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GardenSMART :: Raised Bed Soil Depth: How Much Soil Goes In A Raised Bed

Raised Bed Soil Depth: How Much Soil Goes In A Raised Bed

By Darcy Larum, Gardening Know How
Photograph courtesy of Gardening Know How

There are many reasons to create raised beds in the landscape or garden. Raised beds can be an easy remedy for poor soil conditions, such as rocky, chalky, clay or compacted soil. They are also a solution for limited garden space or for adding height and texture to flat yards. Raised beds can help deter pests like rabbits. They can also allow gardeners with physical handicaps or limitations easy access. How much soil to put in a raised bed depends on its height, and what will be grown.

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About Soil Depth for Raised Beds

Raised beds can be framed or unframed. Unframed raised beds are simply garden beds made of mounded up soil. These are most commonly created for ornamental landscape beds, not fruit or vegetable gardens. The soil depth of unframed raised beds depends on what plants will be grown, what the soil conditions under the berm are and what the desired aesthetic effect is.

Trees, shrubs, ornamental grasses and perennials can have root depths anywhere between 6 inches (15 cm.) to 15 feet (4.5 m.) or more. Tilling the soil beneath any raised bed will loosen it so that plant roots can reach the depths that they need for proper nutrient and water uptake. In locations where the soil is of such poor quality that it cannot be tilled or loosened, raised beds or berms will need to be higher, and require more soil.

How Deep to Fill a Raised Bed

Framed raised beds are frequently used for vegetable gardening. The most common depth of raised beds is 11 inches (28 cm.) because this is the height of two 2 x 6 boards, which are commonly used to frame raised beds. Soil and compost are then added to the raised beds to a depth just a few inches below the rim. A few flaws with this is that many vegetable plants need a depth of 12-24 inches (30-61 cm.) for good root development, rabbits can still get into beds that are less than 2 feet high, and a bed 11 inches high still requires a lot of bending, kneeling and squatting from the gardener.

If the soil beneath a raised bed is not suitable for plant roots, the bed should be created high enough to accommodate the plants.

These plants can have 12- to 18-inch (30-46 cm.) deep roots:

Arugula, broccoli, Brussels sprouts, cabbage, cauliflower, celery, chives, corn, garlic, kohlrabi, lettuce, onions, radishes, spinach, strawberries

Root depth from 18-24 inches (46-61 cm.) should be expected for:

Beans, beets, cantaloupe, carrots, cucumber, eggplant, kale, peas, peppers, potatoes, squash, turnips

Then there are those having much deeper root systems of 24-36 inches (61-91 cm.). These may include:

Artichoke, asparagus, okra, parsnips, pumpkin, rhubarb, sweet potatoes, tomatoes, watermelon

Decide on the type of soil for your raised bed. Bulk soil is most often sold by the yard. To calculate how many yards are needed to fill a bed, measure the length, width and depth of the bed in feet (you can convert inches to feet by dividing them by 12). Multiply the length x width x depth. Then divide this number by 27, which is how many cubic feet are in a yard of soil. The answer is how many yards of soil you will need.

Keep in mind that you will most likely want to mix in compost or other organic matter with regular topsoil. Also, fill raised garden beds to a few inches below the rim to leave room for mulch or straw.


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