Option 1: Topsoil and compost, 50/50
What it is: equal parts screened topsoil and compost, mixed together.
- Pros: the cheapest option for large or deep beds, especially bought in bulk by the cubic yard. Holds nutrients and water well and settles into a stable, soil-like bed.
- Cons: quality varies a lot between suppliers. Ask to see the topsoil; it should be crumbly, not sticky clay, and free of rocks and roots. Heavy and hard work to move.
- Best for: several beds, deep beds, and gardeners on a budget.
Option 2: Square foot gardening mix
What it is: one-third compost, one-third peat moss or coconut coir and one-third coarse vermiculite, by volume. This is the mix popularized by Mel Bartholomew’s square foot gardening method.
- Pros: very light and fluffy, drains well yet holds moisture, and has almost no weed seeds. Easy to plant and weed.
- Cons: usually the most expensive, because peat or coir and vermiculite are sold in bags. Peat and coir are sold compressed and expand when loosened, so check the bag’s loose volume.
- Best for: shallow beds (around 6 inches) and small, intensively planted gardens. Our square foot garden planner calculates the mix as you lay out your beds.
Option 3: Bagged raised bed mix
What it is: a ready-made product sold as “raised bed soil” or “garden soil for raised beds.”
- Pros: convenient: just open and pour. Consistent, and easy to carry in a car.
- Cons: costs add up quickly for large beds, and products vary widely, so read the label.
- Best for: one or two small beds, or topping up.
What to avoid
- Pure compost. It shrinks a lot as it breaks down, and very rich compost alone can hold too much water and supply more nutrients than plants need.
- Plain native clay dug from the yard. It packs down and drains poorly in a bed.
- Bagged “topsoil” of unknown quality as the only ingredient. Some is mostly fine silt or sand.
How deep should the soil be?
About 6 inches of good mix grows most vegetables, and it’s the standard depth for square foot gardening. Deep-rooted crops such as long carrots, parsnips and potatoes do better with 10 to 12 inches. In very tall beds, many gardeners fill the bottom with untreated logs and branches to save soil, since most vegetable roots stay in the top foot.
How much will you need?
Multiply length × width (in feet) × depth (in inches) ÷ 12 for cubic feet. A 4 × 8 foot bed filled 6 inches deep needs 16 cubic feet; at 12 inches, 32 cubic feet (about 1.2 cubic yards). Plan on roughly 10% extra because new soil settles. Our raised bed soil calculator handles several beds, round beds and filler, and compares bagged against bulk cost.
Keeping the bed productive
Beds sink a little every year as organic matter breaks down. Top them up each spring with compost; University of Maryland Extension suggests that about an inch of compost a year helps keep vegetable and flower garden soil productive. Our compost calculator sizes the top-up for you.
Sources
- University of Maryland Extension: Understanding your soil test report
- Oregon State University Extension: How to use compost in gardens and landscapes