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Managing nitrogen (N) fertilization is an important element of input cost control and maintaining optimum yield potential. Timing, rate, source, and method of N application plus N loss mechanisms, all play a role in this balancing act.
Timing of application can be influenced by factors such as weather and workload. It is desirable to apply N as close to the period of rapid plant uptake as possible so that there is reduced risk of N loss prior to plant use. Sidedress applications are preferred over pre-plant, and pre-plant applications are preferred over fall applications in synchronizing N application to rapid crop N uptake and to limit potential loss.1 To reduce spring N loss when N has been applied more than 2 weeks prior to planting, anhydrous ammonia is recommended.1 When considering in-season N applications, the preferred method is injection of ammonia or UAN, followed by UAN dribbled between rows or broadcast urea.2
Sidedressing N applications can help supplement a pre-plant fertility program or supply N to the crop if conditions prevented N application before planting. The first step to addressing potential problems is to determine how much nitrate-N has been lost in the soil. Nitrate is the most plant available form of N.
To estimate how much N has potentially been lost, take into account soil temperatures and days a soil has been saturated. Keep in mind that there will be more nitrate present if there has been a recent application of urea-ammonium nitrate solutions (28, 32% UAN) because one-fourth of the product is nitrate-N.2 Sidedressing N should be targeted for application prior to the V8 stage of growth, when the plant’s demand for N increases rapidly. Adequate N from V5 through V8 is critical because the number of potential ears and ear girth are being determined. Nitrogen sidedress rates are determined by soil nitrate test results.4
Nitrogen application rates based on yield goals have been a common method to determine the amount of N application.1 University recommendations advise to fertilize for normal yields, even in good years, because under those ideal conditions, the microbial activity leads to soils releasing more N to the crop.1
1 Scharf, P. and Lory, J. 2006. Best management practices for nitrogen fertilizer in Missouri. MU Extension, University of Missouri-Columbia Publication IPM1027. 2 Sawyer, J. and Creswell, J. Nitrogen applications. Iowa State University Extension. NMEP7. 3 Sawyer, J. 2007. Estimating nitrogen losses - early spring 2007. Iowa State University. IC-498 (10). 4 Reitsma, K.D. Nitrogen best management practices for corn in South Dakota FS- 941, SDSU Extension. Doc ID 160203133509