Becoming a home gardener with raised beds is like being the chef of your own backyard buffet, and using the right soil (and the right amount of soil) is just as important as buying the right groceries for the recipe. We're covering all the info you need, from measuring your bed and converting the volume to cubic yards, to discussing the best mixture to create a nutrient-rich soil and help you foster a productive garden.

First, what exactly is a raised bed garden?

The term "raised bed" can be used to describe several different things. Yes, we use container gardens. Yes, we have "raised beds" that have a solid base. But different still, a traditional raised bed garden is defined as a planting bed that sits on top of your existing soil. "These beds are ideal for gardeners that live in an area with really poor-quality native soil or that don’t want a permanent garden but require something larger than containers to grow in," shares Amy Enfield, Ph.D., Senior Horticulturist at ScottsMiracle-Gro. "Raised bed gardens are not meant to be walked in but accessed from the sides and can be several inches to several feet in height and not exceeding 4 feet in width. Raised bed gardens have frames, but the material used is totally up to you. Traditional raised beds have sides made of wood or plastic, but you can also use stones, cinder blocks, pavers, corrugated metal, or even bales of straw.

She adds, "I tend to think of a raised bed garden as a hybrid between a container and an in-ground garden. It has defined sides like a container, but an open bottom, and its overall size more closely resembles an in-ground bed."

How to Measure Your Raised Bed for Soil

1. Measure your bed. Use a tape measure to find the length and width of the above-ground planter box, as well as the depth. The soil depth will vary based on what you're planting. Vegetables and herbs generally require a minimum soil depth of six to 12 inches, although root vegetables like carrots and potatoes benefit from more, upwards of 12 to 18 inches. Perennial plants require deeper soil. 18 inches deep to 2 feet deep is recommended.
2. Calculate cubic feet. The amount of soil you need for a raised bed garden depends on the dimensions of the bed and the type of plants you intend to grow. Start with some simple math in feet: Length x Width x Depth. That will give you the volume in cubic feet.
3. Calculate the volume of soil needed as cubic yards. Soil is often packaged or sold in bulk by the cubic yard. There are 27 cubic feet in each cubic yard, so next, convert cubic feet to cubic yards by dividing your number by 27 to determine the total soil volume.

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Choosing the Best Soil for a Raised Garden Bed

In terms of choosing the right soil, a raised bed needs something that’s denser than a potting mix but not as dense as in-ground soil. "A raised bed needs to be filled with a soil that will provide the right balance of moisture retention and aeration for good root and overall plant growth," notes Enfield. "The ideal raised bed soil uses soil amendments, and is a 50:50 blend of compost and sphagnum peat moss (some people will use potting mix). This mixture provides the right density for raised bed gardens but is still well-drained and nutrient-rich. Some people who grow in deeper raised beds (12 inches or more in height) will add topsoil to their compost-peat moss blend." She elaborates, sharing this list that helps us understand why specially-formulated soil benefits gardeners who grow in raised beds:

• Soil Quality: When you grow plants in a raised bed you have better control over the soil quality versus growing in-ground. "You're better able to tailor the soil in raised beds to suit the specific needs of the plants you're growing, whether it's adjusting the pH level or adjusting nutrient levels," she says.
• Drainage: "Because they are elevated, raised beds typically drain better than in-ground gardens," shares Enfield. "Using a well-draining raised bed soil helps prevent waterlogging, which over time can lead to root rot and other issues."
• Aeration: Plant roots need oxygen to grow. "When the soil in a raised bed is too dense (or heavy) it compacts down eliminating needed spaces for air," she notes.
• Nutrient Retention: Raised beds soil usually contains a higher percentage of soil nutrients to help ensure healthy root growth.

Should you mix your own raised garden bed soil, or buy it bagged?

Honestly, the choice is up to you, and often depends on your access to a fresh compost pile and other organic materials. Making your own blend of soil can save you money, especially if you want to customize with additional nutrients, but bags of soil can be more realistic. You can also start by making your own as a base, and top it off with extra garden soil from bags as it settles.

"If you don’t want to mess with mixing your own raised bed soil, the easiest solution is to purchase pre-blended soil bags for raised beds. For example, Miracle-Gro® Organic Raised Bed & Garden Soil, contains quick-release natural plant food that feeds plants for up to 2 months," notes Enfield. "The soil bag blend is peat-free and is formulated with compost from recycled green waste and processed forest products." (Processed forest products are aged byproducts from sawmilling and lumber operations, like shredded bark.)