EDMONTON Ñ Moderate rates of manure can provide most of a crop’s nitrogen needs but too much is not a good thing, says Jeff Schoenau, a soil scientist from the University of Saskatchewan who is monitoring soil changes that take place in plots receiving repeated applications of manure.
“We’re looking at the impact of manure on soil and environmental quality, trying to get a handle on the best management practices for manure,” he told farmers at the recent FarmTech 2005 conference in Edmonton.
“Some of our plots have been in place for eight years. Alberta Agriculture and Ag Canada have some plots in Lethbridge running for more than 25 years.”
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Schoenau considers manure a resource, as a fertilizer that supplies macro and micro nutrients and as a soil builder, providing organic matter.
Because liquid manure is such a diluted product, containing 0.1 to 0.5 percent nitrogen by weight, and feedlot manure is generally less than two percent nitrogen, it’s more difficult to handle than commercial fertilizer such as liquid nitrogen at 28 percent nitrogen.
“The tendency is to want to put as much as you can close to the site of production, but manure seldom has the perfect balance of available nutrients to meet a crop’s requirements,” he said.
“We find we can get the best use of those manure nutrients when we supplement with commercial fertilizer. Some manures are too high in available phosphorus relative to nitrogen. With those manures, we need to apply according to a phosphorus recommendation and provide additional nitrogen with commercial fertilizer.”
He said some hog manure is low in available sulfur relative to nitrogen on sulfur deficient soil. Supplemental fertilizer sulfur helps get the best use out of nitrogen on those soils, he added.
Schoenau said a lot of liquid manure has a high availability of nutrients. Sixty to 90 percent of the nitrogen in liquid hog manure is available to plants in the year of application. In contrast, solid manure, particularly if there’s a lot of straw or wood chips in it, can have low availability in the year of application.
“A lot of the nutrients are tied up in the organic matter and it may not get released right away,” he said.
“It may take two or three years, which can be a benefit.”
All sorts of problems can occur if manure is applied too heavily and too often.
“We end up with lodging, haying off, where the nutrients provide excessive vegetative growth early on, uses up the water and the grain doesn’t fill, salt loading, toxicity with certain elements. Those can be issues that hurt your yield.”
Nutrient loading and pollution can lead to losses of that nutrient, while leaching or surface runoff can affect water quality.
Schoenau said research shows that manure is sustainable and economical when applied at appropriate rates and methods.
“Use the tools available Ñ soil tests, manure tests and equipment available Ñ to get the manure into the ground,” he said.
“That balance of available nutrients is important to get good nutrient utilization from the manure.”
Schoenau said after eight years of applying liquid manure at agronomically reasonable rates of 3,000 gallons per acre or about 80 pounds of nitrogen per acre, there’s no evidence of excess nitrates in the soil profile, down to 1.5 metres.
When he looked at high rates of application of 320 lb. of nitrogen per acre or 12,000 gallons per acre each year, after eight years he had more than 400 lb. per acre of nitrate nitrogen in the top 50 centimetres of soil. Even down to 1.5 m, there was significantly more nitrate nitrogen available than at a more reasonable 3,000 gallons per acre application rate.
“What we’re seeing here with excessive application is a slow migration of nitrate below the root zone. Once that nitrate nitrogen gets below the zone the roots can access it, if you’ve got shallow ground water, it may end up getting into it. That’s why you want application rates in balance with the nutrient removal potential of the crop.”
Cattle manure, applied at agronomic and excessive rates similar to the liquid swine manure, showed much less buildup after eight years.
“With this particular cattle manure, there’s a lot of straw and bedding mixed in with it. We saw little increase in available nitrogen, even at those high rates because we have a lot of that nitrogen still tied up as organic matter that has not yet mineralized to those plant-available forms.”
Schoenau said this doesn’t mean farmers won’t run into problems.
“If you keep doing that, we expect to see those mineralization rates get very high, and continue to release nitrate nitrogen for several years after application has ceased,” he said.
“It’s a slower process but longer studies show you start to see that nitrate come out. And if the crop can’t use it, it’s going to build up in the soil.”
The goal is to maximize crop use. Schoenau calculated the cumulative recovery of nitrogen by the crop during a four year study.
“With swine manure applied at the agronomic rate (100 lb. of nitrogen per acre per year), we got 43 percent back. Compared to urea at the same rate, at 50 percent, we’re doing just about as good with swine manure as with urea,” he said.
“When we put it on at an excessive rate Ñ 400 lb. of N per acre Ñ we’re down to 29 percent recovery. A lot of that nitrogen that we’re adding at that excess rate is getting lost from the system.”
