If farmers continue spreading manure on the land at recommended rates, they risk overloading the soil with phosphorus, says a soil scientist with Alberta Agriculture.
Alberta scientists have been charged with measuring runoff and phosphorus-nitrogen quotients on the land since 1999 after a water quality study indicated agriculture contributes to water quality problems, said soil scientist Barry Olson.
“If we load the landscape with phosphorus, we will see more in the water,” Olson said.
Soil experts say six times more land will be needed to accommodate manure applications if government policy shifts from a nitrogen based formula to a phosphorus ratio.
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Under Alberta’s Agriculture Operations Practices Act, formulas are provided on how much manure from each livestock species may be applied to land.
“One day AOPA will move from nitrogen limits to phosphorus limits,” said Trevor Wallace, nutrient management specialist with Alberta Agriculture.
Farmers must keep records if more than 500 tonnes of manure are spread on the land. They need to record where and when it was placed and they must provide soil test results.
About 80 percent of Alberta’s soils could use manure but most is spread over a limited area because livestock production tends to be concentrated in a small region, said Wallace.
Manure increases crop yields by adding nitrogen and organic material to the soil. However, under current practices, equal amounts of nitrogen and phosphorus are often applied to the land when crops actually need about three parts of nitrogen and one part phosphorus.
If crops receive the ideal amount of nitrogen, then likely the phosphorus load is too high, causing lodging, delayed maturity and too much vegetation. Application guidelines are based on a three year rotation to use up the phosphorus.
Water quality is also a concern.
Phosphorus in water courses causes eutrophication, or phosphorus overload. Phosphorus feeds water plants, which then increase oxygen intake, thereby depriving and killing fish.
“If we over apply manure, nitrogen will accumulate in the soil with the potential of leaching through the soil profile in nitrate form and potentially contaminate ground water,” said Olson. “In the case of phosphorus, it will tend to accumulate in the soil surface and pose a risk to surface water quality in terms of runoff potential.”
He said a phosphorus sorption index is needed for Alberta to assess the soil’s ability to absorb phosphorus in different soil textures and moisture conditions.
Studies on coarse and medium textured soils monitored on plots near Lethbridge showed phosphorus sorption (absorption and adsorption rates) were greater in sandy soil. After eight years of monitoring, the soil was saturated with phosphorus.
The next step is to expand this work to field scale studies.
Wole Akinremi of the University of Manitoba said there is a growing concern with phosphorus in Manitoba’s streams and waterways.
Soil experts know each soil is unique in its ability to deal with phosphorus. The risk is low if soil has high phosphorus retention capacity and low saturation levels and high if the soil has a low retention and high saturation levels.
“Just using soil tests alone is not sufficient,” he said.
