How to calculate volume in square
and rectangular raised beds
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.
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
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.
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 percent topsoil
- 30 percent compost
- 10 percent soilless growing mix—also known as potting soil.
Where to Buy
In our Raised Beds department, you'll find DIY raised beds, raised bed corners, elevated raised beds, watering systems, pest protection, season-extending tools and accessories. For the widest selection of raised beds, visit our partners at raisedbeds.com.
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.
'Can I re-use old potting soil?' Yes!
YES, you can use the old soil in your pots, planters and raised beds. Just start the season with a special boost that ensures good results. With a couple ingredients, you can transform depleted soil into fertile ground for whatever you want to plant.
After a season of growth the soil in this raised bed is a couple inches low.
Top it off: If the level of soil has dropped, add fresh planting mix also known as potting soil. Ideally, the soil comes to within an inch or so of the rim of the planter or raised bed. Use a fork or hand tool to blend the new soil into the old soil.
Blend new soil into the old soil.
All-Purpose Fertilizer and Booster Mix
Recharge it: Measure the square footage of your raised bed or pot to figure out how much of the two "secret ingredients" you need for your bed.
- All-Purpose Fertilizer: Use 1/4 cup per square foot. A slow-release, 5-5-5 granular organic fertilizer that gives plants balanced nutrients for vigorous root growth and improved water penetration all season long. Contains only naturally occurring minerals, peanut meal and animal or vegetable meal.
- Container Booster Mix: Use 3 cups per square foot. This is the secret sauce! This potent, all-organic formula includes a complete diet of nutrients and trace elements to keep plants lush and healthy: concentrated plant and manure compost, washed granite, black rock phosphate, and other natural ingredients.
Fertilizer and booster mix sprinkled on the soil surface.
Sprinkle the granular fertilizer and booster mix on the soil surface. Use a hoe or cultivating tool to incorporate the ingredients.
Use a cultivating tool to mix the amendments into the top few inches of soil.
Plant, feed and repeat! Fill your planter or raised bed and stand back. Great results are sure to follow. During the season, keep your plants happy by feeding them regularly with water-soluble fertilizer, applied at the rate recommended on the package.
Plants grow stronger and faster with regular feeding.