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Effects of Excessive Tillage

The following was a sidebar in my article “Build Better Garden Soil”, published in the April/May 2007 issue of Mother Earth News. I duplicate it here to highlight the importance of minimizing tillage of the soil.

As described in the conclusion to “Soil Ecology: The Basics of Fertility”, the most destructive practices of industrial agriculture are: monoculture, use of destructive synthetic chemicals, and excessive tillage. While homesteaders committed to sustainable gardening avoid the first two of these enemies of soil life, many of them continue to till the soil in ways and at intervals that inhibit longterm soil improvement. Even a spading fork, improperly used, can damage soil life and soil structure.

Tillage of soil releases a “flush” of nutrients, which can give an initial boost to crop growth that is impressive. However, this surge of available nutrients results mostly from the death of large numbers of soil organisms, whose biomass decomposes rapidly into the soil. These nutrients tend to be in soluble and volatile forms, however, and if not taken up immediately by plant roots, are leached to groundwater or outgassed to the atmosphere.

In the meantime, life cycles of many soil species are disrupted—fungal hyphae are broken, earthworm burrows are destroyed, etc.—and it can be some time before their populations recover. If the next tillage occurs before they have done so, we have put in place a cycle which inexorably degrades the health, vigor, and diversity of the soil food web.

One of the most insidious effects of excess tillage is the loss of carbon bound in the soil in the form of humus. Oxygen is necessary to soil life, of course, which is a major reason we work to improve aeration in soil through creation of looser, more open “pore structure.” Excessive exposure of the soil to oxygen, however, as occurs in heavy tillage, leads to oxidation of the carbon (humus) content and its loss to the atmosphere as carbon dioxide. Not only is fertility, so dependent on humus content, impaired: High-tillage agriculture is a major, and growing, cause of accumulation of CO2, a greenhouse gas, in the atmosphere. The amounts of carbon involved are not trivial: It is estimated that every one percent increase of carbon sequestered in a garden’s soil is equivalent to the weight of all the carbon in the atmosphere above that garden, right out to the vacuum of space. By reducing tillage while adding all the organic (carbon-based) matter we can, we reverse CO2 emission: Carbon is sequestered (bound up) in soil in the form of humus. The solution to climate change begins in your backyard.