Estimating Nitrogen Loss and Sidedress Applications

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Wet weather this spring may have led to nitrogen (N) loss through denitrification and leaching and contributed to preventing applications of pre-plant N fertilizer. Determining N loss from denitrification and leaching is not an exact science; however, estimates can be made to provide guidance for determining sidedress N rates.

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The primary mechanisms for N loss after application include denitrification, leaching, and volatilization. Nitrogen in the ammonium form attaches to clay soil particles and organic matter and is not subject to denitrification or leaching in fine-textured soils. However, ammonium can leach in coarse-textured sandy soils. Nitrate-N is subject to loss by denitrification or leaching. Factors influencing N loss include the following: source of N, rate, application timing, method of application, and soil temperature. The first step to determine N loss is to estimate the amount of applied N that was in the nitrate-N form when soil saturation occurred. University of Kentucky research, presented in Table 1, can be used as a guide to estimate percent nitrate-N from applied fertilizer. The conversion of ammonium to nitrate is highly dependent on soil temperature.

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Table 1: The amount of applied fertilizer that is in the nitrate-N form 0, 3, and 6 weeks after application.13;10;13;10;

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Denitrification is the process by which nitrate-N is converted by bacteria into forms of N gas which move freely up through the soil and are lost to the atmosphere. The process is initiated by bacteria that are anaerobic, meaning they are active when oxygen levels are low. The majority of denitrification occurs under saturated soil-water conditions and is considered to be the main process by which nitrate-N is lost, except on sandy soils, where leaching is the major pathway.1 The amount of nitrate-N loss is influenced by several factors, primarily the length of time soils are saturated and soil temperature. University of Illinois estimates indicate when soils are saturated, daily nitrate-N loss by denitrification can be 1% to 2% at soil temperatures less than 55 °F, 2% to 3% if soil temperatures are between 55 and 65 °F, and 4% to 5% at soil temperatures above 65 °F.1

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Leaching occurs when nitrate-N moves downward in the soil profile out of the root zone with excessive precipitation or excessive irrigation. In sandy soils, nitrates may contaminate groundwater sources, but in fine-textured soils leached nitrates typically reach tile lines and may eventually reach surface water.

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Volatilization of urea-based fertilizers can occur when they are surface-applied and not incorporated. Urease enzymes in soil and plant residue convert urea to free ammonia gas. On warm, sunny days, up to 15 to 20% of urea-based N can volatilize within a week of application.2

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Nitrogen Fertilizer for Sidedressing

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Anhydrous Ammonia. Applications should be made under soil conditions that ensure proper sealing of the knife track. Ammonia vapor can escape with shallow injection or if improper sealing occurs, damaging corn leaves. If only a portion of the leaves are damaged, corn plants usually grow out of the leaf damage. Also, sidedressing should occur early to avoid root pruning or anhydrous ammonia “burn�? damage to seedling roots.

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Urea-Ammonium Nitrate (UAN). When using UAN solutions such as 28% or 32% N, injection between the rows is the preferred application method which minimizes volatilization N loss. The probability of N loss is higher if not incorporated. Half of the N content in UAN is urea. Up to 30% of the urea component could be lost due to volatilization with surface application, if no rainfall is received within ten days and temperatures are warm.3

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Broadcasting UAN solutions can be implemented; however, foliar leaf “burn�? may occur, resulting in leaf loss and reduced early growth. Due to injury concerns, it is recommended not to exceed 90 lbs N/acre of broadcast UAN when corn plants are at the V3 (3 visible leaf collars) to V4 growth stage and 60 lbs N/acre broadcast at the V7 growth stage. For applications after V7, use drop hoses for UAN “dribble�? application directly to the soil surface between the rows.

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Urea. Broadcasting urea may cause some leaf “speckling�? or browning of the leaf edge when granules fall into the corn whorl. The potential for this to occur increases with higher application rates and taller plants. However, leaf burning is generally less with broadcast urea granules compared to broadcasting UAN solution, ammonium nitrate, or ammonium nitrate.3 To minimize adhesion of urea granules to the plant, apply when the foliage is dry. Using an urease inhibitor when sidedressing urea early in the season can minimize volatilization loss.

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Options for Nitrogen Fertilizer. According to Iowa State University recommendations, the best options for sidedressing N in order from most preferable to least preferable are:

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  • Injected anhydrous ammonia, UAN, or urea.
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  • Broadcast dry ammonium nitrate, ammonium sulfate, or urease inhibitor treated urea.
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  • Surface dribbling UAN solution
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  • Broadcast UAN 
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  • Broadcast urea
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The potential for N loss after application and plant injury are factors that are considered when ranking N source and application method.  

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