Raised-Bed Soil Calculator

What goes into a good raised-bed mix?

A raised bed is only as good as the soil inside it, and the simplest mistake is filling it with a single material. Straight topsoil compacts into a dense block that holds water on the surface and starves roots of air. Straight compost shrinks fast, holds too much water and can be too rich in salts for young plants. The mix that most extension services and experienced growers recommend is a blend: roughly 60 percent topsoil for mineral structure and weight, 30 percent compost for nutrients and biology, and 10 percent coarse material — perlite, coarse grit or fine bark — to keep air spaces open. That 60/30/10 split is a widely used starting recipe, not a law of nature: heavy clay topsoil needs more aeration, while very sandy topsoil benefits from extra compost. Mel’s Mix, the recipe from square-foot gardening, takes a different approach — equal thirds of blended compost, peat moss or coco coir, and coarse vermiculite, with no topsoil at all. It is lighter, holds water well and needs almost no digging, but it costs more per bed and dries out faster in hot, windy sites.

How deep should a raised bed be?

Ten to twelve inches of soil suits most vegetables and herbs, and it is the depth this calculator defaults to. Shallower beds dry out faster and limit root crops, while beds deeper than 18 inches cost a lot to fill for little extra benefit unless you grow long carrots or parsnips. If your bed sits directly on soil rather than on concrete, roots will push down into the ground below, so a 10-inch bed behaves like a much deeper one — loosen the ground underneath with a fork before filling. On concrete or paving, the bed depth is all the root room your plants get, so go deeper and make sure there are drainage gaps or holes at the base. A bed deeper than 12 inches can also be part-filled at the bottom with coarse woody material or straw to save on soil, but that layer will settle and rob nitrogen as it breaks down, so keep it well below the root zone and top the bed up with compost each year.

Buying soil: bags, bulk and what to ask

For one small bed, bags are the easy choice: a standard 4 by 8 foot bed at 12 inches deep needs about 32 cubic feet, or roughly 22 bags of 1.5 cubic feet. For two or more beds, bulk delivery by the cubic yard is usually much cheaper. When ordering bulk, ask the supplier what is in the blend and whether it is screened — lumps of clay, stones and woody debris are common in cheap mixes. For food beds, ask whether the compost is made from garden waste rather than biosolids, and whether it has been tested. A good supplier will answer plainly; a supplier that cannot say what is in the mix is a reason to shop elsewhere. If you already have a bed filled and just need the volume, the soil volume calculator gives the plain total without the recipe split.

After filling: settle, water and plant

Fresh mix is full of air, so water the bed thoroughly after filling and let it settle for a few days before planting — the level will drop by an inch or two, and you can top it up before the first seeds go in. A thick layer of mulch on the surface keeps the mix from drying out and feeds the soil as it breaks down; the mulch calculator works out how much you need. In later seasons you rarely need to replace the whole mix: remove the top few inches, fork in fresh compost, and re-level. For help deciding what to grow in the finished bed, see the planting calendar, the seed-starting calculator and the full list of garden tools.