The short answer
For a bed on open ground, mix topsoil and compost. The University of Minnesota Extension gives about 2/3 to 1/2 topsoil with 1/2 to 1/3 plant-based compost, and the University of Maryland Extension gives compost to topsoil at 1:2 or 1:1. On a patio or other hard surface, Maryland uses compost and a soilless growing mix in equal parts instead.
The recipes side by side
| Recipe | Parts by volume | Published by |
|---|---|---|
| Topsoil and compost | 2/3 to 1/2 topsoil, 1/2 to 1/3 compost | University of Minnesota Extension |
| Compost and topsoil | 1:2 or 1:1 compost to topsoil | University of Maryland Extension |
| 70/30 | About 70% topsoil, 30% compost | Iowa State University Extension |
| Thirds with sand | Equal parts topsoil, organic matter and coarse sand | Iowa State University Extension |
| Thirds with a drainage material | Equal parts compost; coarse sand, vermiculite, pumice or perlite; native soil or topsoil | Utah State University Extension |
| For hard surfaces | Compost and soilless mix 1:1; up to 20% topsoil in beds at least 16 in deep | University of Maryland Extension |
| Mel’s Mix | 1/3 peat, 1/3 vermiculite, 1/3 blended compost | Square Foot Gardening Foundation |
How to choose
The recipes differ less than they look. Most are topsoil with a third to a half compost; Maryland puts the organic matter in a bed that contains garden soil at 25 to 50% by volume. The sand and drainage-material recipes suit heavy native soil. The soilless recipes, Maryland’s hard-surface mix and Mel’s Mix, leave out topsoil and suit beds with nothing but paving underneath.
Buy what the recipe names. Bagged “organic soil” often contains no topsoil at all, the University of Maryland Extension says, and Washington State University Extension notes that commercial topsoil is usually landscaping fill, a mix of sand and compost. The soil types guide explains the labels.
What not to fill a bed with
Compost alone: NC State Extension advises against it because compost breaks down over time and can lead to disease and watering problems. Potting soil alone: the University of Minnesota Extension says it suits pots and seedlings but dries out too quickly in raised beds.
How much of each to buy
The soil mix calculator has each recipe as a preset and turns your bed into cubic feet and bags of each ingredient. For a 4 × 8 ft bed filled 12 inches deep (32 cubic feet), the 70/30 mix is 22.4 cubic feet of topsoil and 9.6 of compost: 15 and 7 bags of 1.5 cubic feet.
Raised bed soil mix calculator
Open the calculatorReferences
- Raised bed gardens, University of Minnesota Extension
- Growing Vegetables in Raised Beds, University of Maryland Extension
- Creating Raised Bed Planters, Iowa State University Extension and Outreach
- Raised Bed Gardening, Utah State University Extension
- Mel’s Mix Resources, Square Foot Gardening Foundation
- Soil, Plots, and Planters (AG-806), NC State Extension
- Organic Matter and Soil Amendments, University of Maryland Extension
- Raised Beds: Will They Benefit Your Vegetable Garden? (FS075E), Washington State University Extension