by Keith Reid, Soil Fertility Specialist, OMAFRA
Ads are appearing in some farm papers promoting fall application of “protected” forms of nitrogen (N), which has led to questions about whether this is a good idea. Fall applied N is a common practice in the western half of the corn belt, and in the prairies, where the cold over winter tends to be consistent so N applied as ammonium will stay in that form, and where there isn’t much excess moisture around (most years) to leach the N out of the soil profile before crop uptake the next spring. If temperatures warm up in the fall, or in the early spring, then the ammonium can convert to nitrate, which is then subject to losses through leaching or denitrification during the spring before the crop is there to absorb the N out of the soil. These variable fall and winter conditions are much more common in Ontario than in the prairies. This means that the amount of fall applied N that remains in the soil available to a following crop ranges from 100% (rarely) to 0% (frequently).
We do apply manure in the fall, with varying success. Solid manure tends to give better results with fall application, since the organic N in the manure benefits from extra time to mineralize and there isn’t a lot of mineral N to be lost. Fall applied liquid manure gives variable results, since much of the N is already in the ammonium form, but the best results are from manure that is applied after the soil cools down in the fall.
Various products are promoted to delay the release of N into the soil (e.g. polymer coatings), but Ontario trials to date have not been promising. The delay isn’t long enough to ensure the N will be held consistently until spring. Controlled release fertilizer is great in theory, but so far has fallen short in practice under Ontario conditions.
The other consideration is economic…does it make sense to spend dollars in the fall when the same dollars invested in the spring will give a better return, and you won’t have to pay interest for an extra 6 months on borrowed money. There have only been a few times in the last 30 years when the increase in fertilizer prices over winter was enough to justify buying in the fall.
In short, the correct answer to the question, “When should I apply my N in the fall?” is, “Spring.”