At face value, there are several reasons for applying fertilizer post-harvest.
It reduces the workload in spring, the price of nutrients is usually lower in the fall, and it minimizes winter storage risks.
So is fall fertilizer application a no-brainer? Not necessarily, says an agronomist with Alberta Wheat and Barley.
“I would make sure that all other options are entertained beforehand,” said Jeremy Boychyn. “I don’t think fall application should be completely eliminated as an option, but I think all of the risk factors should be looked at.
“The main goal is to ensure producers evaluate the risks and take appropriate action to mitigate those risks.”
Fall application can come back to haunt farmers in the spring, he said. And the biggest risk is nitrogen loss due to denitrification and leaching.
Banding beats broadcasting
There are few, if any, agronomic advantages to fall application. And the time and price savings can be cancelled out by all the things that can go wrong prior to spring seeding, said Boychyn.
Generally, the closer in time a fertilizer application is to seeding, the more effective it will be, he said. Conversely, the further in time away from seeding, the more opportunity there can be for things to go sideways.
Placement considerations in fall are pretty much the same as in the spring — you either broadcast the fertilizer or place it in a band below the soil surface. But there are nuances at play. Boychyn has a hierarchy outlining the worst to the best options.
Typically, surface broadcasting without incorporation is the worst option, he said. Surface broadcasting with incorporation is better, but banding is by far the best choice.
“Broadcasting comes with great risk — run-off and volatilization. Banding can reduce those risks by nearly eliminating both of those factors,” he said. “The risk with (fall) banding is soil saturation in the spring. If the urea or anhydrous encounters saturated spring moisture, the bacteria will convert the ammonium from the urea or anhydrous to nitrates and then be subject to leaching and denitrification, with microbes using the nitrates as an oxygen source.”
Even though banding is the best option, some producers still broadcast in the fall.
“I would not recommend broadcasting urea in the fall without working it in,” he said. “However, working it in destroys soil structure and eliminates the benefits of a no-till or minimum-till system.”
If you’re going to apply in the fall, do so at the right temperature, said Boychyn, who recommends applying at temperatures of 10 C or less, particularly when broadcasting nitrogen on the surface. Otherwise, “you are going to get breakdown of urea and get some loss of nitrogen to the environment.”
Keep soil type top of mind
Some rules of thumb go right out the window depending on where in Alberta you farm. Boychyn recommends producers be more wary of fall fertilizer application in areas where high spring moisture is common.
“The risk of fall banding is much lower in the southern parts of the province as compared to the northern parts, which typically receive more moisture. I am more comfortable with banding urea/anhydrous in the southern part of the province due to less risk of saturated soils. Wetter parts of the province would be at higher risk of denitrification in the spring.”
This is where new tools may come in handy, said Boychyn.
Nitrogen stabilizers such as Agrotain come with claims of reduced nitrogen loss. Companies producing high-efficiency fertilizers such as SuperU say they can reduce ammonia volatilization and prevent denitrification.
Producers may want to consider these in their nutrient management plans, said Boychyn.
“I think there are some opportunities with these tools.”
Coarse soils require particular care when making the decision to fall apply or not, he said.
“Those guys on coarse soil are at higher risk of leaching in wet springs.”
Phosphorus pros and cons
Not all nutrients are created equal. For example, there’s sentiment in some quarters encouraging the fall application of phosphorus. Boychyn said he is much more comfortable with banding phosphorus in the fall compared to nitrogen, which more easily moves around the soil and puts it at risk of loss.
“As we are increasing our yields, producers are trying to add more phosphorus,” he said. “It is becoming harder to do that at seed-safe rates, so displacing some bulk phosphorus application to the fall can be an option.”
Producers should still avoid applying all their P in the fall.
“I always encourage producers to have at least a little bit of phosphorus with their seed,” said Boychyn. “Displacing all of the phosphorus to the fall could potentially have some disadvantages depending on your soil phosphorus levels.
“Research has shown crops seeded into soils with lower levels of soil phosphorus respond better to seed-placed phosphorus.”
Most producers base application rates on historical yields, soil sampling, nutrient carry-over, and expected moisture. However, there is a small minority who combat nutrient loss with overapplication.
This needs to change, said Boychyn.
“I hear producers say, ‘If I’m looking at potentially losing 20 per cent of my nitrogen to volatilization, the price of nitrogen is cheaper so I’ll just purchase more and apply more,’” he said. “I’ve heard that a couple of times, unfortunately.”
It’s not an approach he agrees with.
“When you have a hole in your door in the winter and you know you’re losing 20 per cent of your heat through that door, are you going to increase your temperature by 20 per cent or are you going to try to patch that hole?
“We need to be thinking about how to manage product applications to limit that loss rather than apply over and above what is necessary.”