Healthy soil will help you produce healthy crops. Ideal vegetable garden soil should be loose, deep, and crumbly. It both holds water for root uptake and allows excess rainfall to quickly percolate downward. So how do you fill a raised bed? Should you add topsoil, compost, or potting soil? The answer depends on the situation. Read the scenarios below to learn about options and develop an approach that works best for you!
Raised beds without a wood enclosure- formed by “pulling up” loose, fertile topsoil into a 2-6 inch raised bed with gently sloped sides. Spread 1-2 inches of compost on the area before forming the beds.
Raised bed with an enclosure located in an existing garden- If the soil is in good shape (topsoil intact, not compacted, drains well) increase the soil depth by adding add 4-6 inches of compost (homemade or purchased) and mix it with the top 4-inches of soil. You could also add a 2-3 inch layer of topsoil from the walking space around the raised bed and replace it with woodchips, bricks, or weed barrier fabric.
Raised bed placed on lawn– kill the grass and weeds by covering the area with plain cardboard multiple layers of newspaper, or weed barrier fabric (can take 6-8 weeks). Then fill the bed with a mixture of compost and purchased topsoil in a 1:2 or 1:1 ratio. There are vendors who sell topsoil mixed with compost.
You could also fill the bed with compost and a soilless growing mix in a 1:1 ratio. The latter contain ingredients like peat moss, bark fines, vermiculite, perlite, coconut coir, and compost. As in Scenario 2, you can also remove add the topsoil adjacent to the raised bed.
If the raised bed is at least 6 inches deep and it’s time to plant it’s ok to cut the grass and weeds at ground level and cover the area with the selected growing medium. The vegetation will die and decompose in place.
Raised bed on hardscape or heavily compacted soil- should be at least 8 inches deep for leafy greens, beans, and cucumber, and 12-24 inches deep for pepper, tomato, and squash. Fill the bed with compost and a soilless growing mix in a 1:1 ratio. Topsoil can be added (up to 20% by volume) for beds that are at least 16 inches deep.
Some tips and caveats:
- Submit a soil sample to a soil testing laboratory if you will be using the existing soil (more accurate and complete and usually less costly than DIY soil testers). Pay for a basic soil test and have the lab test the soil for leadif you plan to grow food in your raised bed. The lab will send back results (soil pH, nutrient levels, etc.) and fertilizer and soil amendment recommendations.
- Topsoil sales are not regulated in Maryland. Go to a reputable nursery or topsoil dealer and ask questions about where the soil comes from, how it’s been treated, and soil test results. Examine the soil before purchase or delivery. Topsoil should be dark and crumbly with an earthy smell. Do not purchase if the soil is foul smelling, has grayish mottling or a chalky texture. Some sellers have a mix of topsoil and compost which can make an excellent growing medium for raised beds.
- Minimize the amount of digging and tilling needed to prepare the soil for planting. Disturbing the soil brings weed seeds to the surface where they can more readily germinate.
- Over time the quality of the native soil below the raised bed will be improved through the addition of organic matter and root growth of crop plants.
- Organic matter improves the structure of soils that are high in clay so that roots can better grow and take advantage of available water, air, and nutrients. It can be grown (cover crops and living plant roots), added as fresh organic materials (manure, grass clippings, leaves), or added to soil as compost. You need 3 cu. yds. of compost to add a 1 in. layer to 1,000 sq. ft. of ground or 8.33 cu. ft. (12, 5-gallon buckets) to cover 100 sq. ft.
By Jon Traunfeld, Extension Specialist