Double-digging is not the same as mechanical subsoiling, which sends a tractor-pulled implement below the topsoil to break up the subsoil. What subsoiling and double-digging have in common, though, is that unless done properly, they can result in bringing subsoil to the surface, or mixing topsoil and subsoil. Both are to be avoided.
Manual double-digging depends on removal of the topsoil to the subsoil level; the topsoil is usually placed to the side on a tarp or in a wheelbarrow. The subsoil is then broken up to a spade’s depth, a layer of well-rotted manure or compost is laid on top, and the topsoil from the next row, or “spit,” to be dug is turned onto the compost. Thus, subsoil and topsoil are not “churned” together and earthworms and weather action help the compost to be incorporated
Why do it?
Double-digging improves the aeration of the soil, facilitates root penetration, and is especially recommended for crops such as carrots that root deeply, helping to prevent forking. It’s beneficial for new garden beds with long-term plantings, such as vegetables, perennials, cane fruits, and shrubs. If the soil needs amending, these plants will benefit from double-digging before planting.
Double-digging also improves soil drainage by breaking up compaction and removing hardpan on soil that has been subjected to repeated rototilling. However, it is claimed that deep digging disrupts the soil life, breaking delicate strands of mycorrhizal fungi on many plant roots and disrupting earthworm and beetle habitats. Also, so few research studies have been done that the benefits and deficits of the process are largely anecdotal: “I don’t like double-digging,” says Linda Chalker-Scott, Ph.D., an urban horticulturist for the Washington State University Extension. “It’s one of the practices that sound good, but for which I can’t find any reliable scientific evidence. We do know, however, that working the soil unnecessarily brings weed seeds to the surface, destroys soil structure, disturbs the roots of nearby woody plants, and harms or kills soil organisms. As a cheap and lazy gardener, I prefer to let nature do most of the work for me using thick layers of coarse, organic mulch.”
The key word in her remark is unnecessarily, for if the soil is already friable, drains well, and is rich in organic matter, it doesn’t require such intensive therapy. Eileen Weinsteiger, horticulturist at the Rodale Institute, agrees; she advises that gardeners who don’t have the time or strength to double-dig try growing yellow blossom sweet clover (Melilotus officinalis), a biennial legume that helps build organic matter while its long taproots penetrate heavy and compacted soil, opening pore spaces that improve drainage. (M. officinalis is considered invasive in certain areas; check with your local Cooperative Extension office.)
There is no denying that double-digging can be hard, time-consuming work, and if the soil is full of boulders or heavy clay, it would probably be best to choose one of the following alternatives:
- Build raised beds.
- “Single-dig” amendments into the topsoil just one spade deep. This is especially true if the only crops to be grown will be shallow-rooting leaf crops, beans, and so on.
Mulch repeatedly with decomposable materials. This will eventually generate a deep tilth, which, if not walked upon, will remain open and free draining.
Douglas Welsh, Ph.D., a Texas A&M University Extension horticulturist, comments, “With the terrible soil I have gardened in over the past 40 years, none were conducive to double-digging: black, sticky, gumbo soil; clay concrete; and 4 inches of soil and 60 feet of rock. So my strategy has been to build up, not dig down.” This is particularly good advice when the loamy soils are only a few inches deep over heavy clay subsoil. With such shallow topsoil, the action of double-digging might incorporate the clay subsoil. Better to actively incorporate organic matter on the surface and so build up the soil. Says Welsh, “You can create 12 inches of garden soil by incorporating 6 inches of organic matter into the top 6 to 8 inches of soil.” This is the approach used by Mark Smallwood, executive director of the Rodale Institute: “Instead of digging, I simply topdress my garden with good balanced compost to create and maintain biological diversity in my soil,” he says.
So the jury remains divided, but one thing they can all agree upon is that double-digging has no noticeable benefits for crop production of shallow-rooting, leafy vegetables, which would seem to indicate that gardeners should make up their minds according their needs—and the strength of their backs. Digging, like all garden activities, is healthy work—but only if done correctly.