N rate (EONR) based on delta yield in Pennsylvania.
To determine the accuracy of N Tool and PSNT fertilizer recommendations, seven farmers across Lancaster County participated in on-farm experiments to determine the economic optimum sidedress N rates in their cropping systems in 2023 and 2024. In the on-farm experiments, instead of using the farmer’s typical sidedress N application rate, six different rates of N fertilizer between 0 and 260 lbs N/ac were sidedressed to determine a yield response curve.
In addition to measuring the N response curves, field-specific soil, cover crop, and crop production data was collected at each site to use as N Tool inputs to generate a predicted N sidedress rate. Soil samples for the pre-sidedress soil nitrate test (PSNT) were also collected at each site in early June to determine sidedress N recommendations based on the new PSNT calibration.
Applying sidedress N treatments with liquid UAN in Lancaster County in June 2023. Credit: Andrew Lefever
Publications completed for this work
Arrington, K. E., Ordóñez, R. A., Rivera-Ocasio, Z., Luthard, M., Tierney, S., Spargo, J., Finney, D., Kaye, J. P., & White, C. M. (2024). Improving a nitrogen mineralization model for predicting unfertilized corn yield. Soil Science Society of America Journal, 88(3), 905–920. https://doi.org/10.1002/saj2.20665
Sanders, Z. P., & White, C. M. (2023). Can nitrogen recommendations for corn production be improved through spatially explicit crediting of cover crops and soil organic matter? Smart Agricultural Technology, 6, 100336. https://doi.org/10.1016/j.atech.2023.100336
White, C. M., Finney, D. M., Kemanian, A. R., & Kaye, J. P. (2020). Modeling the contributions of nitrogen mineralization to yield of corn. Agronomy Journal, 113(1), 490–503. https://doi.org/10.1002/agj2.20474
White, C. M., Sigdel, S., Karsten, H. D., Meinen, R., & Spargo, J. T. (2023). Recalibrating the Pennsylvania pre-sidedress soil nitrate test recommendations for modern corn production. Agronomy Journal. https://doi.org/10.1002/agj2.21426
