The kind of soil used in any garden can vary depending upon plants, region, and availability. But there tends to be a formula used most often.
The type of soil you need for your raised garden beds is usually a mix made up of 60% topsoil, 30% compost, and 10% potting soil (that contains perlite, peat moss, and/or vermiculite). Some people also add organic fertilizer. The depth of that soil depends upon what you plan to grow in it.
A good, healthy soil mixture that promotes the natural ecosystem that plants thrive in is essential to raised bed gardening. If you want to know about this mix as well as some other suggestions, keep reading!
How Much Soil Do I Need for My Raised Garden Bed?
First off, since you’re buying for a bed or multiple beds, you may want to purchase soil in bulk. It’s a lot cheaper and you can always mix it at your home and then if you have extra, it’s there to be mixed in later. After all, there’s always going to be some settling.
You can usually find pre-prepared compost in bulk as well. If you’re wanting a good price on compost as well as mulch, check with the local landfills and waste management companies. Perhaps your city waste removal department has a lead. In our area, the local town landfill sells both by the cubic foot, often at half the price, you would pay elsewhere.
When buying your soil in bulk, it comes in cubic feet or cubic yards. That means you need to break out that high school math again! Remember, when it comes to the standard raised garden bed, you’re handling a rectangle.
Remember how to get the area of a rectangle? Side x Other Side = Area²
EXAMPLE: 4feet x 6feet = 24feet²
You do that but then multiply this area by the depth of the space you need to fill. And remember we’re using feet as the measurement. So when you measure out the inches, you’ll need to divide that by 12 (because there are 12 inches in a foot).
EXAMPLE: We have a taller bed, so I need 24 inches deep of soil on top.
24inches ÷ 12 = 2feet now 24feet² x 2feet = 48feet³ or 48 cubic feet.
If you need to change this to cubic yards, just remember that 27 cubic feet = 1 cubic yard. So you need to divide the number you got above cubic feet by 27.
EXAMPLE: 48 cubic feet ÷ 27 = 1.78 cubic yards
Did You Know?
Sometimes topsoil mixed specifically for plants is called LOAM. Loam is soil made with a balance of the three main types of soil: sand, silt, and clay soil. Because this combination of soil types is known to be able to hold proper moisture, oxygen, and nutrients, it creates the perfect soil texture for plant growth.
What Is the Best Formula for Soil Used in a Raised Garden Bed?
- 60 percent topsoil/loam
- 30 percent compost
- 10 percent potting soil (you want a potting soil mix that contains perlite, peat moss, and/or vermiculite)
Just remember that these proportions are approximate, but I’ve scoured a lot of extension service publications and gardening websites to find this out, and the percentages tend to stay right around this. I’m going to use it for my own beds and see what happens.
If you want to add fertilizer to this mix, just add ½ cup per 5 square feet of soil.
What Are Some Things to Know About Your Soil?
- Some places may not have access to quality topsoil. If that’s your case, then you may want to use an alternative. Tossing a 50% – 50% blend of compost and potting soil can take its place. Or if you have some topsoil but it’s a limited amount, you can always mix it with the 50% – 50% blend to give it a refresh.
- Speaking of refreshing soil – you can definitely refresh old soil. I tend to find old soil when I transplant other plants from one place to another. Also, the soil in the raised bed can settle over the season, and then, as you take out various plants, you lose soil volume as well.
When I find old soil I dump it into the bed I want to top off. Then, I grab up enough new potting soil to fill the bed as I like it. I use a yard rake for the shorter/larger beds and a hand tool for the taller beds to blend all of those soils together.
- You can also add peat moss to your raised beds. However, it shouldn’t be any more than 20% of your total mix. Why? Because peat moss is naturally acidic. So you probably don’t want to grow your veggies in it.
- The point of raised beds is to make gardening less labor-intensive by limiting the weeding and helping by giving your beds the best, richest soil. So don’t step on or compact the soil! The fluffy, well-drained soil that you can develop is a huge advantage that can provide you with vigorous plant growth. If you compact it, you will reduce aeration, and slow the activity of earthworms and beneficial microorganisms under the soil’s surface.
- You can add a layer of fine wood chip mulch to the top of your soil for better moisture retention. As the wood chips break down naturally, they add more nutrients and bulk to the soil.
As I learn things for my own backyard gardening, I’m sharing them with you. I hope you found this article helpful. If you want to know more about layering the rest of the materials in your raised garden beds along with more insightful information, I wrote an article all about that.
I also wrote an article about what you should use for your base material in a raised garden bed. Be sure to check it out!