A raised garden bed is only as good as what you fill it with. Native soil packed into a raised bed compacts quickly, drains poorly, and limits root growth, which are exactly the problems you are trying to solve by building a raised bed in the first place. Getting the soil mix right from the start sets up productive growing seasons for years before you need to replenish it significantly.
Native yard soil compacts when wet, especially in a confined bed. Compacted soil restricts root penetration, drains slowly, and tends to form a hard crust on the surface that sheds water rather than absorbing it. Raised beds that perform well are filled with a loose, well-draining mix that retains moisture without becoming waterlogged. Native soil can be part of the blend but should never be the majority of the fill, especially in beds deeper than six inches.
One of the most widely used raised bed formulas is equal parts compost, peat moss or coco coir, and coarse vermiculite or perlite, often called Mel's Mix after the Square Foot Gardening method. This blend drains well, never compacts, and provides consistent nutrients as the compost fraction breaks down. Vermiculite holds moisture and nutrients while maintaining an open, aerated structure. Coco coir is a sustainable alternative to peat moss with similar water retention properties and a more neutral pH. This is a more expensive fill than buying bulk topsoil, but for beds under 4x8 feet it is manageable and produces noticeably better results.
A more economical approach for large or multiple beds is a blend of 60 percent topsoil, 30 percent compost, and 10 percent perlite or coarse sand. Avoid fine play sand because it actually worsens drainage when mixed with soil. Use coarse horticultural or builder's sand if sand is part of the mix. Compost is the most important ingredient; do not skimp on it.
Volume is calculated by length times width times depth, all in feet, which gives you cubic feet. Divide by 27 to convert to cubic yards, which is how bulk soil is typically sold. For a standard 4x8-foot bed filled 12 inches deep, you need about 32 cubic feet, or just under 1.2 cubic yards. For the same bed at 6 inches deep, that is 16 cubic feet, or about 0.6 cubic yards. If you are buying bagged soil and compost, a standard bag is typically 1.5 or 2 cubic feet. Calculate how many bags you need before you load them into your car. Buying in bulk is significantly cheaper for anything over a couple cubic yards, and many landscape suppliers deliver.
Freshly placed soil mix will settle 10 to 20 percent in the first season as it compresses and the organic material begins to decompose. Fill your bed slightly above the rim at installation time, and it will settle to the right level within a few weeks of watering. Top-dress with compost each spring to replenish what has decomposed.
Beyond the base mix, several amendments improve performance for specific crops. A balanced granular fertilizer worked into the top few inches at planting time gives new beds a nutrient head start. Worm castings mixed into the top layer provide micronutrients and beneficial microorganisms. For acid-loving crops like blueberries, acidify the bed with sulfur before planting. For most vegetables, a pH of 6.0 to 7.0 is ideal. Test your mix with an inexpensive pH meter and adjust accordingly before the growing season.
Raised bed soil depletes faster than in-ground beds because the limited volume is heavily cropped and the organic matter breaks down continuously. Add one to two inches of finished compost to the top of each bed every spring before planting. Every two to three years, work it in lightly with a broadfork or hand cultivator to break up any compaction and improve structure. Avoid rotary tilling, which destroys the soil structure and beneficial fungal networks that develop in undisturbed beds. With annual compost top-dressing, a well-built bed maintains its structure and productivity for many years without major renovation.
