Wet conditions are causing a lot of concern over what’s happening with nitrogen in those sodden soils this spring.
John Heard, soil fertility specialist with Manitoba Agriculture and Rural Development, told the weekly Crop Talk webinar May 11 that this unusual year is causing farmers to contact him with a couple of areas of concern.
First, there’s whether their fall nitrogen application is still going to be in place and available for a growing crop this summer.
“A lot of farmers had the chance to put down what I consider reasonably priced nitrogen in the fall, and now they’re wondering if it’s still there after all this wet weather,” he said.
And farmers in drought areas who had high fall nitrogen residual levels in the nitrate form are wondering if that source would still be effective.
Fall nitrogen
When it comes to fall nitrogen applications, the question is whether they’ve stayed in the stable form they were first applied in, such as urea, which contains ammonium.
If the applications are done correctly, Heard says the majority of it should still be in the ammonium form.
“That stuff is not lost,” he says. “We consider it stabilized when it’s in that form in the soil.”
It comes down to the process of nitrification – a biological, microbial activity that’s dependent on temperature.
“Work done in Manitoba shows that if banded urea is put down in the fall and temperatures are 10 C, it takes 40 days for that banded urea to convert 100 per cent to nitrate,” Heard said. The conversion rate is reduced to essentially zero in frozen soils.
Since most Manitoba soils were snow covered, wet, and around 0 C until the end of April, nothing was happening. However, as soil temperatures rise, so does the speed at which urea converts to nitrates.
“So that fall nitrogen put down, providing it was in an ammonium form, was not doing anything,” Heard said. “But things are getting revved up now. You can see those temperatures have gone up, and so now those bugs will be working and starting to convert ammonium to nitrate, which is good for crop use, but it also moves it into a vulnerable form,” Heard explains.
Residual nitrogen
Last growing season’s drought has also cast a long soil-fertility shadow.
“Last fall we had a lot of residual nitrogen, sometimes two to four times more than normal,” Heard said. “The worse the drought, the more carry-over. That is actually money in the bank. That was nitrogen that was available for this year’s crop.”
But unlike fall-applied nitrogen, this is in the nitrate form, so it’s vulnerable to losses. The concern, during the recent wet weather, is nitrogen leaching. Leaching can happen any time water is moving through the soil, and it’s something that coarse or sandy soils are more susceptible to.
“If the ground was not deeply frozen and water was moving through it, anything in the nitrate form could be moving,” explains Heard. “It might have moved from the supersurface to the subsoils, hopefully staying within rooting depth. But again, it’s moving further from the surface.”
However, once it warms up, microbes once again become a factor. Heard points out that a made-in-Manitoba rule of thumb is that on saturated soils or flooded soils in the spring when there’s no oxygen in the soil, once things warm up (over about 5 C), losses of two to four pounds of nitrogen per acre per day are possible through denitrification.
“This one is the real whopper with our clay soils in the valley and in other areas – wherever water is going to be standing and oxygen levels depleted,” he said.
Managing losses
Heard points out some options for farmers who suspect they might have experienced nitrogen losses.
“You may wish to do or have your agronomist do a re-soil sampling of a few areas where you had high nitrogen carry-over,” Heard said. The strategy is to do it now before seeding and then adjust nitrogen rates later. Unfortunately, this isn’t easy right now because of the wet soil.
“I’m trying to do that, but it’s just too darned sticky out there,” he said. “I’m sure agronomists are out there trying to do it, but it is a challenge.”
Heard also warns against doing this if nitrogen was applied in the fall because the testing measures nitrates and will not detect the ammonium that has yet to convert.
“I think you’ll just frustrate yourself if you sample where fall nitrogen has been applied,” he said.
The other option is to use a nitrogen-rich strip.
“We put down either paths with more nitrogen at seeding or broadcast them in there, and then compare early-season growth or colour and top-dress if warranted. If you don’t see a difference come June, then your current rates are probably good to go.”
