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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.

How much compost for my garden?

Compost makes a great addition to any garden plan.  But how much compost do you need?

A new plot in sand may require wheelbarrows of the stuff.  But if you are digging up a patch of lawn that has seen repeated compost applications over the years, the soil beneath the sod should be in pretty good shape.  A sprinkle might be all that’s needed.

How can you tell if the soil is good?  

The best method is soil testing.  (Contact your county Cooperative Extension Service for more information).  But you can use visual clues, too.  

Weeds like purslane, crabgrass, and dandelion are signs of a troubled soil.  

Stick a spade in the ground and turn over a shovelful of soil.  If it’s sticky and looks like modeling clay or dry and resembles beach sand, you’ve got big problems.  Fortunately, your soil is probably somewhere between these two extremes. 

Is it dark brown and loose?  Are there earthworms?  That’s what you want to see.  

How much compost do you need for a garden?

If building raised beds or container gardening, the soil blend should be about 30 percent compost.  When breaking new ground, incorporate 2 to 3 inches into the top 6 to 8 inches of soil.  

If your soil is very hard,  and you are planning deep rooted vegetables like tomatoes,  consider digging a little deeper.  Maintain the compost-to-soil ratio at about one part compost to two parts soil.

For an established garden with decent soil, just rake an inch or two into the surface before planting.   A 1/8 to 1/4 inch layer of compost sprinkled on the surface as needed throughout the growing season can revitalize flagging rows or containers.  The compost will feed your plants when you water. 

Three to 4 inches of compost can also be used as mulch during the growing season or as blankets when putting beds to sleep for the winter.  However, don’t pile compost up against tree trunks and stems of woody ornamentals.   

Our compost calculator can help you determine how much to buy.