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N-Based Manure-Spreading Regulations Create P Overload

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“Those levels were in the top 15 cm (six inches) of soil… But, when we looked more closely, we saw that the highest P levels were in the top few cm of soil.

barry olsen

alberta agriculture research scientist

af contributor

It’s a problem that’s already caused grief for Manitoba hog farmers. Much of their industry expansion in the 1990s was based on manure-spreading regulations based only on nitrogen.

But at these rates, phosphorus (P) was not being taken up by the crop, and excess was leaching into waterways. Many existing barns are now not in compliance with the regulations. Hauling manure to areas where there is more available spread land is uneconomic. A moratorium has been placed on new hog operations in much of the province.

A similar problem may be facing Alberta. The levels of manure approved under current regulations may be too high, according to research by Alberta Agriculture research scientists at Lethbridge. Barry Olson and Ross McKenzie, working with Agriculture Canada researcher, Frank Larney, have found there’s a risk of phosphorus moving off cropland after applying manure or compost, even at recommended rates with immediate incorporation.

The problem is that nutrients in compost or in manure don’t match crop nutrient needs. Current recommendations for compost and manure application rates are based on the nitrogen content only. Phosphorus was assumed to be tied up in the soil, and that it would not move off the field. Unfortunately, that’s not entirely true.

Phosphorus is only slightly soluble, but a small portion of phosphorus dissolves in soil water and can move off the field in run-off. Also, phosphorus binds to clay and organic material in the soil and can move off the field if those soil particles are eroded by wind or water. And, if levels of P are high, even the small amount that dissolves in soil water can leach into groundwater, at levels that eventually become significant .

Once it’s moved off the field, water or soil particles carrying high levels of P land in ditches or fencerows and those high levels move into water that eventually drains into a creek or river.

Excess phosphorus in rivers and lakes boosts production of algae and other plants, which decay and take up oxygen, leading to “eutrophication.” This phenomenon is responsible for the “dead zone” in the Gulf of Mexico, and closer to home, the massive algae blooms which now cover most of Lake Winnipeg by the end of summer. Lake Winnipeg is the destination for most of the water draining from Canadian Prairies, including Alberta.

Compost also a problem

Olson applied compost and manure at rates that matched crop needs based on crop needs for either P or N, and added N fertilizer to balance the nutrient supply after P-based applications. He also applied manure and compost at triple the P needs every three years. The manure was incorporated twice using a double disk, working the plots both ways.

Over the next six years, soil N levels were the same whether the plots were fertilized with inorganic fertilizer or natural products, or cropped without added nutrients.

Where compost or manure was applied at rates calculated to match plant N needs, P accumulated. After six years, soil P was eight to 10 times higher where manure applications were based on crop N needs, around 215 lb./ acre (121 parts per million). Meeting crop N needs with compost resulted in even higher soil P levels, over 280 lb./acre (157 parts per million).

“Those levels were in the top 15 cm (six inches) of soil,” says Olson. “But, when we looked more closely, we saw that the highest P levels were in the top few cm (about an inch) of soil. That’s where nutrients in soil and run-off water interact or wind can move exposed soil particles. P levels in that top inch of soil were 500 lbs./ acre 248 to 310 parts per million, enough to cause problems in lakes and rivers.”

Crop yields weren’t affected by the nutrient sources. P buildup was reduced with less-frequent applications, Applying manure or compost at three times the crop’s P needs every three years didn’t lower yields or build up excessive P.

Basing maximum manure applications based on P needs of crops would lower the maximum per-acre levels for spreading manure. It would also change the economics of growing crops with nutrients from manure or compost. Sustainable application rates, to reduce the risk to water quality in our rivers and lakes may require manure be spread over a larger land base.

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