Double-digging is one of gardening’s most time-honored techniques, the need for which, like so many other practices, is being questioned. Researchers at the University of Missouri Extension have given double-digging a thumbs-up, writing in one of their publications: “Double-digging involves removing the topsoil the depth of a spade, setting the soil aside and then loosening the subsoil another spade’s depth. Finally, the topsoil is returned with added amendments, such as compost, manure or fertilizers. This labor-intensive soil-preparation method provides an excellent rooting zone for plants.” Organic Gardening, too, has recommended the method. Furthermore, double-digging is a key component of the biodynamic method of cultivation.
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.