Conventional wisdom says less tillage is better when it comes to soil structure, but the issue is more complex when comparing organic soil health to zero till, according to Agriculture and Agri-Food Canada researcher Dr. Alan Moulin.
Moulin’s team looked at “soil aggregates,” or how soil particles bind together into larger groups, under high-input conventional till, reduced-input no till and organic production, where tillage was the main means of weed control.
The analysis was part of an 18-year study comparing different levels of input and tillage with three six-year crop rotations (a low-diversity rotation, a mix of annual crops and a rotation including both annuals and perennial forage). The team explored soil aggregation during the second of three six-year rotations.
Less tillage means a ‘stronger’ soil structure that’s less prone to wind and water erosion, the thinking goes, and, in fact zero-tilled plots returned the highest stability.
Reduced-input plots averaged 54.4 per cent wet aggregate stability across rotations, compared to 42.6 per cent in high-input and 40.6 per cent in organic systems.
That does not mean that tilled organic soils are automatically unhealthy, he said.
“When you consider aggregation in organic soils, you’ll often find a lower proportion of large aggregates but those aggregates are quite resistant to pressure — quite stable — and the reason that they’re stable under organic agriculture, at least agriculture that we did in this particular study, is because of the tillage,” he said. “The tillage dissolves or affects those aggregates that aren’t very stable; they don’t have a lot of cohesiveness. They disintegrate quickly.”
- Read more: Can organic no till work in the field?
At the same time, results showed a much higher accumulation of large aggregates at the surface in zero till, an indicator of soil health that is in line with conventional wisdom on preventing erosion.
Organic production beat out high-input systems on soil aggregate size, but still fell well behind reduced-input no till. An average 23.6 per cent of aggregates in organic soil were under five millimetres and just over 31.5 per cent of aggregates lay within the 1.3-millimetre erodible soil fraction.
In comparison, just over 21 per cent of reduced-input aggregates measured under five millimetres and only 27.9 per cent of aggregates lay under the 1.3-millimetre threshold. More tellingly, 28.7 per cent of aggregates measured between 12.7 and 38 millimetres, the most out of the three cropping systems (25.7 per cent in high input and 22.9 per cent of organics).
Between rotation systems, annual grains in reduced-input no-till plots had the highest number of large aggregates (over 12.7 millimetres) and the smallest number of fine, easily wind-eroded, aggregates (smaller than 1.3 millimetres) in the first five centimetres of soil, something the study attributed to both cropping diversity and the use of tillage.
Both soil carbon content and soil aggregation are among Moulin’s top indicators of soil health, he said, and yet both aggregate stability and the number of large aggregates may be considered signs of healthy soil.
The report eventually concluded, however, that reducing or eliminating tillage, along with increased cropping frequency and “appropriate combinations of fertilizer input and diversified cropping,” would benefit soil.
Moulin added that the length of the study might have weighed results against organic production. There have been many changes to the industry since his study started in the mid-’90s, he said, directly referencing Entz.
“The impact of organic systems may not be as dramatic as the systems that we included in this study that was started in 1994 and I think cover crops, to a certain extent, may have some significant influence on sustainability and soil health in these organic systems,” he said.