Optimal Soil-to-Compost Ratio for Raised Beds

Garden Soil

• Ordinary garden soil or topsoil used alone in a raised garden bed crusts over, settles and shrinks away from the sides of the bed. Rainfall or irrigation may cause it to compact too tightly around plant root systems. Controlling the content of the soil mixture ensures better drainage and better moisture retention. Loose soil with an optimal compost ratio enhances root development. Roots cannot spread easily through compacted soil, nor cling well to loose soil that does not have enough organic matter.

Recommended Ratio

• The optimal soil-to-compost ratio for a raised bed is 1 part organic compost, 1 part sand or perlite and 2 parts garden soil. Commercial organic compost contains plant residues, animal manure, fish waste products and mined mineral byproducts that increase soil fertility. Look for the Organic Materials Review Institute (OMRI) label on commercial compost products to ensure safe organic content. Perlite consists of pebbles made from natural volcanic glass. They expand in the soil and create air-filled pore space. Air space in the soil allows roots to absorb oxygen. Growstones are a less-expensive alternative to perlite. They are made from ground, recycled glass formed into small pebbles of various sizes. Sand is available from home centers and garden supply stores.

Compost in the Mix

• The 1 part organic compost used in your mixture provides many benefits to the soil of a raised-bed garden. It contains billions of microorganisms that are invisible to the eye but provide all the essential nutrients for healthy plant development. Tiny bacteria and fungi break down plants and insects that are in the ordinary garden soil portion of your mixture, turning it into more productive soil. It is also the microorganisms in compost that help retain water. Compost degrades wood preservatives, pesticides and other soil contaminants.

Ongoing Care

• Soil in a raised bed dries out more quickly than soil at ground level. This may be a benefit in early spring but not in the summer. Water carries away nutrients, and plants absorb nutrients from soil as they grow. Once the raised bed is filled with the correct soil-to-compost ratio, it still needs regular fertilization. Fertilize with organic fertilizer or homemade compost. Till in plant residue at the end of the growing season to further increase soil fertility.