Each farm will base its nitrogen application practices on time, labour, equipment and cost. Often losses in efficiency in one area can be compensated for by improvements in another. The “best” nitrogen (N) system is not universal, but will depend on the major limiting factors on each individual farm. Here are key points to consider for the six most common systems.
1) Pre-plant banding
–Banding N in concentrated rows below the soil surface tends to be the most efficient form of application under Western Canadian conditions. Placing fertilizer below the soil surface protects the ammonia portion from volatilization losses. Placing the fertilizer in a band reduces the contact between the fertilizer and the soil microorganisms, reducing immobilization of both ammonium and nitrate. Banding also slows the conversion of urea to ammonium and ammonium to nitrate, which can reduce losses by denitrification and leaching. Ideally, bands should not be disturbed by pre-seeding tillage or seeding operations.
–All forms of N fertilizer perform well when applied as a spring pre-plant band, provided that the fertilizer is separated from the seed. Anhydrous ammonia (NH3) should be placed at least 4 inches below the soil surface and, if possible, seeding should be done perpendicular to ammonia bands. There is no need to delay seeding after application if anhydrous ammonia is placed at recommended depths, especially on moist clay soils.
–Pre-plant banding may delay seeding and dry and disrupt the seedbed, especially in clay soils.
2) Surface applications immediately before or after seeding
–Broadcast applications cover a lot of acres in day.
–Because of the high potential for volatilization and immobilization losses, surface applications of N tend to be less efficient than in-soil banded applications. Efficiency of surface applications tends to improve in higher rainfall areas. Efficiency is lower on high pH soil, since high pH encourages the production of ammonia gas.
–Urea or UAN sources of N can be lost by volatilization until they are incorporated or move into the soil with precipitation. Tillage during conventional seeding operations is generally sufficient to incorporate urea or UAN solution and reduce volatilization.
–Ammonium or nitrate sources in close contact with crop residues may be subject to immobilization as the residues decompose.
–Stranding of broadcast fertilizer in dry soil above the active portion of the crop’s root system may be a problem in some weather conditions.
–High rates of broadcast urea-N applied without incorporation on fields seeded with disc-openers may concentrate pellets in the seed-furrow and cause seedling damage to sensitive crops like canola.
–For post-emergent N delivery, dribble-banded UAN, which reduces contact with crop residues and soil, will generally be a better choice than broadcast urea for surface applications. Urease inhibitor can improve results for urea (see next paragraph). Volatilization losses with dribble banded UAN will be lower than with urea, both because the UAN provides a portion of the N as nitrate and because UAN does not increase initial pH at the application site to the same extent as urea. Use of a dribble-band rather than a spray application reduces contact between the fertilizer and crop residue, reducing immobilization.
–Urease inhibitors such as Agrotain applied with urea or UAN slow the conversion of urea to ammonia and ammonium. This allows more time for the urea to move into the soil before it is converted into ammonia and ammonium. The slower conversion also reduces the concentration of ammonia at the soil surface, reducing the rate of volatilization. As volatile losses from UAN are generally lower than from urea, the benefit of using the urease inhibitor may be lower with UAN than with urea.
–While a higher rate of fertilizer may be required to compensate for the reduced fertilizer use efficiency for surface applications, this may be a practical compromise for some growers.
3) Placement in the seed row
–Excess N with the seed can lead to seedling damage due to combination of salt and ammonia toxicity. This damage can reduce crop yields, reduces response to nitrogen fertilizer and reduces nitrogen use efficiency. Seedling toxicity may also delay crop emergence and reduce crop vigour.
–Safe rates of seed-placed N depend on a number of factors including environmental conditions, soil type, width of the seed/fertilizer band, row spacing and fertilizer source. Lower the rate in light-textured soils, low soil organic matter, cool growing conditions, low soil moisture, in the presence of salts or free lime, or with the use of wide row spacing. For more detail about determining safe rates of N fertilizer that can be applied with cereals and canola, please refer to the Manitoba Soil Fertility Guide.
–Placement of fertilizer in the seed-row eliminates an extra pass for fertilizer application. It also eliminates the extra expense, draft requirements and soil disturbance required to side-band the fertilizer requirements.
