FAQ: Can I use potting soil for raised beds?
Using potting soil for raised beds is not so much a question of whether you can, but whether you should. A product designed for a raised bed, added to a very large container, might work. But the inverse is not advised.
Like many things in life, “one size fits all” may not be the best soil strategy for container-grown, raised bed, and open ground/row crop plantings.
Soil has a number of functions. It acts as a substrate to support plants. Air, water, and microbes move through its pore spaces. Nutrients are stored until needed by those plants.
But the manner in which that soil is contained (or not) may require some adjustments to ensure optimum plant health.
Roots need wiggle room
Think about your feet in a pair of tight-fitting shoes. There is not much space for your toes. Air doesn’t circulate very well. Your feet may sweat a bit.
A plant in a container can experience similar problems.
For one thing, containers need to be portable. That dictates a lighter soil than might be found in the garden or raised bed.
The confined space physically restricts root development, too. So the lighter soil makes it easier for the plant to send out roots.
A typical container is watertight with a small drainage hole at the bottom. Lighter soil helps move water quickly from top to bottom to prevent waterlogging.
But this rapid drainage may also lead to less water retained – one reason to make sure your potting mix includes the moisture-holding properties of compost.
Raised beds are little different. They are generally much larger than a container and do not have an impermeable surface between the bed and the native soil.
A raised bed is like a comfortable pair of boots – more room to wiggle toes, but still a confined space.
Rain and irrigation water have the space to spread laterally before percolating down through the bed. Roots are able to do the same.
But media mixes for planting beds often have wood chips or other materials added to improve drainage. Because growing space is still confined in a raised bed, the soil needs to have a lighter density than the adjacent lawn or garden.
That garden soil can be much heavier because roots have unlimited space to stretch out to seek water and nutrients. In the landscape, it needs to have the ability to support big shrubs and trees, too.
But do note that “heavier” is used here as a density comparison to the lighter potting and container soils. A heavy garden soil is not desirable, either. It requires amendment to lighten things up. Otherwise, roots will not develop properly.
Assuming that garden soil is a good density, it might be likened to going barefoot – plenty of room to wiggle the toes. Or, in this case, plenty of space to spread roots.
Make your own or buy pre-made
If mixing up your own container or raised bed media, know that compost can be substituted 1:1 for peat moss in your favorite soil recipes.
And if using your garden soil as a base, sterilize in the microwave or under plastic before adding the compost. Sterilizing compost will kill the beneficial microorganisms that make compost such an ideal soil amendment.
For readers along the U.S. East Coast, McGill does not make a potting mix, but we do offer a bulk landscape mix for raised beds. Ask for it at your local landscape supply yard.