How Much Topsoil For 8 Feet X 4 Foot Garden Bed 6 Inches High

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

By: Darcy Larum, Landscape Designer

raised bed

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 adding height and texture to flat yards. Raised beds can help deter pests like rabbits. They also can allow gardeners with physical handicaps or limitations easy access to their beds. How much soil goes in a raised bed depends on the bed height, and what will be grown. Continue reading for more information on raised bed soil depth.

About Soil Depth for Raised Beds

Raised beds can be framed or unframed. Unframed raised beds are often called berms, and are simply garden beds made of mounded up soil. These are most commonly created for ornamental landscape beds, not fruit or vegetable gardens. Unframed raised bed soil depth 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 up 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 created higher, resulting in more soil needing to be brought in.

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 2×6 inch boards, which is commonly used to frame raised beds. Soil and compost is then filled into the raised beds to a depth just a few inches (7.6 cm.) below its rim. A few flaws with this is that while 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 (61 cm.) high, and a garden 11 inches (28 cm.) high still requires a lot of bending, kneeling and squatting for 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. The following plants can have 12- to 18-inch (30-46 cm.) roots:

Arugula
Broccoli
Brussels sprouts
Cabbage
Cauliflower
Celery
Corn
Chives
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
Squash
Turnips
Potatoes

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 raised 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 top soil. Also, fill raised garden beds to a few inches below the rim to leave room for mulch or straw.

This article was last updated on 07/30/20