–Seed-row placement is a form of shallow banding, and will reduce N losses compared to broadcast.
–A reasonable compromise may be to apply a portion of the fertilizer with the seed and broadcast or dribble-band the remainder.
–A controlled-release product like ESN, may increase the rate of N that can safely be applied with the seed. At 100%, ESN can be used at 3 times the safe urea rate. A blend of half ESN and half urea can be used at 1.5 times the safe urea rate, assuming a low level of damage to the ESN protective coating.
4) Side-banding or Mid-row banding at seeding
–Side banding N beside and below the seed will decrease the risk of ammonia toxicity compared to seed-row placement.
–Often the entire N needs of the crop can be met through sideband placement, but recent research indicates that placement 1” to the side and 1” below may NOT be sufficient separation for crop safety. Therefore, if the entire N needs are to be applied, the side band should be at least 2” from the seed row for solution or dry fertilizer and at least 2-3 inches from the seedrow for anhydrous ammonia.
–Mid-row banding the N between every second row at seeding maintains the greatest degree of seed safety. Less soil disturbance and hence more moisture retention would be achieved with a disk type mid-row bander unit compared to a shank-type.
–Application equipment for fluids is often easier to work with and cheaper to modify than equipment for granular or ammonia application.
–Anhydrous ammonia is a relatively low-cost nitrogen source and can be safely applied using side-band or mid-row band equipment, as long as the seed-fertilizer separation is adequate. Wing-tip injection of anhydrous ammonia on sweep openers has performed well for cereals on heavier soils. However at the shallower seeding depths required for canola or flax, there may not be sufficient soil coverage to prevent ammonia escape to the surface.
–Cost of this type of equipment with capacity to deliver seed and fertilizer separately can be high. Draft requirement and seed-bed disturbance may also increase and trash clearance may become a problem.
–Recently there have been concerns over shallow placement of urea or UAN in the soil and volatilization potential. Field studies in eastern Manitoba have shown urea placement at ½ to 1” depth had slightly more volatilization than at 1.5 to 4” placement, but yields were unaffected. Proper coverage and packing may help minimize any potential for loss.
5) Banding ammonia immediately after seeding
–Banding anhydrous ammonia immediately after seeding may have some advantages over topdressing in terms of cost and efficiency.
–If such a strategy is attempted, ensure that anhydrous ammonia is placed perpendicular to direction of seeding, using a narrow knife or low disturbance opener to minimize destruction of the seedbed. Also ensure that anhydrous ammonia is injected at the recommended depth to minimize the potential for seedling damage and to prevent ammonia escape from the trench.
6) Post emergence or mid-season applications
–Generally, applying all or part of a crop’s N requirements after emergence does not produce higher yields than pre-plant or one-pass applications.
–In some cases, producers may not be able band their N fertilizer prior to seeding due to time limitations or risk of seedbed quality problems. In other cases, producers may want to delay applying a portion of their N fertilizer until they have better estimate of their crop’s yield potential. In these cases, top-dressing is often a reasonably efficient method of applying nitrogen fertilizer if rainfall is received soon after application.
–Likelihood of a benefit from post-seeding applications increases with the likelihood of receiving significant in-season precipitation.
–If no N is applied at or before seeding, apply to canola prior to bolting.
–Losses will be higher on high pH soils.
–UAN is well-adapted to post-seeding N applications if it is dribble-banded, dropped below the leaf canopy or injected using coulter applicators after crop emergence. If applyied in a full-coverage spray, UAN may result in leaf burning and significant losses of N. Adding a urease inhibitor to UAN will further help to reduce foliar damage and volatilization losses from post-emergence applications. Similarly, adding a urease inhibitor to urea or using a urea fertilizer that contains a urease inhibitor (e.g., SuperU) will help to reduce volatilization from broadcast granular fertilizer.
Thanks to John Heard, crop nutrition specialist with Manitoba Agriculture, and Don Flaten, soil sciences prof at the University of Manitoba, for this content. Read their full report: Heard Flaten Spring N options 2017