Fall ammonia applications on dry soils

With rapid crop maturity and harvest many farmers will be looking to the next job on their list – fall fertilization. Typically, fall nitrogen is applied by 34-46% of Manitoba farms, and the most popular source is still anhydrous ammonia.

Last year in SW Manitoba, an inadvertent application of ammonia to excessively dry soil caused distress to adjacent landowners, so let’s review a few of the basics regarding N application.

Can soils be too dry for anhydrous ammonia?

Although soil moisture is low, it doesn’t take much moisture for the chemical reaction of ammonia (NH3) with H+ ions from water to convert to ammonium (NH4+).  This positively charged ammonium cation is then held on the exchange complex of clay and organic matter.

But dry soils do affect the physical closure of injection slots and may allow physical escape of ammonia gas. When soils are dry, big clods of soil may form and leave large channels for ammonia to move quickly and escape to the atmosphere. In those situations, ammonia loss can be substantial.

Clay soils that are very dry will be cloddy or lumpy and may permit too much gaseous ammonia to escape (Figure 1). The zone of ammonia dissipation from the injection point is larger in dry soil, so although the soil may be difficult to work, deeper injection may actually be required. However, deep tillage of dry clay soils may simply produce larger clods. Lighter textured soils will have better tilth than dry clay soils and will be more likely to produce a good seal to retain the ammonia.

 

What can one do if soils are dry?

Slot closure may be better on previously worked than on uncultivated soils if the soil flows and seals better. Such is not the case if soils were cloddy.

Deeper application may help put the ammonia closer to moisture and prevent the dissipation zone from reaching the surface. In the cornbelt where high N rates are applied on 30” spaced shanks, recommendations for dry soils are to place ammonia 6-8” deep, whereas typical ammonia injection depths are 3-4” on the Prairies. Attempts to place ammonia so deep here on clay soils may just produce larger clods.

Modifications to injection knives may offer some help. In-crop ammonia application for corn often uses closing disks or sealing wings (“beaver tails”) on the knives to aid coverage/closure of injection slots.

However, in most cases the farmer is best to wait for rainfall to improve soil tilth.

How do I know losses are unacceptable?

The only way to assess your soil conditions is a test run with your applicator. An application pass without N will indicate whether soils are too cloddy and injection slot closure is inadequate.

If after making a round with N, you can still smell ammonia from the previous application, make adjustments in depth or closure modifications. Or wait for rainfall to improve soil structure.

The “white puffs of smoke” are not ammonia gas, but clouds of water vapour. As long as ammonia smells do not persist after application, these white clouds should not be a major concern.

Will fall ammonia banding make my soils drier?

Fall banding can have contrasting effects of soil moisture. Under very dry conditions where snow-cover is limited, the loss of standing stubble through this banding tillage reduces snow trap on the field and may leave the field susceptible to evaporative losses.

However, in areas where snow cover is more reliable, fall banding may provide better moisture than a spring banded application. Spring banding can dry the seedbed, reducing available moisture and seedbed quality.

Additional information on fall N application is posted at:https://www.gov.mb.ca/agriculture/crops/soil-fertility/pubs/fer01s01.pdf

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Herbicide Application in Dry Weather – Rules of Thumb

See Dry Weather Weed Control on Manitoba Agriculture website for more details and complete recommendations and cautions.

Dry weather means both weeds and crops shift gears. Weed spectrums can be different,  post-emergent herbicides can  be less effective because weeds may have smaller leaves and/or thicker cuticles (waxy layer) that slows the penetration of herbicides.

Some herbicides withstand dry weather better than others so choose your product carefully. Here are some general guidelines on weed control during a dry period.

1. Remove weeds early.

2. Know your crop stage.

3.  Review the “Effects of Growing Conditions” section of each product in the Manitoba Agriculture Guide to Field Crop Protection to determine likely outcomes.

4. High Daytime temperatures can trigger crop injury in some herbicides.

5. Use full rates of herbicide.

6. Use higher water volumes.

7. Use split applications of broadleaf and grassy herbicides rather than tank mixing if the Guide to Field Crop Protection warns that antagonism can occur.

8. Check the forecast for rain – shallow, stressed crops roots may be impacted by herbicides moving into the root zone.

9. Compare the risk of crop injury to the risk of yield loss due to weed pressure.

 

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Can Sidebanded Nitrogen Cause Injury in a Dry Year?

Submitted by: John Heard, Manitoba Agriculture, Soil Fertility Specialist

With a lack of seedbed moisture, there are justified concerns about seedplaced fertilizer injury to canola and other crops.  How safe is sidebanded nitrogen? Research studies by  Dr. Cindy Grant documented considerable canola stand thinning when high rates of sidebanded urea or UAN solution were applied.  Agrotain (AT) served to reduce stand injury, but is no longer supported for this use by the manufacturer. 

Points:
  • Stands were thinned at even modest N rates, on a clay loam soil.  At high rates stands were reduced to 50%
  • Crop growth compensated for reduced stands and generally produced as good a yield as the Agrotain protected stands, except at the highest rate.
For more detailed analysis and discussion on the issue see the full .pdf
More topics on soil fertility can be found on Manitoba Agriculture’s Soil Fertility webpages.
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