A series of looming challenges makes planning now for spring nutrient applications crucial, two Manitoba soil scientists say.
A wet fall and stretched-out harvest kept many producers from applying nitrogen fertilizer. While spring nitrogen application is not unheard of, it will then have to compete with many other tasks, said University of Manitoba soil science professor Don Flaten.
That’s going to mean a big adjustment for many producers, since typically 45 per cent of nitrogen for canola production in Manitoba is fall applied, and around a third for both wheat and corn.
This may be compounded by logistics, said Flaten, as manufacturers, dealers and farmers attempt to haul nitrogen along with the usual phosphorus and potassium fertilizers.
“It’s going to strain the whole system,” he said.
As a result, Flaten said some agronomists are advising their clients to get their nutrition inputs in place now so they’re not left in the lurch this spring.
To get all nutrients in place, farmers will need to be prepared to use practices other than what they’re used to, Flaten said.
When deciding how to apply nitrogen this spring, or into the summer, producers have several methods and sources to work with.
Seven options are laid out in a bulletin prepared by Manitoba Agriculture and Resource Development crop nutrition specialist John Heard and Flaten.
Banding nitrogen in concentrated rows below the soil surface tends to be more efficient than broadcasting on the soil surface under western Canadian conditions, say Heard and Flaten.
This method protects the ammonia portion from volatilization loss (in which ammonia vaporizes off). It minimizes contact with soil micro-organisms, which can tie up and immobilize the nitrogen fertilizer. It can also slow the conversion to ammonium, and then to nitrate, reducing loss to denitrification and leaching in wet soils.
All forms of nitrogen fertilizer do well when applied as a spring pre-plant band if the fertilizer is separated from the seed. Anhydrous ammonia should be placed at least four inches below the soil surface and, if possible, seeding should be done perpendicular to ammonia bands.
Producers won’t need to delay seeding after application if the anhydrous ammonia is placed at recommended depths, especially on moist clay soils. However, pre-plant banding may delay seeding and dry or disrupt the seedbed, especially in clay soils.
Where spring tillage is required to manage ruts, spring banding fertilizer with a cultivator-style air seeder may accomplish both jobs. Low-disturbance disk drill seeders can also band granular fertilizer before or after seeding. Urea should be banded as deep as possible for seed safety and on an angle to seed rows.
Surface applications immediately before or after seeding
While broadcasting is a quick application method, Heard and Flaten say, granular urea or urea ammonium nitrate (UAN) solution sources of nitrogen can be lost through volatilization unless they are incorporated. Tillage during conventional seeding is usually enough to incorporate urea or UAN solution.
However, if any form of N fertilizer is in close contact with crop residues, it may be immobilized as it can be used by micro-organisms during the decomposition of those residues.
Where fields are rutted and tillage is needed before seeding, it makes sense to broadcast fertilizer beforehand. Generally, cultivators and disks will incorporate fertilizer to a depth of three to four inches. Harrowing or shallow vertical tillage may not be deep enough to eliminate volatilization loss of urea, or for placement of immobile nutrients like phosphorus and potassium.
Because of greater risk of loss to immobilization or volatilization, surface applications of nitrogen tend to be less efficient than in-soil banded applications. Rainfall soon after broadcasting N can increase efficiency, while soil with a high pH can decrease efficiency.
Surface applying N without incorporation may play a role in fertilization of forages, winter cereals and for post-emergent nitrogen delivery, but lack of incorporation will increase risk of loss.
If dribble banded to reduce contact with crop residues and soil, UAN solution will generally be a better choice than broadcast urea for surface applications. In several Manitoba field studies, surface dribble-banded applications of UAN were nearly as effective as in-soil banded applications.
Another option to reduce the risk of volatilization loss is to add a urease inhibitor to urea or UAN solutions. This type of inhibitor slows conversion of urea to ammonium, allowing more time for the urea to move into the soil with rainfall.
Placement in seed row
Placement of fertilizer in the seed row is an attractive option, Heard and Flaten write, since it eliminates an extra pass for fertilizer application. Seed-row placement is also a form of banding, so it is efficient for reducing nitrogen losses.
However, applying excess nitrogen with seed can lead to seedling damage due to a combination of salt and ammonia toxicity. This can limit response to nitrogen fertilizer, delay emergence, and decrease crop vigour.
