Article by Dale Cowan
Agronomy Strategy Manager and Senior Agronomist
AGRIS Co-operative - CENTRAL OFFICE
Understanding the depth and quality of your topsoil is critical for maximizing productivity and sustainability on your farm. A 2020 report titled “Soil Organic Carbon Content: Decreases Partly Attributed to Dilution by Increased Depth of Cultivation in Southern Ontario” (C.J. Warren, D.D. Saurette, and A.W. Gillespie) analyzed changes in topsoil depth, soil organic carbon (SOC), and total carbon stock over the past 70 years.
Key Findings
The study revealed that from 1950 to 2019:
- SOC levels decreased: From 2.85% (1950–1980) to 2.34% (2000–2019).
- Topsoil depth increased: From 17.8 cm (7 inches) to 25.1 cm (12 inches).
- Carbon stock increased: From 65.9 t/ha to 79.0 t/ha.
At first glance, the drop in SOC may seem alarming, but the researchers attribute this to dilution. As cultivation depth increased with larger, more powerful equipment, rich topsoil mixed with lower-quality subsoil, resulting in a lower percentage of SOC but a greater overall volume of topsoil.
Farming Practices and Their Impact
Over the past 30 years, changes in tillage and residue management practices have helped stabilize this trend. According to census data:
- Residue left on the soil surface increased from 17.8% in 1991 to 35.3% in 2021.
- No-till farming grew from 4% to 30.4% during the same period.
These regenerative practices, such as leaving crop residue on the surface and reducing tillage depth, have likely played a role in slowing SOC dilution.
Why Soil Organic Carbon Matters
SOC plays a vital role in soil health. The loss of organic matter at the surface can negatively affect:
- Soil structure and aggregate stability.
- Water infiltration and retention (each 1% of soil organic matter stores approximately 1 inch of water).
- Nutrient availability, especially nitrogen.
What’s Next for Your Fields?
This summer, one of our intern projects will investigate topsoil depth on selected client farms. By using soil sampling tubes, we’ll measure the “A” horizon—the darkest, most fertile layer—and record data to map and analyze soil carbon content. This baseline will help us understand how soil health varies across field positions and guide future recommendations for improving carbon retention.
Take Action
Curious about the depth of your topsoil and its impact on your yields? Contact your local AGRIS Co-operative branch to learn more about how we can help you assess your soil’s health and carbon potential. Let’s work together to build resilient, productive soils for the future.
Contact Us to get started!