Features Agronomy Tillage
No reason to till a no-till field

For many no-till farmers, it would be heresy to pull out the cultivator. Perhaps some farmers might get boxed into a corner because of heavily rutted fields due to a wet harvest, such as in the Interlake of Manitoba heading in to 2009, or a weed problem that got away. But for the most part, there just is not a lot of reason to commit the ultimate no-till sin: tillage.
 

Tillage does not provide an agronomic benefit on no-till soil. Photo by Bruce Barker.

But what if it happened?  What if the neighbour’s GPS failed, the tractor driver was asleep, and the cultivator drifted across the headland onto another grower’s no-till field? Separate research studies by the University of Saskatchewan and Agriculture and Agri-Food Canada at Brandon sheds some light on a one time tillage event. They show that the world will not end, but occasionally tilling no-till land would require some pretty extenuating circumstances. “Strategic tillage really has very little effect on most of the soil properties,” says soil scientist Jeff Schoenau at the University of Saskatchewan, who supervised the work conducted by graduate student Chris Baan. “But I would be hesitant to recommend it as a practice unless there was a very good reason.”

The Saskatchewan study examined the effect of tillage on soil conditions and crop growth at three long-term no-till sites, one in each of the Brown (Central Butte), Black (Rosthern), and Gray (Tisdale) soil zones of Saskatchewan. The sites were previously managed as no-till for 10 years or more. In this study, the sites were subjected to a one year cycle of tillage at three levels of intensity involving spring cultivation only (MT), fall plus spring (CT), and fall plus spring plus disc (MXT), and a no-till (NT) control.
 
The Central Butte site had been in a cereal-chemfallow rotation. The Rosthern and Tisdale sites were in a cereal-oilseed rotation.

Physical soil parameters relatively unaffected
Soil cores were taken in the spring and fall after imposition of the tillage treatments. Total and particulate soil organic carbon, pH, electrical conductivity (EC), and mean weight diameter of the soil aggregates generally were not significantly affected by the tillage operations. However, there was a trend at all sites for tillage treatments to have lower mass of organic carbon in the zero to 10 centimetres (zero to four inch) depth, which suggests that if the cycle of tillage continued, the decline in organic carbon could become significant.

Tillage tended to reduce the bulk density at the five to 10 centimetre (two to four inch) depth and resulted in a small increase (approximately one degree C) in spring soil temperature at the five centimetre (two inch) depth.

Straw incorporation affected N availability
Schoenau points out that two differing responses were observed on the plots. On the Brown soil chemfallow plot, where very little stubble was present, the tillage tended to increase soil nitrates slightly, indicating increased mineralization of crop residue and soil organic matter.

NT – no-till control CT – fall plus spring MT – spring cultivation  MXT – fall plus spring plus disc Source: Baan, Grevers and Schoenau. U of S.

Conversely, on the Black and Grey soil plots, where tillage incorporated a large amount of cereal crop residue into the soil, tillage had a detrimental effect.
 
“One of the things that happens when incorporating fresh cereal straw residue with a wide carbon: nitrogen ratio is it caused immobilization,” explains Schoenau. “Incorporating that residue had a negative effect on nitrogen availability, and some negative effect on yield in the Black and Grey soil plots.”

The research also looked at the effect of tillage on phosphorus (P) stratification. Concerns about P stratification in long-term NT soils have been raised as a reason to conduct tillage in some circles.  Indeed, all three study sites demonstrated some P stratification in the NT treatment with concentrations of extractable P in the zero to five centimetre (zero to two inch) depth that were about three times or more the concentrations at the 10 to 15 centimetre (four to six inch) depth.
 
Sampling of the treatments showed that tillage, particularly the MXT treatment, had some effect on reducing P stratification at all the experimental sites. However, a general lack of significant effects of tillage treatments on P concentrations and uptake among treatments suggests that while the tillage reduced P stratification, its impact on P availability was minimal. “I must say that tillage did reduce the stratification to some extent. There is more uniformity with increasing tillage, but in the end, in terms of the impact on supplies of available P and uptake of P, and yield, we didn’t see much difference among treatments,” explains Schoenau.

The Saskatchewan findings were similar to Irvine’s work at Brandon
Byron Irvine from Agriculture and Agri-Food Canada, conducted a three year study near Brandon, Manitoba with two rotations: canola-wheat-pea and canola-wheat-flax. The objective was to determine the impact of tillage in a long term zero tillage system on crop yields, weed numbers, foliar disease and nutrient uptake. The tillage treatments included low disturbance direct seeding as the control; tillage with a heavy duty cultivator once in the spring prior to seeding; low disturbance direct seeding with heavy harrow the previous fall, and a Phoenix harrow in the fall prior to seeding once in three years.

While wheat yields were about two percent greater when low disturbance seeding was used, compared to low disturbance after cultivation or heavy harrowing, the difference was not significant. Heavy harrowing and Phoenix harrows had little or no impact on wheat yield.
 
Flax, pea and canola yields during the three years tended to be lower when planted after strategic tillage rather than low disturbance seeding however this was significant only with dry pea. In the year following tillage only pea yields were lower on land, which had received tillage. Irvine explains these differences may be due to larger numbers of pre-spray weed numbers in tilled ground.

Those pre-spray weed numbers were about 30 percent higher in the year where tillage occurred.  These numbers remained at this level in the year following tillage but returned to similar levels to continuous low disturbance planting two years after the tillage event.

Carbon, N and P levels remained stable, and the impacts on soil quality were minimal.

Ultimately, these two studies showed that tilling a long-term no-till field didn’t do much damage but the underlying message is that there would have to be a drastic reason to do it. “While soil quality isn’t affected much by a one time tillage event, my biggest concern on a field scale, is losing that protective cover. I’m worried about the potential erosion from wind or water that isn’t reflected in a small plot study. I have to emphasize the importance of that surface layer to protect the soil against erosion by wind and water,” says Schoenau. “There would have to be a pretty big reason to go in with the cultivator on no-till soils.”

Peter Gamache, Team Leader with Alberta Reduced Tillage LINKAGES in Edmonton, Alberta agrees. He says that there are so many benefits to long term no-till that a one-time tillage operation would have to be a drastic last resort. “There are tools to deal with just about every situation. There are farmers out there who have been in no-till for many years and they haven’t had to pull out the cultivator.”