A reasonable compromise may be to apply some of the fertilizer with the seed and broadcast or dribble band the remainder.
The amount of seed-placed fertilizer that can be safely applied depends on a number of factors including environmental conditions, crop grown, soil type, width of the seed/fertilizer band, row spacing and fertilizer source. Small-seeded crops like flax or canola are more sensitive to seedling damage than wheat or barley.
The rate applied with seed must be decreased with coarse (sandy) textured soils, low soil organic matter, low soil moisture, in the presence of salts or free lime, or with the use of wide row spacing. Since the risk of seedling injury is dependent on the soil type and growing season weather conditions, a rate that causes no damage in one field or year may cause damage the next.
With cereal crops, urea is generally more damaging to crops than ammonium nitrate (which is not commonly available anymore), while damage from UAN solution is moderate since it’s a blend of urea and ammonium nitrate.
The use of a controlled-release product like polymer-coated ESN increases the rate of nitrogen that can safely be applied with the seed.
Side banding or mid-row banding at seeding
Compared to seed placing, banding nitrogen to the side and below seed will decrease the risk of salt or ammonia toxicity.
Often the entire nitrogen needs of the crop can be met through side-band placement, but Manitoba research has shown that placement one inch to the side and one inch below may not be enough separation for crop safety. Therefore, in order to apply all the nitrogen required, the side band should be at least two inches from the seed row for solution or dry fertilizer and at least two to three inches from the seed row for anhydrous ammonia.
Mid-row banding nitrogen between every second row at seeding gives the most seed safety.
Banding nitrogen immediately after seeding
Some research has shown that banding ammonia immediately after seeding can have advantages of cost and efficiency over top dressing. This research was done many years ago, on heavy clay soils seeded with diskers or air seeders that scattered the seed behind their openers.
If producers attempt this, they must ensure that ammonia is placed perpendicular to the direction of seeding, using a narrow knife or low-disturbance opener to minimize destruction of the seedbed. Also ensure the ammonia is injected at the recommended depth to minimize potential for seedling damage and to prevent ammonia from escaping from the trench.
As mentioned earlier, low-disturbance disk drills could be operated after seeding to band urea or UAN solution into the soil with minimal stand disruption or injury.
Post-emergence or mid-season applications
In some cases, producers may want to delay applying a portion of their nitrogen fertilizer until they have a better estimate of their crop’s yield potential or because of constraints on the amount of N fertilizer they can apply before or during seeding. Historically in Manitoba, applying all or part of a crop’s nitrogen fertilizer has not produced higher yields than pre-plant or one-pass seeding and fertilizing application. However, new fertilizer products and practices have improved the efficiency of post-planting applications.
Top dressing N fertilizer is often a reasonably efficient method of applying nitrogen if rain falls soon after application. However, post-seeding surface applications will be subject to the same considerations as surface applications prior to seeding. UAN solution is well adapted to use for post-seeding nitrogen applications if it is dribble banded or injected using spoke-wheel or coulter applicators after crop emergence, or if a urease inhibitor is added to reduce the risk of volatilization. Applying UAN in a full-coverage spray may result in leaf burning and significant nitrogen losses.
Similarly, granular urea can be broadcasted after planting, but adding a urease inhibitor is recommended, unless rainfall is imminent shortly after application.
Generally, post-emergent nitrogen should be applied to cereals at or before the three- to five-leaf stage, and to canola prior to bolting. Recent research on split application of urea treated with a urease inhibitor produced highest spring wheat yield when 25 to 50 per cent of the N was applied at stem elongation; however, in all of these trials, more than five millimetres of rain were received within five days of application.
As adverse weather may delay post-seeding applications, some nitrogen should be applied at seeding, especially if available soil nitrogen is low.
Enhanced-efficiency fertilizers (EEF)
The directed use of EEFs has already been mentioned related to reducing volatilization losses (urease inhibitors) and improving seed safety (polymer-coated ESN).
Several EEFs also contain nitrification inhibitors, which slow the process where ammonium is converted to nitrate, decreasing potential leaching and denitrification losses under wet conditions. These nitrification inhibitors include nitrapyrin and DCD.
Similarly, polymer-coated fertilizer such as ESN controls the release of urea and slows the accumulation of nitrate-N. Such products may be beneficial when N fertilizer is exposed to prolonged excessively wet conditions and when crop uptake is low.