Why don't we apply N in the fall, like the Prairies?

Monday, October 7, 2013

The answer lies in our climate and soil, which are quite different from those found in the Prairies and which, in turn, dictate a different schedule

by KEITH REID

If combines aren't rolling across the countryside now, they will soon as the fall harvest gets underway. If you were farming in the Dakotas or in Saskatchewan, the combines would be followed by the anhydrous applicators putting down the nitrogen (N) for next spring's crop. Why is fall N application so popular in the West and why don't we do it in Ontario?

The advantages to the fertilizer industry of fall N application are clear. A dealer can get the same amount of fertilizer applied with half the equipment if the application is spread out between fall and spring, rather than all being done in a short window at planting. It also reduces the need for storage capacity and helps out with cash flow.  

There are also benefits for the farmer who applies N in the fall, although they come along with some drawbacks as well. The biggest advantage is time management, with one of the big spring jobs out of the way so planting can get done quicker. There will also be significant discounts on the price of the fertilizer, so it can look like fall application is a better deal. There are, however, a couple of major drawbacks to balance these advantages.

The first is that farmers give up some flexibility to adjust N rates according to the conditions in the spring.  A warm, dry spring can often mean more nitrogen is available from the breakdown of soil organic matter, reducing the amount of N fertilizer that is needed. If the full rate is applied in the fall, the opportunity to adjust rates has been lost.

The second and more significant drawback is that nitrogen applied in the fall may not be used as efficiently as N applied in the spring, so the crop will require more nitrogen to achieve the same yield. This drop in efficiency gets worse as you move away from a dry climate with cold winters into the more humid parts of the country, which is the biggest reason that fall application of N is not recommended in Ontario.

It might be worthwhile to review why fall application does work in the West. A number of factors have to come together if nitrogen is going to stay in the soil for six months from the time it is applied and still be available for crop uptake. First, the climate should be dry enough that there is not a lot of water leaching out of the rooting zone during the winter. Second, the nitrogen should be applied in the ammonium form (anhydrous ammonia or urea), so it will bind to the clay and organic matter in the soil.

Finally, the soil should be cold enough to keep the nitrogen in the ammonium form, rather than converting to nitrate, and it should stay that way from the time of application through the entire winter until just before spring planting starts.  

These conditions exist most years in the Prairies when the N is applied late enough in the fall that the soil has cooled below 10 C. Earlier N application into warm soil can result in large losses, because the ammonium converts quickly to nitrate, which can then be lost through leaching or denitrification. Even with proper application timing, however, there is the risk of a warmer than normal winter accompanied by excess rainfall in the spring that creates the conditions for excessive losses. If current trends continue and winters keep getting warmer, the losses from fall applied N may get larger and more frequent, particularly towards the southern end of the zone where this practice is common.

Contrast the ideal conditions for fall N application with the normal conditions in Ontario.  The amount of snow received over the winter is much more than is common on the Prairies. The onset of winter is quite variable from year to year, and there are often a number of warm spells after the soil has cooled down in the fall. Waiting until late fall to avoid this is not reliable because, in years when snow comes early, you will be shut out of the field. An added complication is that the soils get wet as the fall progresses, so they may be too soft to travel over by the time the soil has cooled enough for safe application.

Even if conditions do allow for application in the fall under good conditions, there is no guarantee that there won't be a number of thaws during the winter that allow the soil to warm up, or an early spring thaw followed by a couple of months of warm weather when the N could be lost.  

Our soils and climate are both quite different from those of the Prairies, so it makes sense that our nitrogen management should be too. There are good reasons why our nitrogen fertilizer should stay in the bin (or the tank) until spring. BF

Keith Reid is Soil Scientist with Agriculture and Agri-Food Canada in Guelph.