The era of black summerfallow is over, and direct seeding and zero tillage have pretty much solved problems of soil erosion on the Prairies. Or so goes conventional wisdom.
Not so, says David Lobb, a professor in the University of Manitoba’s department of soil science and senior research chair for the Watershed Systems Research Program (WSRP).
“I think there has to be a whole new generation of tillage equipment developed in the next five to 10 years,” Lobb said in an interview Feb. 4 after speaking at the Manitoba Soil Science Society’s 59th annual meeting in Winnipeg. “It has to be if you want to deal with the fact that we have highly variable, highly degraded landscapes.
“High-disturbance direct seeding, which is fairly common on the Canadian Prairies, can actually result in as much tillage erosion soil loss as a mouldboard plow. Because of the speed, and variability of speed, it moves soil much greater distances and with much greater variability.”
For years farmers and soil experts put most of the blame for soil erosion on wind and water, but research has shown the biggest culprit is tillage. Even though the research proving that has been around for almost 25 years, Lobb said it’s still news to many people.
“You should always think of any soil-engaging tool as a road grader with holes in the blade,” he said. “It’s exactly the way they behave. So anything you can do to accentuate that levelling of the landscape is going to cause soil loss in one area to accumulate in another.”
Even zero-till farmers can erode soil, Lobb said.
“The reality is they may not be moving as much soil but the nature of the disturbance is so variable they’re causing degradation, they’re causing loss. They don’t appreciate that.”
Putting it back
After 30 years of zero till, many farmers still have degraded hilltops because they were already eroded and any soil building due to zero till is probably lost because there’s still been some erosion from seeding, Lobb said.
The good news is that with tillage erosion, most of the soil is still nearby, usually in lower areas of the field. The soil can be scraped and dumped back on hilltops.
“It only takes about 10 cm of topsoil and when you do that, you get positive change in the wet years and the dry years,” Lobb told the meeting. He said studies show the cost can be recovered in four to six years.
“Its highly economic — probably the most economic land management practice that farmers have access to.”
Tilling and seeding aren’t the only contributors to tillage erosion — manure and fertilizer injection and row-crop tillage erode soil too.
“Root crop harvesting, like potatoes, will cause as much tillage erosion as all other forms of tillage combined,” Lobb said.
High and variable speeds contribute to soil movement. For example, equipment will go faster downhill than up, resulting in more soil going down than up. To compensate, farmers need either smaller tillage and seeding equipment or bigger tractors, or they have to slow down, but no one wants to do that.
Flat fields erode too
While gravity helps move soil from hilltops to low areas, tillage erosion happens in flat fields too. That’s why farmers in the famously flat Red River Valley keep having to clean out their surface drains, Lobb said.
Farmers should select tillage equipment that incorporates a little bit of crop residue and loosens the seedbed but doesn’t go excessively deep.
Given that wind and water don’t account for as much soil erosion as earlier believed, it raises questions about how much crop residue is necessary.
“So-called conservation tillage, such as chisel plowing, may not be very effective at protecting soil, particularly when you have other practices that might actually degrade it,” Lobb said. “And so-called conservation tillage systems may not actually protect water quality either.”
Lobb also questioned the value of riparian zones (vegetated areas around waterways), which are promoted to trap soil running off fields and filter out nutrients. But Lobb said not much soil is washed from fields and riparian zones don’t filter nutrients because run-off usually flows in small streams directly through the riparian area into the waterway.
“Water blows through those systems,” he said. “They do not filter. They cannot filter. They are not effective.”
To be effective, riparian zones would have to be wider and run-off would have to soak into the ground. Riparian vegetation would also have to be harvested.
“Because if you just keep putting nutrients into the vegetation and the vegetation is bleeding all those nutrients you’re not going to have any positive effect in the long run.” Lobb said. “You have to remove the nutrients by harvesting the vegetation and that’s something that people who promote riparian areas don’t want.”
Lobb is studying capturing a farm’s run-off in a large dugout, then either using it for irrigation or releasing it later in the season. Not only could nutrients and water be recycled, but the system could mitigate flooding.
Another strategy is to reduce run-off at the plant level by making soil more absorbent, Lobb said. That requires getting more organic matter and microbial activity in the soil, which comes from producing healthy crops and reduced tillage. It also requires drainage to remove excess water.