Container gardening is become more and more popular. Planting in containers (that you can place on top of a stand or table) makes it easier to reach the plants without bending over and straining your back. And they’re not just for apartment dwellers anymore!
Even gardeners with plenty of outdoors space are starting gardens on their patios – or even inside the house. Containers mean that we can start planting earlier, or even garden year-round. But a closed-bottom container – with its limited soil depth – restricts the types of crops we can grow.
The solution is simple: a raised garden bed! Because of their open bottoms, raised beds let the plant roots reach deep. But their elevated height makes it easy to tend your crop without risking injuries. They can be bought or built at little cost and in many sizes – which means they’ll fit into any space – even the tiniest gardens.
And, when you consider how much soil you need for the same number of plants grown in containers, a raised bed needs very little extra earth. But, exactly how much will you need? Well, that depends on the size of your bed – and the type of soil you choose.
What is the standard size of a raised bed?
Many builders are selling pre-made raised beds. These wooden garden frames are usually 3’-4’ wide by about 6’-8’ long. But, if you build your own bed, you could make it any size you’d like.
Take a look at these two great examples of raised bed frames.
The only rule of thumb is to be sure that you can easily reach into the center of the raised bed to tend the plants there without stepping or kneeling in the soil. The raised bed can be any length – though it’s best to stick to standard lumber sizes to reduce building costs or cutting.
The bed depth can also vary but it should be at least 6” deep from the surface of the ground. Anywhere between 12”-24” inches is ideal – it leaves plenty of space for long-rooted plants or root vegetables to grow.
How do I work out how much soil I need for my raised bed?
You will need to do some quick math to determine how much soil you’ll need for your raised garden bed.
- Multiply the width by the length and then by the desired soil depth to obtain your cubic area. For example, if your raised bed is 10 foot long by 4 foot wide by 1 foot deep that calculation works out
- 10′ x 4′ x 1′ = 14 cubic feet of soil
- Many garden centers sell soil in large bags which hold 1 cubic yard. There are 27 cubic feet in 1 cubic yard. For the example above we could make two raised beds of this size with one cubic yard bag of soil.
Again, the exact amount of soil you’ll need for a raised bed depends not just on the shape, width and length of the bed, but also on its depth.
You could use this handy calculator from Gardener’s Supply Company which also determines how much fertilizer should be added to the soil.
How much soil do I need for a 4×8 raised bed?
You’ll need to consider the depth of your planter in order to calculate how much soil you’ll need for your 4’x8’ raised garden bed. It’s not enough to simply say 32 cubic feet because this doesn’t account for the height of the walls.
If the sides are only 6” high, then 2/3 of a cubic yard of soil will fill your bed. Doubling the depth doubles the amount of soil to 1¼ cubic yards and a 24”-deep bed needs almost 2½ cubic yards of soil.
What type of soil do you put in a raised garden bed?
One of the biggest benefits of a raised garden bed is that you can completely control the soil they hold. But which soil is best? Sadly, there’s no secret formula for the perfect soil. You’ll need to consider what you’ll be planting, as well as the local climate to determine what type of soil will give the best results.
Some growers insist that potting soil is best for raised beds. But others swear by a “triple mix” – an even mixture of topsoil, Charlie’s Compost, and Black Gold or Peat Moss. Many horticulturists suggest this soil mix for the best results.
It’s also recommended that you “double-dig” or deep rototill the ground beneath the beds before filling them. That means going down two shovel blade lengths – or about 24” in depth. We can then see if any roots are encroaching into the growing space. It also lets us remove any rocks and debris from the soil – or check the underlying soil and determine if it needs any amendments before filling in the raised bed.
Where do you get the soil for a raised garden bed?
If you’ve removed soil from elsewhere on your property, feel free to recycle it into your raised bed. If you do, be sure to add the proper amendments (compost, fertilizer, vermiculite, or sand to improve the airflow and drainage, including peat for water retention in the soil).
Any garden center and most home-improvement stores sell bagged potting mixes or soil – choose the best mix for your particular application and your plants’ needs. You may also be able to buy soil in bulk from your garden supply center. I have linked to examples above which can be bought online with home delivery.
Can I use regular garden soil in raised beds?
It’s totally possible to use regular garden soil in your raised garden bed. As long as it isn’t heavy clay soil, you should be able to use it straight away. Take a read at this article about determining the type of soil you have. This will allow you to add the above amendments to make it great for growing plants.
Remember to amend the soil as needed to keep it at its best – this will optimize your growing crop. Add green compost to your raised bed in the fall: chopped leaves or mulched grass works well. And in the spring, add a layer of compost.
Another great tip: if you’ve cut away a rectangle of sod under your raised bed, you can flip the pieces upside-down and use them to fill the bottom of the raised bed. Because there will be quite a bit of soil attached to the sod, you’ll need less soil to fill the bed. Also, as the grass breaks down over time, it will add nutrients to your raised bed.