Great things can come from small beginnings. Square foot gardening lets you grow a lively assortment of vegetables and other plants in tiny spaces. An essential element of square foot success is carefully prepared soil. Square foot soil combines excellent drainage and moisture retention with balanced nourishment for plants. Fill your square foot beds with the combination of ingredients usually referred to as "Mel's Mix," in honor of the Mel Bartholomew, founder of modern square foot gardening method.
Square foot beds should be deep enough to hold 6 inches of soil. For a four-square bed, measuring 4 feet on each side, you will need 8 cubic feet of soil to produce a depth of 6 inches. If you want to grow potatoes, carrots or other deep-rooted crops, plan on another 0.5 cubic foot of soil for every 1-foot square box you stack up to make soil deep enough for long roots. The standard formula for square foot soil is one-third baled or uncompressed peat moss, one-third coarse vermiculite and one-third mixed compost varieties. This means you will need 2 2/3 cubic feet each of the three necessary materials.
Peat moss keeps soil light in texture and holds moisture until needed by plant roots. Gardeners concerned about the sustainability of peat moss resources substitute more renewable coconut coil fiber. If you buy compressed peat moss, the actual volume is double the amount printed on the package label. A package with a printed volume of 3.8 cubic feet actually contains 7.6 cubic feet of peat moss.
Vermiculite is a heat-expanded silicate product that maintains the air spaces growing plant roots require in addition to moisture. Crinkly and lightweight, vermiculite is a frequent ingredient in potting, seed starting and houseplant soil mixtures and is manufactured in several sizes. Square foot soil formulas specify coarse-grade vermiculite, the largest size. Some gardeners find coarse vermiculite difficult to buy in local garden centers and may need to resort to special-order or catalog shopping.
The organic materials in compost provide the nutritional content of square foot soil. Ideally, this portion of the soil is made up of five varieties of compost to assure a complete array of nutrients. Commercially available composts include aged poultry, cow or sheep manure, kelp or seed meals, bat guano, worm castings, rotted sawdust, rotted leaves or mushroom compost. If you make your own compost, use it as one of your five components, provided it has been aged enough to kill weed seeds. With five varieties in your compost mix, your garden will not need added fertilizer.
An old tarp and a bar rake make soil mixing easy. Put peat moss down first, so you can separate matted or clotted parts with the rake. Add vermiculite and your five kinds of compost, and blend it together with your rake or pull up the tarp edges to roll the mixture around.