Still Lots of N Fertilizer Options when Seeding is Delayed

Prepared by John Heard, MB Ag Soil Fertility Specialist (FPE)

In a wet spring when seeding is delayed, field operations should be minimized to permit seeding as soon as possible. Preplant applications of N, in particular may be compromised to advance seeding dates.

There are a wide variety of options for applying nitrogen fertilizer efficiently during a late, wet spring. Some suitable options for applying N fertilizer under these conditions are discussed below. However, producers should not compromise phosphorus application since banded or seed placed P is most efficient and options for post seeding are limited to spoke wheel injection systems.

Preplant banded immediately prior to seeding

Nitrogen fertilizer may be banded immediately prior to seeding using all nitrogen forms, including anhydrous ammonia (82-0-0) or urea (46-0-0), provided that the fertilizer is separated from the seed. Anhydrous ammonia should be placed at least 4" (10 cm) below the soil surface, and ideally seeding should be done perpendicular to anhydrous ammonia bands. There is no need to delay seeding after application if anhydrous ammonia is placed at recommended depths, especially in moist clay soils. The average efficiency for spring banding is rated higher than spring broadcasting. However, these efficiency ratings do not consider the potentially negative impact of preplant banding on seeding date and seedbed quality.

Preplant broadcast

Tillage during conventional seeding operations is generally sufficient to incorporate either urea (46-0-0), UAN solution (28-0-0). If these fertilizers are left on the soil surface, there is a risk that nitrogen will be lost due to volatilization or "gassing off" using an urcase inhibitor with broadcast urea or UAN would minimize volatilization loss. The average efficiency for broadcasting is rated lower than spring banding, but similar to fall banding for Manitoba soils. However, as mentioned previously, these efficiency ratings do not consider the impact of preplant banding on seeding date and seedbed quality.

Seed row application

Typically, up to half of the cereal crop N needs may be met with seed row applications, depending on the spreading width of seed and fertilizer, row spacing, soil type and moisture levels at seeding. Moist clay soils, for example, allow higher rates of N to be placed with the seed, compared to dry or sandy soils. Remaining N could be delayed until your spring soil test results are available. Refer to the factsheet Guidelines for safely applying fertilizer with seed for specific rates.

Side banded at seeding

Full crop requirement for nitrogen fertilizer might be applied as a band beside the seedrow if adequate separation exists between the fertilizer and the seed row. N must be placed at least 1 inch ( 2.5 cm) to the side of the seed row if solution or dry N is used, and 2-3 inches ( 5-7.5 cm) below the row if anhydrous ammonia is used. Several side banding openers are available to provide adequate anhydrous separation. Seeders equipped with mid-row banders allow full rates of N to be applied at seeding without endangering the seed.

Broadcast immediately after seeding, prior to crop emergence

Solid or liquid forms of nitrogen could be broadcast and harrowed in after seeding as part of seedbed firming and levelling. Shallow incoporation of urea by harrowing is insufficient to prevent volatilization loss. High rates of urea-N (>90 lb N per acre) applied without incorporation, especially on drill-seeded fields, may cause seedling damage to sensitive crops such as canola.

Banded immediately after seeding, prior to crop emergence

Limited research has indicated that post seeding banding of anhydrous ammonia may have some advantages over top dressing in terms of cost and efficiency. Research was conducted on heavy clay soils seeded with diskers or air seeders. If such a strategy is attempted, ensure that anhydrous ammonia is placed with a narrow knife or low disturbance opener to minimize disruption of the seedbed. Also, ensure that the anhydrous ammonia is applied at the recommended depth (at least 4" (10 cm) deep) to minimize the potential for seedling damage.

Top-dressing after crop emergence

Top-dressing allows for uncomplicated, quick seeding; it is also an efficient method of applying nitrogen fertilizer if rainfall is received soon after application, which is common in the spring in the Red River Valley. In addition, this method of application may allow farmers to soil test after seeding to determine the availability of fall applied N. However if conditions favor volatilization, loss of urea N may be very high (AAFC studies).

Ammonium nitrate (no longer readily available) is the most reliable source for top-dressing, since it is less subject to volatilization than urea or UAN solution. However, urea's average performance is still acceptable given good soil contact and favorable moisture (Table 1). Band stripping of dry N will minimize surface contact and may reduce volatilization potential.

The performance of broadcast UAN solution under adverse conditions may not be as high as for other forms of N (see Table 1). However, UAN solution is well-suited to spoke wheel injection or dribble application for improved performance under favourable conditions.

Table 1. Effect of Pre-and Post-Emergent Applications of Several N Fertilizers* on Yield of Barley in Manitoba (University of Manitoba)

Fertilizer Source Yield of Barley (bu/ac) Time of Application
At seeding 2 Weeks After Emergence 4 Weeks After Emergence
Ammonium nitrate (34-0-0) 57 54 58
Urea (46-0-0) 59 54 56
UAN solution (28-0-0)** 54 51 50

* Fertilizer application rate was 52 lb N/acre in all treatments. Check yield without N fertilizer was 42 bu/ac. ** In other experiments in Manitoba, post-emergent application of UAN solution was similar in efficiency to application at planting.

The success of top dressing N fertilizer is affected by many factors. If rainfall is not received after broadcasting, any form of N fertilizer may remain stranded at the surface, and be vulnerable to volatilization loss. Following are conditions affecting the potential for N loss through volatilization.

Table 2: Conditions that Affect Volatilization Losses of N Fertilizer

High Loss Potential Low Loss Potential
Moist conditions, followed by rapid drying Dry conditions
High soil temperatures Low soil temperatures
High soil pH (>pH 7.5) Low pH (>7.5)
High lime content in surface soil No lime at soil surface
Coarse soil texture (sandy) Fine textured soil (clay)
Low organic matter content High organic matter content
High amount of surface residue (Zero Till) Conventional tillage

The risk of volatilization loss of urea or the urea portion of UAN solution (28-0-0) can be reduced through the use of Agrotain urease inhibitor. This additive will delay volatilization of urea for up to 2-weeks, thereby increasing the probability of receiving sufficient rain for incorporation.

Early applications will produce the greatest yield benefit since crop yield potential is determined early. Ideally N should be applied to cereals at or before the 3-5 leaf stage and prior to bolting of canola. Later applications may result in increases in protein rather than yield.

For more information contact your local MB Ag Office.