Soil organic carbon (SOC)—carbon that derives from organic materials and is stored or sequestered in soil—accounts for approximately 58 percent of the total organic mass found in soil. It is the largest global pool of terrestrial carbon. But historic levels of soil organic carbon far exceeded modern-day levels. Globally, most agricultural lands have already relinquished approximately 50 to 70 percent of their initial SOC stores. This loss of SOC contributes significantly to the levels of climate-altering carbon dioxide in the atmosphere; approximately 792 billion tons of carbon emissions from 1750 to 1999 can be attributed to the loss of SOC. The depletion continues today, fueled by land-use changes and the regular plowing and tilling of agricultural fields.
When a field or garden is tilled, previously buried organic matter—including carbon—is brought to the surface and exposed to oxygen in the air. Oxygen is necessary for most soil microbes to digest carbon and other organic compounds. By bringing buried SOC to the surface, tilling accelerates the rate of decomposition and carbon dioxide exhalation.
Because no-till agriculture significantly reduces the frequency and severity of soil disturbance, early proponents believed the rates of carbon sequestration would far exceed the rates observed in tilled agriculture. But studies have not always supported this expectation. Jeff Moyer, farm director for the Rodale Institute near Kutztown, Pennsylvania, explains that conventional no-till agriculture rarely results in carbon sequestration. “Our research clearly documents that conventional systems are lucky to hold steady and not lose carbon over time, even under the best management practices.”
According to Moyer, the carbon equation changes dramatically for organic agriculture. “In organic systems, just the opposite appears to be the case; carbon is sequestered at greater depths in no-till systems than tilled systems. The extended root production, along with the increased soil macro- and microbiological life, sequesters the carbon deeper in the soil.”
Long-term field tests at the Rodale Institute already show that every hectare (approximately 2.5 acres) of land in organic tilled agriculture has the potential to sequester more than 1,000 kilograms of SOC annually. This is equivalent to capturing approximately 3,500 kilograms—nearly 4 tons—of carbon dioxide per hectare per year. Now, researchers at the Rodale Institute and elsewhere are beginning to quantify the cumulative effect of combining organic agricultural practices with no-till.
“We can assume that when no-till is added to an organic system, even greater amounts of carbon will be sequestered,” says Moyer, who explains that preliminary results support this assumption.
Healthy, undisturbed soil may be the secret to carbon sequestration. Organic agriculture encourages a robust population of soil organisms, including beneficial mycorrhizal fungi, which may facilitate the accumulation of SOC. No-till agriculture, meanwhile, reduces the loss of SOC by reducing the rate of decomposition. By combining the health of organic soil with the stability of untilled soil, we may be able to maximize carbon sequestration.
While the research focuses on agriculture-scale production, the results can be applied to the home garden, as well. As Moyer explains, carbon sequestration is not the only reason to practice no-till cultivation in the home garden. “We, as gardeners and farmers, need to be aware that building the health of the soil is priority number one, and that if we do a good job of that, the crops we choose to grow will do well.
There is no doubt that healthy, well-structured soil supports robust plants and abundant harvests. Untilled soil supports numerous worm holes, ant nests, and other subterranean tunnels that increase water penetration. Soil aggregates—clumps of soil particles held together with organic matter—also tend to be larger and more abundant in untilled soil, which improves the water-retention capability of the soil. Finally, untilled soil tends to have a healthier population of plant-supporting soil organisms than tilled soil.
Moyer agrees. “It isn’t hard to imagine that, for the living organisms in the soil, any day that includes tillage is a bad day.” By simply setting aside the tiller, gardeners may be able to simultaneously decrease their chore list and increase their harvest. It’s just a bonus that organic no-till is also good for the planet.