Raised Bed Soil Calculator

Filling a raised bed is a volume problem with a soil-physics constraint attached. Volume is length times width times depth for a rectangular frame, or pi times radius squared times depth for a round one, with depth converted from inches by dividing by 12; 27 cubic feet make one cubic yard, the unit bulk suppliers price in. A 4-by-8-foot bed filled 12 inches deep therefore needs 32 cubic feet, about 1.19 cubic yards. Weight follows from bulk density, and moist blended garden soil runs near 80 pounds per cubic foot, so that same bed carries roughly 2,560 pounds, a load worth checking against the frame and against any deck or rooftop beneath it. Lightweight peat-and-perlite mixes fall closer to 40 to 50 pounds per cubic foot, dense wet topsoil closer to 100. Depth should be matched to rooting habit: 6 to 8 inches suits lettuce, herbs, and radishes, 10 to 12 inches suits peppers, beans, and cucumbers, and 12 to 18 inches suits tomatoes, carrots, potatoes, and asparagus, with 18 inches the practical minimum over a sealed surface such as concrete. Mix composition matters as much as volume, because native topsoil alone compacts badly in a confined frame; a common general-purpose blend is about 60 percent topsoil, 30 percent compost, and 10 percent perlite or vermiculite. Expect 10 to 20 percent settling over the first season as irrigation consolidates the mix, and plan on an annual inch or two of compost as top dressing.

Raised, deeply worked beds are an old market-gardening practice rather than a modern invention. The maraichers who supplied nineteenth-century Paris grew intensively on raised beds of manure-rich soil under glass cloches, taking several crops a year from very small plots and returning enormous quantities of horse manure to the ground. That French intensive tradition was transmitted to the United States by the English horticulturist Alan Chadwick, who combined it with biodynamic ideas while building the student garden at the University of California, Santa Cruz from 1967 onward. John Jeavons, working with Ecology Action in California, systematized the approach as biointensive growing and published How to Grow More Vegetables in 1974, giving home gardeners double-dug beds, close plant spacing, and explicit yield targets. The soilless fill that most bagged raised-bed products now imitate came from Mel Bartholomew, a retired engineer whose 1981 book Square Foot Gardening and accompanying public television series promoted shallow framed beds laid out on a one-foot grid. His later editions specified the blend widely known as Mel's Mix, equal parts by volume of peat moss or coconut coir, coarse vermiculite, and compost drawn from several different sources, which is the origin of the one-third-each proportion still quoted in raised bed guidance.