There are many benefits to conservation tillage, but in Manitoba, it may not be the best way to retain moisture and nutrients
Conservation agricultural practices have come to be widely accepted as a good way to prevent erosion, improve soil health and reduce nutrient loading while maintaining moisture on the landscape. Conservation tillage, for example, has many benefits including reducing erosion — especially during rainfall events — reducing nitrogen losses to water, increasing water retention from rainfall, increasing biodiversity, improving soil organic matter, sequestering carbon and providing wildlife habitat.
Dr. David Lobb of the University of Manitoba, however, believes that these practices, while having ecological benefits, may not be terribly effective for moisture or nutrient retention under Manitoba conditions, where snowmelt accounts for 80 to 90 per cent of surface water supplies.
A study of two side-by-side tillage systems in the South Tobacco Creek watershed in southwest Manitoba found that although nitrogen and sediment losses were smaller, phosphorus losses from conservation tillage were greater than from conventional tillage. In both systems, erosion of soil particles was a minor contributor to phosphorus loss. In that study and several others in the Prairies, researchers found that dissolved phosphorus in runoff is the main form of phosphorus loss.
These results are similar to recent findings in Ohio’s Maumee River watershed that flows into Lake Erie. Researchers conducted a long-term study of the effect of different best management practices on dissolved phosphorus losses. They found that nutrient and manure management systems provided the largest reduction in phosphorus losses (40 per cent and 28 per cent respectively), but conservation tillage increased losses by five per cent.
The problem, simply put, is that runoff from snowmelt does not penetrate very far into the soil because the ground is still frozen when most of the melting occurs. Lobb and his colleague, Dr. Don Flaten, also of the University of Manitoba and Dr. Jane Elliott with Environment Canada, have found that 80 per cent of runoff during snowmelt occurs when the ground is impermeable, which means most of that snowmelt continues downstream until it reaches a river or lake. The proportion of dissolved phosphorus in that snowmelt runoff is relatively high and stable as it picks up soluble reactive phosphorus from thawing and frozen vegetative crop residues on the surface of the frozen soil.
To compound the problem, on the Prairies periodic flooding — often related to excess water flows caused by rapid snowmelt — causes phosphorus loading to increase. Natural or man-made areas such as ditches or retention dams that hold back runoff and retain nutrients during dryer times will over flow during flood events and contribute an even more concentrated nutrient load to the overall amounts ending up downstream.
“Even if nutrient application to agricultural land was to stop tomorrow, nutrients would continue to run off the land for many years,” says Lobb. “It’s a legacy of agriculture.”
Flaten and Lobb emphasize the need for more effective ways to reduce phosphorus sources on zero-tilled land, which might include periodic tillage, some form of vertical tillage, or removal of modest amounts of crop residues. “It’s important for farmers to be able to enjoy the benefits of zero tillage, while reducing their expectations that zero tillage is a universal solution to problems with water quality, especially in areas where dissolved phosphorus loss is a major concern and water erosionis a minor concern,” saysLobb. †