If you have a patch of soil that you'd like to turn into a well-draining garden bed with deep, loose soil, then you'll need to put in some elbow grease and double-dig the bed. Here's step-by-step instructions for creating a great new garden bed:
What is double-digging?
Loosening the soil more than 12 inches down creates conditions under which plants' roots thrive.
How do you double-dig: Begin at one end of the bed and dig a 1-foot-wide by 1-foot-deep trench across the bed's width, placing the excavated dirt in a wheelbarrow. Next, work a garden fork into the floor of the trench and slowly rock it back and forth to loosen the soil. Continue until the soil in the excavated area is loosened. Dig a second, similar-size trench next to the first, this time placing the excavated soil in the first trench. Loosen the soil at the bottom of the second trench with the garden fork. Dig another trench and backfill the second trench, loosen the bottom of the third trench, and continue this process until you reach the end of the bed. Fill the last trench with the soil excavated from the first.
Why you should double-dig: Carrots, potatoes, beets, and other root crops need deep, loose soil to grow well. More important, double-digging is the first step in creating the most productive garden bed possible, insists John Jeavons, author of How to Grow More Vegetables. "Double-digging adds air deep into the soil and enables roots to grow and the microbes to create good soil structure," Jeavons says. This is, no two ways about it, a labor-intensive approach. But if the soil where you want your garden is very dense or hard-packed, making the effort to do this will pay you back handsomely as your garden grows.
(by Dan Sullivan found here)
Double digging is typically done when cultivating soil in a new garden, or when deep top-soil is required. On poor or heavy soils, or for vegetable gardens, double digging might be required every 3-5 years. In other cases, double digging is only really needed on starting a new garden, or on total replanting.
For photos in the step-by-step process, visit the Royal Horticultural Society site.