The latest soil-management recommendation will come as a surprise: In some cases, producers should consider periodic tillage to reduce the risk of phosphorus (P) loss from conservation tillage systems.
According to Don Flaten, a professor in the University of Manitoba’s Department of Soil Science, zero tillage actually increases P loading from soils to surface water — P that drains off the land each spring during snowmelt.
Flaten says much of the research contributing to best management practice (BMP) recommendations for conservation tillage has historically been done under non-Prairie conditions, examining rainfall runoff rather than spring snowmelt runoff. In the Prairies, 80 per cent of our runoff occurs during snowmelt.
“Because the process of snowmelt runoff is so different from rainfall runoff, the fundamental controls on those two runoff systems are completely different, especially when it comes to nutrient loss,” he says. “When summer rainfall is occurring, it’s more evenly distributed and soil is thawed, so there’s infiltration, and the vegetation is growing and alive.”
Flaten says soils are actually very efficient at retaining vegetative P during the growing season. When P washes out of vegetation during summer rainfall events, very little of that P will make its way into surface water, he says.
By contrast, snowmelt runoff occurs overtop frozen soils that do not allow infiltration. Vegetation is dead or dormant, and does not intercept water or nutrients. Instead, they “bleed” into runoff water.
One runoff simulation experiment done under laboratory conditions in Pennsylvania examined runoff P in a soil tray containing manured soil and a cover crop, says Flaten. Under room temperature conditions, the cover-cropped soil “did a great job” of intercepting P and reducing erosion. But when the soil and cover crop were frozen, P losses were 10 times greater than when there was no cover crop.
The experiment “completely flipped a BMP into a bad management practice from a P loss perspective,” says Flaten, despite the many other benefits of cover cropping.
Flaten and colleagues at the University of Manitoba, Agriculture and Agri-Food Canada, Environment Canada and the Province of Manitoba, as well as local farmers, have been conducting research at the South Tobacco Creek Model Watershed for many years. He says they’ve learned much that has come as a surprise to the research and extension community.
“When we look at biophysical measurements, we’ve found that zero tillage, although it has lots of benefits and reduces sediment and nitrogen loading, increases P loading,” he says. “Periodic tillage, or intermittent tillage in the fall, can reduce the amount of P loss from those conservation tillage systems.”
The researchers have made other surprising findings: perennial alfalfa forage fields lose, on average, approximately 2.5 times more P than cultivated annual fields. They are also examining the effects of bale grazing on nutrient loss.
“A variety of practices have been evaluated, and we frequently come up with findings that don’t match conventional thinking. But all of these differences have to be considered in light of the very different process we have in the Prairies,” says Flaten.
This isn’t to say producers should throw out the BMPs with the runoff water. Flaten says there are many other benefits to BMPs that should be retained.
“We certainly want to minimize the buildup of soil test P in our soils, and we want to practice conservation tillage in a way that protects us from erosion,” he says. “But one of the most important things to remember is that a lot of our BMPs have a whole range of benefits. There may be detrimental factors associated with those BMPs, but we don’t want to throw away the benefits because we’re focused on P loss.”
The answer is not eliminating conservation tillage, but fine-tuning BMPs to minimize tradeoffs. Examples of other BMPs that are important include managing inputs and timing with respect to P fertilizer management, applying P after snowmelt and injecting nutrients whenever possible.
Producers can also minimize green vegetative material that’s susceptible to large losses during the snowmelt event. “For example, in our alfalfa trial, if we had harvested some of that alfalfa in October it wouldn’t be hanging around waiting to get into trouble during spring snowmelt losses,” Flaten says.
Other potential solutions include storing water upstream in wetlands or reservoirs that can be recycled and reused on agricultural land. “As we start seeing more long-season crops like soybeans that do better with a good supply of late season moisture, maybe there’s an opportunity to put that stored drainage water back on top of nearby land and increase yields,” he says.
“But to put all of our faith in one water-management practice, let’s say re-establishing wetlands with no economic return—it’s unlikely farmers will move very far in that direction. We’re trying to come up with market-driven solutions for water management.”