Great growing mixes balance three jobs: compost supplies nutrients and soil biology, a base (topsoil, coco coir, or peat) provides body and moisture retention, and an aeration component (perlite, vermiculite, or coarse sand) keeps roots oxygenated and drainage sharp.
Popularised by Mel Bartholomew's Square Foot Gardening, this is equal thirds compost, peat moss (or coco coir), and coarse vermiculite. It's light, water-retentive, and fertile enough to grow intensively in small spaces — the default for new raised beds.
Pots dry out faster and their roots can't reach subsoil, so container mixes lean heavier on base (60%) with less compost (30%) and a modest 10% aeration — nutrient-rich compost in high proportions can hold too much water in a closed pot. This is also why raised bed soil and potting soil aren't interchangeable: bed soil leans on heavier topsoil for structure, while potting soil stays lighter and fluffier for a container's tighter space.
Another widely used ratio: 40% topsoil, 40% compost, and 20% coarse sand or perlite for drainage and airflow. You can buy ready-mixed "raised bed soil" at a garden center, but mixing your own from local topsoil, quality compost, and coarse sand is usually more cost-effective. It's close to our Container Mix preset above, just with more compost and less base.
Containers are even more space-constrained, so the mix has to work harder — lean on a lightweight, well-draining potting mix rather than heavier bed soil. Compost, peat or coconut coir, and perlite make a solid DIY base, or a pre-mixed bag works just as well.
Ready to calculate your own mix? Head back to the calculator, or check the FAQ for bed depth, fertilizing, and NPK precision notes.