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University of California
UCCE Master Gardeners of Orange County
To improve your soil, you'll need to add six to eight inches of organic matter to the entire bed. Help figuring out how much that is.
You can add any organic matter you choose. Spread your organic matter on top of the soil. Here's where the manual labor comes in. The organic matter needs to be mixed into the top six to twelve inches of soil. Digging it in and mixing it with a shovel is a great way to do this, as it moves a lot of earth without pulverizing the soil particles the way tilling can. However, if digging is just too hard on your back, using a tiller is a fine method.
When you're finished, your garden bed will be several inches higher than it was originally. It will settle some over the course of a season, but the soil structure will keep improving as microorganisms in the soil work to break down all of the organic matter you've added. The bed can be planted immediately, however. You'll be adding more organic matter on the top of the bed as a mulch once or twice a year. This will continue the process of improving soil structure and offset any settling that happens.
Improve soil for established plants
It is more difficult to improve soil around trees and other established plants than unplanted soil in, for example, a vegetable bed. To improve the fertility and the structure of the soil, consider using a thin (1 to 2 inch) layer of compost as mulch around the drip line of the tree or plant. Mulch is used on top of the existing soil surface.
Be careful not to cultivate the soil (dig in) around the tree or plant, since many feeder roots are near the surface and cultivation can harm them. The compost mulch can be reapplied as needed.
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