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May 29, 2020

Gardening by the Numbers: How to Calculate How Much Soil Your Raised Bed Needs

raised bed garden

By Amanda Rose Newton

photo courtesy of Brian Bentz

Starting your garden with quality soil is the most important factor in producing great vegetables.

Part of the advantages of a raised bed is that you are not stuck with the less than stellar soil in your yard as you can amend, mix, and create your perfect blend. The small size makes it easy to add in amendments like mushroom compost or earthworm castings.

The one downside to raised beds is figuring out how much soil you need! Since raised beds come in different shapes and sizes, determining the amount needed varies!

The Math – How to calculate soil for a raised bed

If using a formula to complete a calculation makes you want to break out into a cold sweat, do not despair! It is as simple as:

                                                                Length X Width= Area

                                                Area X Depth= Amount of soil needed.

Example: How Much Soil Do I Need for a 4×7 Bed with 10-inch sides?

To fill a 4×7 bed with 10″ sides, you will need 15 cubic feet of blended soil. To create the blend, use the following quantities:

4×7= 28

10 in= 0.83 Ft

28 ft X 0.83 ft= 24 cubic ft

Soil is either sold in bags by the cubic foot or by the yard if buying in bulk. You can easily convert your feet to yards by remembering there are 3 feet in a yard.

Therefore, 24 cubic ft would be 0.9 cubic yards. Easy!

Calculating soil amount for Geometric Shaped Beds

There are no rules saying all raised beds must be square! Be cutting edge with a fun circle, hexagon, or triangle-shaped raised bed! Of course, this does change the math a bit! Use the formulas bellow, and make sure to convert your feet to yards!

Circle= Circle Area = πr²

Hexagon=Hexagon Area = 3/2 * √3 * a²

Triangle= Triangle Area = b * h / 2

The Right Blend of soil for a raised bed

Once you have figured out the amount of soil you need, it’s time to purchase it! Vegetables, herbs, and fruit require a rich, light, and loose soil, and you can certainly create your own mix using the following general ratios:

  • 60 percent quality soil
  • 30 percent composted material
  • 10 percent soilless add-ins such as perlite.

If doing the calculation to figure out your soil needs was the most math you plan on doing for a while, Rockledge Gardens custom Garden Soil is ready to add to your raised bed- no blending required!

Our custom garden soil consists of all the nutrients and symbiotic mycorrhizae needed to promote healthy root growth in young plants. It also contains pine bark, peat moss, and mushroom compost that will slowly release nutrients and aid in adequate water retention.

Soil Season to Season

After a full growing season, you can “top off” your beds with fresh garden soil and blend in into the rest of the soil. Reusing old soil is perfectly acceptable.

Consider starting a compost pile as that will allow you to not only reduce waste, but also enable you to directly input nutrient-rich amendments to your raised bed gardens. This will aid in reducing the amount of fertilizer you will need to use.

Finally, to ensure your gardening success, be sure to take advantage of our success kit, which includes Espoma Biotone to give your plants the best possible start!