GLENLEA, Man. — Farmers who apply solid manure to their land shouldn’t expect an immediate return on investment because only a fraction of the nitrogen in the manure is available to the crop in the first few years following application.
Manitoba Agriculture guidelines suggest that 25 percent of the nitrogen in solid pig and dairy manure is available to the crop in the first year, but University of Manitoba scientists have determined the actual figure is much lower.
“After six years of annual applications of solid pig and dairy manures at high rates, the cumulative efficiency of all the solid manure N (organic plus ammonium N) was only five percent to seven percent,” the scientists concluded in a paper summarizing their research.
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Trevor Fraser, a U of M soil fertility technician, said using the measuring sticks of nitrogen and yield, solid manures performed poorly in field trials at the Glenlea Research Station south of Winnipeg.
“The liquid pig (manure), in both rotations (annual and perennial), did very well. It was almost the same as synthetic fertilizer,” Fraser said at a Glenlea field day in early August.
“The solid pig manure and solid dairy manure… were almost no better than the control in the first few years. We didn’t see any real nitrogen coming out those (manures)…. These types of manures, the solid manures, don’t release as much nitrogen as we give them credit for.”
During the field day, Fraser pointed to a rapeseed field to explain why so little nitrogen is immediately available to the crop. Residue from the manure was still visible on the soil surface, even though it was incorporated into the earth with a cultivator.
“Looking at that plot and seeing how much straw is on the surface, that gives you an idea of how slow the decomposition of those manures can be,” Fraser said.
Don Flaten, a U of M soil scientist who collaborated with Fraser on the research, said the full story is more nuanced than nitrogen availability.
“To look at it just from the nitrogen perspective is probably being overly narrow. There are a lot of other benefits to manures,” he said. “Solid manures are relatively rich in other mineral nutrients, like phosphorus and potassium…. Which is a blessing if you are working in cropping systems that regularly deplete and re-move more phosphorus…. Canola and soybean rotations, for example.”
When farmers apply solid manure, there are also long-term physical and biological improvements to the soil, Flaten said.
Mitchell Timmerman, Manitoba agriculture nutrient management specialist, said producers might opt to use solid manures strategically to address a specific fertility problem.
“If the solid manure is not a great nitrogen fertilizer, it could be a good source of another major macronutrient… that could be phosphorus,” he said. “It may justify delivering it to a field, a little farther away, if it’s to address that particular deficiency.”
Solid manure could also be used to remediate hilltops where the soil is thin and infertile.
“Strategic placement on the knolls (is useful) to try and remedy the damage (from) years of tillage, where the topsoil has moved down slope,” Timmerman said.
Flaten, Fraser and Timmerman all made the same point: applying solid manure isn’t a quick fix. It’s a long-term strategy to enhance soil properties and overall health.
Consequently, producers may need to temper their expectations.
“It takes time for these manures to release their nitrogen. Once it starts releasing you have no idea how much nitrogen you’re going to get out of them,” Fraser said, noting the amount of nitrogen released from solid manure may increase over time.
“Applying them to nitrogen rate might not be the best way to manage them. It might be better to apply them at a slightly lower rate and then try to get those long-term effects coming out.”
