Surprise! We found no change in soil organic matter when comparing the incorporation of a green manure by full-inversion tillage versus a no-till method over the subsequent three years of an organic crop rotation. This result was surprising given the differences between the two systems, and the benefits of no-till systems shown in other research.
As part of Organic Science Cluster 2 (OSC2), Dr. Derek Lynch with myself, as a then Phd student, investigated how no-till termination of green manure impacts soil health.
We followed soil carbon in a four-year organic grain rotation. A mixture of hairy vetch and oat were used as a full season green manure, which was terminated either by full-inversion tillage, or using a crop roller as a no-till termination. In the no-till treatment, a heavy layer of mulch remained on the soil surface compared to the bare, highly disturbed soil of the tilled green manure.
Tillage in organic agriculture provides multiple services. It provides weed control and incorporates amendments, such as green manure residue or compost, which provide fertility to the following crop. However, tillage can be degrading to soil, resulting in loss of organic matter. It has also been shown to decrease microbial biomass and enzyme activity. This has led to criticism of organic agriculture for being degrading to soil health.
No-till or minimum till agriculture has been adopted as a best soil management practice. It has found success in conventional systems by building organic matter and reducing erosion. Minimizing tillage is one of the three pillars of conservation agriculture supported by the Food and Agriculture Organization of the United Nations, and one of the five core principles of regenerative agriculture supported by a new initiative from General Mills.
However, caution should be exercised in applying the results of research done in conventional production systems to organic agriculture. Organic and conventional systems often look very different. Many research studies comparing soil health in organic versus conventional systems shows no detrimental effect or even a slight improvement in the organic systems regardless of tillage levels.
This is not the only research to have shown soil health benefits under organic agriculture when tillage is used. A possible explanation could be the organic matter being added in organic systems, such as the green manure biomass used in the above study. This could provide a ‘buffer’ against the detrimental effects of tillage, by creating larger pools of soil carbon, and making the soil more resilient to disturbance.
I would also point out that there are two other pillars to conservation agriculture that organic agriculture does very well; using diverse crop rotations and keeping the soil covered, whether that’s with a cover crop or just some weeds! Soil health is not held up by just one pillar.
There is a new and growing body of research questioning some of the claims made about no-till agriculture. Flaws in measuring, such as not accounting for changes in bulk density caused by tillage, could be leading to over estimation of soil carbon stocks under no-till management. The shallow sampling depth of many studies also means we are not seeing the full picture. Carbon at deeper soil depths can form a large percentage of soil carbon stocks, and doesn’t necessarily have the same response to tillage as surface soil carbon.
While full adoption of no-till management in organic production may not be the final answer on soil health in organic production, there are other benefits to reducing tillage, such as reduced fuel costs. There are also many options for reducing soil disturbance without becoming fully no-till, such as moving to chisel till, which research has shown has less impact on soil degradation.
Current research funded by Organic Science Cluster 3 (OSC3) continues this theme and will add more insight into the impacts of tillage in organic production. Drs. Derek Lynch and Caroline Halde are investigating how tillage intensity impacts soil carbon and soil health on organic farms in Quebec as part of Activity 27: Soil health in organic tillage-based systems.
(OSC3) Originally published in Organic Science Canada magazine, Issue #2, Spring 2020
