In many gardening and farming books, you'll see reference to "double digging." Double digging is a way of preparing a garden bed by - you got it, digging. Using a spade, you remove the first layer of topsoil one spade deep (8 to 12 inches) and set it aside. Then you dig a second 8- to 12-inch layer of soil (the subsoil). When this second layer is loosened, amendments such as compost, aged manure, peat moss, and/or leaf mold are added to improve the soil. After amending, the top layer is put back in place.
Why Double Dig?
Double digging is typically done on a small-scale farm. If you are growing a large amount of crops, you will likely want to use a tractor to cultivate the soil and apply any amendments. If you are gardening on a hobby farm or homestead, double digging can provide an inexpensive way to improve the soil without relying on expensive equipment.
Double digging relieves subsoil compaction - it loosens and aerates the lower layer of soil where the plants will grow. Compaction happens when tractors or construction equipment compress the soil, but it also happens naturally with layers of clay or silt in some soils. In compacted soil, plant roots and moisture are unable to penetrate the hard layers, so plants are stunted and water does not drain well.
Plant roots need oxygen and water, and double digging ensures that the soil has these elements for healthy, happy crops.