Double digging is a traditional, intensive method of deep soil preparation used in horticulture and gardening. This technique involves manually working the soil to a depth of two spade blades, typically reaching 20 to 24 inches down into the soil profile. Cultivating this deep layer aims to permanently improve the physical environment of the garden bed. This deep cultivation process is generally a one-time effort for a new bed, setting the stage for years of productive growing.
How Double Digging Transforms Soil Structure
Disturbing the subsoil layer triggers the physical and biological transformations that benefit the garden bed. Breaking up the lower soil, often referred to as the second spit, fractures hard layers of compaction that typically form just below the depth of standard tilling. This mechanical loosening dramatically increases the soil’s macroporosity, which is the space available for air and water. Improved macroporosity allows water to drain more effectively, preventing waterlogging while promoting better water infiltration during rainfall.
The enhanced structure allows greater air penetration, or aeration, which directly supports beneficial soil microbes and fungi. These organisms require oxygen to thrive and perform functions like decomposing organic matter and cycling nutrients. By extending the biologically active zone deeper than the topsoil, double digging allows plant roots to access a larger reservoir of nutrients and moisture. This deeper access is particularly helpful for deep-rooted crops, such as carrots, parsnips, and potatoes, allowing them to develop unobstructed and resulting in healthier yields.
Step-by-Step Guide to the Technique
The double-digging process requires a garden spade, a sturdy garden fork, and a wheelbarrow to temporarily hold soil. The execution is systematic, ensuring the topsoil and subsoil layers remain largely in their original strata to preserve biological activity. The first phase involves marking out the bed and digging a starting trench along one end, making the trench one spade’s depth and width.
The topsoil from the first trench must be removed from the bed area and set aside, as it will be used to fill the final trench. The second phase focuses on the exposed subsoil layer at the bottom of this trench. A garden fork is used to loosen this subsoil to the depth of the fork’s tines, approximately 9 to 12 inches, without inverting or bringing the lower soil to the surface.
The third phase is amending the soil by incorporating organic matter directly into the loosened subsoil at the trench bottom. Adding materials like well-rotted compost or manure enriches this deeper layer, providing long-term fertility and improving the soil’s ability to hold water and air. Once the subsoil is loosened and amended, the fourth phase begins by digging the next parallel trench, one spade width away from the first.
The topsoil removed from the second trench is immediately placed into the first, now-empty, trench, covering the loosened and amended subsoil. This progression continues across the entire bed: digging the topsoil from the current trench to fill the previous one, then loosening and amending the exposed subsoil. When the final trench is reached, the reserved topsoil from the initial trench is used to fill it, completing the process and leaving a deeply cultivated, level bed.
When and Where to Use This Method
Double digging is a significant labor investment, making its application most justified where intensive soil rehabilitation is necessary. It is highly recommended when preparing a new garden bed or plot for the first time, especially on land heavily compacted by construction or machinery. This method is particularly effective for breaking up dense, heavy clay soils, which are notorious for poor drainage and creating a hardpan layer that restricts root growth.
The technique is also beneficial for areas designated for specific types of crops, such as deep-rooted vegetables or permanent perennial plantings that require deep anchoring. While mechanical tillers or single-digging only cultivate the top 8 to 12 inches and can sometimes create a plow pan, double digging is a deep, foundational investment. It generally only needs to be performed once, or perhaps every three to five years, to maintain the soil’s improved condition.