Before managing nitrate leaching in your soils, you must first have a general understanding of nitrogen and the nitrogen cycle. Nitrogen (N) is all around us in various forms and is one of the most volatile yet necessary yield-limiting nutrients for crops.
Soil receives N from the atmosphere through rain, biological and industrial fixation, animal and plant wastes, and manufactured fertilizers. In the soil, N goes through an anaerobic process called nitrification in which soil organisms convert ammonia and ammonium to nitrite and then to nitrate, which is the form plants can uptake and use.
Nitrate leaching in soils is the process of it gradually being moved down into the soil via soil moisture. It is determined by soil characteristics and water concentrations. Movement rates out of the root zones will vary. The rapidity of movement is considerably higher in sandy soils compared to clay soils. Knowing your soil types and zones within your fields are essential to managing leaching. Soil data can be found for free in the Soil Survey Geographic Database (SSURGO) on the U.S. Department of Agriculture website, or soil sampling can be done to improve accuracy.
Considerable advancements have been made over the last 30 years in N application timing and method, which have helped to mitigate leaching. Application timing and knowing your annual rainfall patterns (Figure 1) is very important, especially in areas with lighter soils. Some farmers depending on equipment setup, and irrigation abilities are opting for very minimal N applications ahead of planting or during planting. Many are now favoring more side-dress applications at various stages of plant growth or irrigation applications through center pivot injections systems (Figure 2).
Farmers with center pivot irrigation systems, especially in lighter soils, have reduced leaching by having more frequent applications with less injected N per application. This allows the ability to “spoon feed” the N and accomplish watering at the same time. The benefit of keeping the N near the top of the root zone is great; however, the downside is if soil moisture conditions do not require a watering and N still needs to be applied, you may have to irrigate just to apply it. Many things need to be considered when making your fertilizer plan for the year; therefore, balance what is best for your operation and how to maximize N efficiency.
Advancements in precision agriculture are making it possible to apply variable rates of N fertilizers through prescriptive applications. Reducing or increasing application rates, depending on need by zone, have proven to make a large difference in both yield, N use, and reduced nitrate leaching due to proper placement and use. In fields with high variability, precision mapping and guidance along with agronomic testing (soil and tissue sampling) have given farmers the ability to write prescriptions with programs such as the Climate FieldView™ platform. Within the FieldView, “Scripting Tool” farmers have the functionality to create zones by many data layers either preloaded with available soil data or uploading their own. The easy-to-use functionality of FieldView makes this an ideal digital platform for “do it yourself” farmers.
Nitrification inhibitors and other special fertilizer solutions have been around for many years and have advanced considerably. While modest in proportion to reducing nitrate leaching, they can have an impact. Proper timing of the inhibitors with application are very important. Be sure to work with your supplier for proper use and effectiveness.
With growing regulations and awareness around nitrate leaching into groundwater and surface water tributaries, a significant amount of research has been done and is ongoing through various university systems around the country. Be sure to reach out to your local extension experts in your area to see what research has been done.
USDA_NRCS. 2014. Soil nitrogen. Soil Health – Guides for Educators. United States Department of Agriculture and Natural Resources Conservation Service. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051575.pdf.
Wortmann, C., Puntel, L., Iqbal, J., and Maharjan, B. 2020. Agronomic management for reduced nitrate leaching. CropWatch. University of Nebraska-Lincoln. https://cropwatch.unl.edu/.
Thompson, L. 2020. Precision nitrogen management on-farm research project. https://cropwatch.unl.edu/.
Websites verified 12/23/2020.