Recent heavy rains may have caused nitrogen (N) loss from denitrification or leaching.
Fertilizer sources and temperatures affect N loss and growers should re-evaluate N availability in corn fields.
Rates, timing, and fertilizer products of sidedress applications can be determined after N analysis.
Flooding Effects on Nitrogen in Soil
Water may soak into soil, runoff or pond during heavy rains. Fields that are submerged for more than two days could suffer significant loss of N through denitrification or leaching. Saturated soils result in denitrification, which tends to be more prevalent in heavier-textured soils, whereas leaching is more prevalent in sandy soils. Denitrification begins after soils have been saturated for 2 to 3 days.1 Research conducted in Illinois indicated approximately a 4 to 5% nitrate-N loss via denitrification for each day that silt loam and clay loam soils were saturated during late May and early June.2 Loss of nitrate from denitrification may be less in cooler soils. University of Nebraska research estimated denitrification caused a 2 to 2.5% loss of nitrate when soil temperature was between 55 and 60° F.2
Nitrogen from Fertilizer
In addition to temperature, conversion to nitrate-N, or nitrification, occurs at different rates depending on fertilizer type (Table 1). Fertilizer products with a nitrification inhibitor can slow conversion to nitrate-N. Nitrification of ammonium-based fertilizers (anhydrous ammonia, urea, or ammonium sulfate) does not occur in saturated soils. Therefore, minimal N would be in the nitrate form that is susceptible to loss if anhydrous ammonia was applied shortly before a wet period (within the last two weeks).2 Nitrogen fertilizers with N partly in the nitrate form (urea-ammonium nitrate (UAN) solution or ammonium nitrate) would already have N in a form susceptible to loss. Nitrate-N is more susceptible to loss; however, soils that are dry may have less leaching or denitrification that could lead to N loss.
Compared to pre-plant fertilizer applications, sidedressing can occasionally lead to greater yield potential on medium and fine-textured soils when there are excessive early-season rains.3 Some considerations for rate, timing, and sidedressing products are discussed below.
Rate. Fields should be evaluated to determine the amount of N loss. A calculation can help determine N loss based on Table 1 and the estimation of a 4 to 5% nitrate-N loss per day of denitrification. Nitrate-N tests may be preferred to help avoid over application of N in fields with variable N loss. Soil samples should taken down to a 12-inch depth. Several samples from both high and low areas should be collected per field and mixed. A nitrate analysis of a subsample will guide sidedressing rates of N. Nitrogen rate can also be adjusted based on early indications of yield potential.
Timing. Sidedress applications ideally coincide with N requirements of corn 6 to 10 weeks after planting.3 However, yield loss from N deficiency is a concern if wet conditions continue and sidedressing is delayed. Consider early sidedress applications as seedlings have exhausted nutrients stored in the seed and are increasingly dependent on soil supplied nutrients at the V3 growth stage. Adequate N from V5 through V8 is critical because this is when the number of potential ears and ear girth are being determined.
Products. Anhydrous ammonia can be injected into moist soils as long as root pruning is not a concern. Injecting UAN solution (28%) is a good choice to avoid volatilization, or UAN could be dribbled as a band between rows. Broadcast UAN solution can burn foliage of large corn and should be avoided.
1Murdock, L.W. Estimating nitrogen losses from wet soils. University of Kentucky.
2Sawyer, J. 2007. Estimating nitrogen losses — early spring 2007. Iowa State University. IC-498(10).
3Vitosh, M.L. et. al. 1995. Tri-state fertilizer recommendations for corn, soybeans, wheat,and alfalfa. Michigan State University, The Ohio State University, and Purdue University. E-2567.
4Russelle, M. et al. 1983. Nitrogen accumulation rates of irrigated corn. Agronomy Journal.Vol. 75:593-598.
5Sawyer, J. 2000. What about N losses in 2000? Iowa State University.IC-484(16).