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Nutrient management in the fall is an annual function of production agriculture. Nutrient inputs are critical for optimal corn production, even when corn prices are low. The 4Rs of nutrient management provides guidelines to help maximize the potential benefits of fertilizer applications and reduce potential loss to ground water and larger water resources.
The 4R stewardship program educates and encourages the Right fertilizer source at the Right rate at the Right time and in the Right place. The U.S. Environmental Protection Agency (EPA) has shown growing concerns about surface and groundwater contamination in state waters, calling for the reduction of nitrogen (N) and phosphorus (P) loads entering the Gulf of Mexico. Nutrient stewardship should be practiced on all crop production acres in the Upper Mississippi Basin to help shrink the hypoxia zone.
The Right place involves placing the nutrients where they have the best opportunity to remain in the soil for the plant. This can include strip till placement of dry fertilizer, or knifing in liquid N applications with the proper equipment. The 4Rs provide flexibility in a nutrient management system that enables nutrient product selection and applications that can be customized to each field or crop and consideration for the availability and cost of various nutrient sources.
Soil sampling along with evaluating nutrient removal by the previous crop are critical first steps that help define the Right fertilizer rate for N, P, and potassium (K), micronutrients, and to manage soil pH. The crop nutrient removal calculator (http://ipni.info/calculator) can be used to estimate crop nutrient removal of N, P (expressed as P2O5), K (expressed as K2O), and sulfur (S) for many field crops.
Variable Rate Technology is an enhanced 4R tool that allows nutrients to be applied at rates that match the requirements of 2 1/2 acre grid soil test results. To reduce runoff potential from rapid spring thaw events, fertilizers should not be applied to snow covered or frozen ground. Application on frozen ground is neither the right time or the right place.
Midwest land grant universities conduct extensive field research and recommend a Maximum Return to Nitrogen (MRTN) rate based on soil type and economic factors such as the price of N and expected grain selling price per bushel. The Right time is also critical not only for maximizing yield potential, but minimizing environmental concerns from N loss. For fall applied N, waiting until soil temperatures fall to 50 °F at the four-inch depth helps reduce nitrification and thus loss potential. Inhibitors can provide protection against loss from early spring applied N as well. Using proven nitrification inhibitors, splitting N applications so that the right rate is applied over multiple applications helps provide N to the plant at various growth stages and mitigate catastrophic N losses from unexpected weather events, such as flooding or drought. Corn extracts only 15% of required N prior to the rapid vegetative growth stages of V5 to V8; however, N should be applied prior to V5 for the plants to be fully nourished. A corn plant will need the most N at the V10 growth stage, which occurs about 40 days after plant emergence.
Phosphorus can be applied either in the fall (Figure 1) prior to tillage, or in the spring. Incorporation within the corn root zone is recommended because P is nearly immobile in the soil. Potassium deficiencies can occur when crop residue is removed from the field through silage or the baling of corn stalks. Additional P and K information can be found in the agKnowledge Spotlight Importance of P and K in Corn and Soybean Development (http://www.aganytime.com/Documents/ArticlePDFs/ImportanceofPandKinCornandSoybeanDevelopment_ASDK_Spotlight.pdf).
Although nutrient uptake S, zinc, and manganese make up less than one percent of the fertilizer applied in corn, they are critical for corn development and it is important to identify and manage deficiency symptoms.1
Additional fertility information can be found in the agKnowledge Spotlight Corn Fertility in Medium-to High-Yield Environments (http://www.aganytime.com/Documents/ArticlePDFs/CornFertilityinMediumtoHighYieldEnvironments_DK_Spotlight_DEC14.pdf).
Sources:1Mallarino, A.P., Oltmaus, R.R., Prater, J.R., Villavicencio, C.X., and Thompson, L.B. 2011. Nutrient uptake by corn and soybean, removal, and recycling with crop residue. Iowa State University. 2011 Integrated Crop Management Conference: 103-113. http://www.agronext.iastate.edu; Ehmke, T. 4R nutrient management. Right source. Right rate. Right time. Right place. Right now. American Society of Agronomy. (Mr. Ehmke, contributing writer, Crops & Soils magazine). Web sources verified 10/7/17. 171016091129