A raised bed is a large container, typically built with sides of wood, metal, or stone, placed on top of the native ground. This method offers improved drainage and better control over soil quality, which benefits plant health. Determining the exact volume of material needed is the first step in setting up a new bed successfully.
Calculating Soil Volume
The calculation for determining the necessary soil volume requires a straightforward mathematical formula. To find the volume of a rectangular raised bed, multiply the length, width, and height together, using feet as the unit of measurement for all three dimensions. The result is the total volume in cubic feet.
If the bed’s height is in inches, convert it to feet by dividing the number of inches by 12 before multiplying. For example, a raised bed measuring 4 feet long, 4 feet wide, and 12 inches high is calculated as 4 x 4 x 1 = 16 cubic feet. A 6-inch high bed would be 4 x 4 x 0.5 = 8 cubic feet.
Since bulk soil is typically sold by the cubic yard, converting your cubic feet total is often necessary. One cubic yard equals 27 cubic feet. To convert your volume to cubic yards, divide your total cubic feet by 27. Using the previous example, a 16 cubic foot bed requires 16 / 27, or approximately 0.59 cubic yards of soil.
Ideal Soil Mix Composition
The growing medium should be light, well-aerated, and rich in organic matter to encourage strong root development and proper water movement. A high-quality blend is generally composed of three primary ingredients: a base, organic matter, and an aeration component.
Compost provides essential nutrients and beneficial microorganisms, improving the soil’s structure and ability to retain moisture. The final mix should contain between 25% and 50% organic matter by volume. Topsoil or a quality soil-less potting mix provides the bulk and stability for the bed.
The third component is an aeration material, which prevents compaction and ensures good drainage for gas exchange in the root zone. Coarse sand, perlite, or vermiculite can serve this function. A balanced recipe often includes equal parts of topsoil, compost, and a draining material like coarse sand.
Strategies for Using Less Purchased Soil
Filling a large raised bed entirely with purchased soil can be costly. Strategies to reduce the volume of commercial material often involve creating a layered system, which substitutes a portion of the expensive top layers with organic filler. This method leverages materials already available in the yard or that can be sourced inexpensively.
The bottom layer should be filled with coarse, untreated organic matter that decomposes slowly. Suitable materials include logs, thick branches, and sturdy yard waste. This layer can occupy up to a third of the bed’s depth, promoting drainage and slowly releasing nutrients as it breaks down.
On top of the coarse material, finer organic debris can be added. Examples include grass clippings, straw, shredded leaves, or partially broken-down compost. This middle layer fills the air gaps created by the larger pieces and provides a richer material for the final soil layer to rest upon. This layering process replaces bulk volume with decomposing matter, significantly reducing the quantity of purchased soil needed.