The success of a raised garden bed relies almost entirely on the quality of the material used to fill it. Unlike traditional in-ground gardening, a raised bed provides a closed system where the gardener controls the growing medium from the start. This requires specific planning to source the right volume and composition of materials. A properly filled raised bed ensures plants have the consistent drainage, aeration, and nutrient supply they need to thrive.
Determining the Needed Volume
The initial step in acquiring soil involves accurately calculating the volume your raised beds require. For a rectangular bed, measure the length, width, and desired depth, converting all measurements into feet. Multiplying these three dimensions (Length × Width × Depth) provides the total cubic volume in cubic feet. Because soil, especially mixes containing organic matter, will compress and settle over time, increase the total by at least 10% to account for this compaction. When purchasing material in bulk, convert the total cubic feet into cubic yards by dividing the number by 27.
Understanding the Ideal Raised Bed Mix
A customized raised bed mix is engineered to optimize conditions, a combination significantly different from common native garden soil or commercial potting mixes. Native soil often compacts too easily, restricting root growth and drainage, while standard potting soil can be too light and expensive to use in large volumes. The ideal medium is a blend of three functional components to balance structure, nutrition, and aeration.
The structural component is often a high-quality loam or screened topsoil, providing a mineral base of sand, silt, and clay particles. This balance allows for both water retention and adequate drainage, preventing anaerobic conditions that inhibit root function. The second component is organic matter, primarily compost, which is the nutrient engine of the mix. Compost is rich in micronutrients, improves water absorption, and supports the beneficial microbial activity that makes nutrients available to plants.
The final component is an aeration amendment, typically perlite or vermiculite, used to maintain a loose structure. Perlite is an expanded volcanic glass that creates permanent air pockets, ensuring sufficient oxygen reaches the roots and facilitating rapid water drainage. Vermiculite, a hydrated mineral, functions similarly but holds substantially more moisture, making it a better choice for beds prone to drying out quickly. A common starting ratio is a 50/50 blend of screened topsoil and high-quality compost, with aeration amendments added based on the specific needs of the plants or the local climate.
Sourcing Options: Bagged vs. Bulk
Deciding where to purchase the material depends heavily on the project size and the total volume calculated. For smaller projects, such as one or two average-sized raised beds, bagged materials are often the most convenient choice. Bagged soil components are readily available at garden centers and big-box stores, are easy to transport, and simplify blending specific amendments like perlite or peat moss.
However, bagged soil is significantly more costly per unit of volume than bulk material. For larger projects requiring more than one cubic yard of material, purchasing in bulk from a local landscape supply yard or specialized soil producer is far more economical. Bulk soil is sold by the cubic yard (27 cubic feet) and can cost less than half the price of the bagged equivalent.
Sourcing bulk material requires arranging for a dump truck delivery, which necessitates a clear, accessible drop-off location. A major benefit of bulk suppliers is the ability to inquire about the exact composition of their raised bed mixes, which often consist of a professionally blended ratio of screened topsoil and compost. Always confirm the supplier’s mix ingredients to ensure the material meets the structural and nutritional requirements of a high-performance raised bed.
Preparing and Filling the Beds
Once the correct quantity of material is acquired, the process of filling the beds should be done systematically to ensure long-term performance. If using unmixed components, employ a layering technique, often called the ‘lasagna method,’ by placing coarser organic materials, like branches or leaves, at the bottom. This bottom layer acts as a reservoir, slowly decomposing to release nutrients and improve deep drainage, saving the more expensive blended soil for the top.
The customized topsoil and compost mix should be reserved for the upper 6 to 12 inches of the bed, where the majority of plant roots will establish. As material is added, water each layer thoroughly to help the soil particles settle naturally and eliminate any dry pockets or air voids. Attempting to compress the soil manually can destroy the structure and negate the benefits of the loose mix.
The soil level should be filled close to the top edge of the frame, leaving only an inch or two of headspace. Even with watering during the filling process, the organic matter will continue to decompose and compact over the first few months. This settling will result in a lower soil line by the next season, which is corrected by annually topping off the bed with a fresh layer of compost.