How to Calculate Soil Volume in Raised Beds
If you have several raised beds, it makes sense to order your soil in bulk. Some garden centers even offer soil blends that are made specifically use in raised beds.
Example: How Much Soil Do I Need for a 3x6 Bed?
To fill a 3x6 bed with 10″ sides, you will need 15 cubic feet of blended soil. To create the blend, use the following quantities:
- 9 cubic feet of topsoil (9 20-quart bags)
- 4.5 cubic feet of compost (4.5 20-quart bags)
- 1.5 cubic feet of soilless growing mix
- 1.5 cups Gardener's Supply All-Purpose Fertilizer
The Raised Bed Booster Kit includes two 6-quart bags of Booster Mix (compost, Chilean nitrate, black rock phosphate and other organic nutrients) and a 1 pound shaker of organic All-Purpose Fertilizer (5-5-5). Treats 18 square feet (a 3 x 6-ft. bed) for one season.
Good soil is the single most important ingredient for a good garden. Raised beds give you an immediate advantage over a regular garden, because when you fill your raised bed, you can fill it with a blend of soil that's superior to the native soil in your yard. Soil that's loose and rich with nutrients and organic matter will allow the roots of your plants to grow freely, and ensure that they have access to the water and nutrients they need to sustain healthy growth.
Before placing your raised beds in their permanent location, be sure to remove any grass or perennial weeds from the area. Use a garden fork or shovel to loosen the native soil to a depth of 6-10″. This will improve drainage and moisture retention in the raised beds. It also means that even with a 5″-high raised bed, your plants will think they're growing in a bed that's 12-18″ deep — plenty of room for carrots, potatoes, full-size tomato plants and most any other vegetable you'd ever want to grow.
'The Recipe'If you'll be filling more than one raised bed, you may want to buy your soil in bulk — by the cubic foot or cubic yard. Use the Soil Calculator to figure out the total amount of soil you'll need for each bed. For most situations, we recommend these proportions:
- 60% topsoil
- 30% compost
- 10% soilless growing mix that contains peat moss, perlite and/or vermiculite, such as ProMix
Keep in mind that proportions are approximate because soil volume varies from source to source. For instance, if the calculator specifies .444 cubic yards of soil for your bed, go ahead and round it up to a half yard.
If you do not have access to quality topsoil, an acceptable substitute would be a 50-50 blend of soilless growing medium and compost. If you want to add peat moss to the bed, it should not be more than 20 percent of the total mix. Peat moss is naturally acidic and is not a good medium for growing vegetables.
Recharging an Existing Bed- Pull any weeds that have overwintered.
- Use a hoe to loosen and fluff the soil because winter snow and rain can compact it.
- If the soil has settled, top it off with an inch or two of fresh compost and mix it into the existing soil.
- Replenish nutrients and re-energize the soil with the Raised Bed Booster Kit.
Gardener's tip: If you are not planting within a week, cover the bed with a layer of straw to protect the soil. This will, however, prevent the soil from warming up and drying out. So you'll need to uncover the bed at least a week before planting warm-season crops.
