Raised bed soils are made with a combination of sand and compost. These are called, in Viragrow terms, “soil mixes”. Soils in raised beds shrink and become more compact as the compost content of this soil decreases. They become lighter in color as well. This soil becomes more and more sandy as the compost in it dissolves into plants and melts into the surrounding soil.

What Does the Compost Do?

Compost does two things; it changes the physical structure of the soil by making it more” fluffy” (more porous) and changes the soil’s chemistry (pH, salinity). Monitoring changes in soil chemistry is measured with instruments or plants since it cannot be seen. The easiest way to judge a soils chemistry is to grow fast germinating seed (like beets or lettuce) and room temperature. Seeds are sensitive to a soil’s chemistry. Measuring and counting the quick germinating vegetables seeds in a soil mix you invented is called a “bioassay”.

As the compost disappears, fertilizer is needed to substitute for the nutrients supplied by the compost. Garden fertilizers are needed as this compost slowly disappears.

Some garden plants need more compost in the soil than others. At a bare minimum, about 2% organic matter is needed mostly for root crops like carrots, onions and garlic. Other vegetables like tomatoes, peppers and eggplant growing in raised beds might need 4 to 6% organic content in the soil. Generally speaking, you will never go wrong with maintaining a raised bed with around and an 8% organic content.

Compost is also needed to help keep the soil “fluffy”; with large air spaces between soil particles. These larger air spaces are important for the roots some vegetables.

Add Compost to Raised Beds

If you want to rely on the nutrients primarily supplied by the compost then mix a 1-inch layer of compost to the soil in raised beds once a year. When this is done, all that is needed is an occasional “bump” in growth supplied by high nitrogen fertilizers. If you want to rely on nutrients from fertilizers then add compost to these raised beds (to maintain the structural component) every second or third year.