How Much Soil Is Needed For 8 By 4 Raised Garden Bed

One of the major benefits of gardening in a raised bed is you get to control the soil. This is especially advantageous for those whose property has hard-packed or clay soil, issues with tree roots, or concerns about pollutants. And since good soil is the foundation of a healthy garden, you want to make sure you're setting your veggies up for success. So, what is the best soil for a raised garden bed?

Raised beds can be any size, but for a standard, rectangular bed, I recommend about three to four feet wide by six to eight feet long and 10 to 12 inches high. Those dimensions allow a gardener to reach in to plant, sow, and weed, without having to walk through it. This leads to another benefit of raised garden beds in comparison to gardening in-ground in traditional rows. The soil in a raised bed will remain loose and friable, rather than being hard-packed over time by footsteps. We also know there is a whole web of micro-activity happening, so it's best not to disturb and compact the soil for that reason, too.

How much soil do you need?

Filling a raised bed will likely require more soil than you think. A soil delivery might make the most sense economically. However, if it's not practical logistically, you'll need to purchase it in bags. You could also find an area in your yard from which you can move topsoil. There are some great soil calculators online that can help you figure out the amount you need.

If you happened to cut out the sod underneath where your raised bed will go, flip the pieces, grass-side-down to fill the bottom of your raised beds. There is lots of soil attached and the grass will break down over time. This also means you'll require less soil to fill the raised bed.

If you dug up sod to make room for a raised bed, flip the pieces upside-down and use them to fill the bottom.

The best soil for a raised garden bed

When I built my raised beds, I called around and ordered what I thought would be a good-quality triple mix. In Ontario where I live, triple mix is generally top soil, compost, and peat moss or black loam. A 50/50 mix seems to be more common in the U.S., which is a blend of top soil and compost.

If you're ordering a soil delivery, try to find out where your soil has come from. Topsoil is often taken from land being developed for new subdivisions. It might have sat for a long time and can be devoid of nutrients.

If you're purchasing bags of soil, look for labels like organic vegetable and herb mix or organic garden soil for vegetables and flowers.

Whatever you end up using, you want to make sure you amend it with compost. All that rich organic matter is an important component that will hold moisture and provide nutrients to your plants. Compost is an essential ingredient in the best soil for a raised garden bed, no matter which mix of ingredients you choose.

I filled my beds with about 3/4 triple mix, and even though it had compost in it, I top-dressed the garden with about ¼ compost. If you don't have a compost pile, there are all sorts different types of compost on the market. Garden centres sell everything from mushroom or shrimp compost, to composted manure or bags labelled "organic vegetable compost." Your municipality might even have free compost giveaway days in the spring.

Amending the soil in your raised bed

If you don't have a compost pile, keep some compost on reserve throughout the gardening season. If you're pulling out your spent pea plants mid-summer, not only are you removing a bit of earth, but those plants will have depleted the soil of nutrients. Topping up your beds with compost will add nutrients back into the soil to prepare it for whatever you plant next.

I like to add chopped leaves into the soil in the fall. Run them over with your lawnmower and sprinkle into your beds to break down over the winter. I have a compost pile where all the other leaves go. When they're ready, I'll use the leaf mould to spread in my gardens. To maintain the health of even the best soil for a raised garden bed, adding organic matter every year is essential.

In the springtime, I also will amend the soil with compost. I find the soil levels in my raised beds are usually lower from the weight of the snow. This fills them back up to the top.

Additional soil tips

If you have smaller containers to fill, check out Jessica's recipes in her DIY potting soil article
It's a good idea to do a soil pH test from time to time, so you can make the necessary amendments that will help your crops to flourish.
Growing cover crops is also a great way to add nutrients back into the soil.
If you are growing berries, like strawberries and blueberries, which like a more acidic soil, you can purchase soil that's been specially formulated to grow them, or adjust the level with elemental sulfur or aluminum sulfate.

Looking for raised bed inspiration?

Planting a raised bed: Tips on spacing, sowing, and growing in raised bed gardens
Raised bed designs for gardening
Elevated raised bed gardening
6 things to think about before preparing a raised bed garden

Find out the right soil for filling your raised beds