The Dirt on Soil. What is the Best Soil Blend for Raised Beds?

Filling your raised beds with a high-quality soil blend is non-negotiable if you want to set your garden up for success. While it’s an upfront investment, getting it right the first time will save you time, energy, and money in the long run.

You'll create a solid foundation for your plants to thrive, requiring little to no additives other than a bit of mushroom compost.

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When I first started gardening, I created a complicated soil blend as advised by a gardening influencer purchasing approximately 60% topsoil, 10% sand, and 15% mushroom compost in bulk from one supplier. Then, I added 5% bagged composted manure from Home Depot, 2-3% worm castings from Amazon, and 7-8% leaves collected from my in-law’s yard.

Despite all the work researching this blend and then hand mixing it myself, my plants struggled with disease and serious pest pressure that summer.

I eventually learned that the main problem was either the topsoil or mushroom compost I purchased in bulk (or both). I never tested that soil blend, but it was clear that it was poor quality. If I had gotten quality topsoil and mushroom compost and followed that same formula, my garden would have thrived. I learned, however, that you don’t need to obsess over a formula if you’re sourcing good-quality soil to begin with.

What is the best soil blend for my raised beds?

The best type soil for your raised bed is one that is tested. You can test it yourself after you buy it to determine if it needs any additives, or you can source pre-tested soil from the start that meets certain quality requirements. Without testing the soil, it’s a shot in the dark whether or not it will be good for your plants.

The ideal soil blend for your raised beds is a rich, sandy loam mix that is dark, nutrient-packed, and able to retain moisture while ensuring proper drainage. This blend combines healthy topsoil, coarse sand, and compost (ideally mushroom-based), with a bit of organic matter like shredded leaves, composted manure, or worm castings. Organic matter is ideal, but you can go without it and still have a healthy garden.

One option is to source and purchase each component separately and mix everything yourself which can be a lot of work and probably more money.

Another option is to buy a pre-mixed loamy garden soil blend from one supplier, which is my preference. Just make sure it's a high-quality mix.

Where can I source a quality-tested soil blend?

The US Composting Council is a great resource to help you find local compost providers. Suppliers that are certified by the US Composting Council (UCC) receive a Seal of Testing Assurance (STA) for following the UCC’s long list of STA Requirements. Some providers offer a loamy garden soil blend with UCC-certified compost, which is what I recommend for your raised beds.

I sourced a UCC-approved soil blend for a client, and she successfully grew over 90 cucumbers in her 2x4 bed. In contrast, I spoke with another first-time gardener who used a cheap, low-quality mix and couldn’t grow a single cucumber in her garden. I’m telling you, it’s all in the soil!

If you don’t have a UCC-approved provider near you, I would recommend contacting a local Kitchen Garden Consultant or Landscaper to have them source a blend for you. If you’re sourcing it yourself, I would encourage you to contact the provider directly to understand where they source their soil, what they test for (if anything), and how often.

Buying in bulk will be cheaper if you’re filling multiple raised beds. If you have a small container garden, you can buy your soil blend in bags. You could buy a raised bed soil blend specifically or do 33% Topsoil 33% Sand 33% Mushroom compost (with a dash of worm castings). Just make sure whatever you’re buying in bags is OMRI Certified unless you can confirm from the provider it’s of high quality.

I want to add a disclaimer that while using quality-tested soil gives you the best chance for healthy plants, it's never a 100% guarantee.

In the summer of 2024, Delaware had hotter-than-normal temperatures. By the time I replenished my beds in the fall with my quality soil blend, it had dried out completely, and I had to add a few bags of mushroom compost to restore its nutrients. It didn’t break the bank and wasn’t labor-intensive - In the end, my garden still thrived.

Should I fill the bottom of my garden beds before adding the soil to save money?

My short answer is no. It's best to fill your beds entirely with your soil blend to keep them level.

My four garden beds are 4'x8'x3', which require a large amount of soil. (A 3-foot high raised bed is way too high, but that’s a topic for another post!)

After already spending so much money on my beds, I didn't want to spend more on soil, so I came up with the 'brilliant' idea of filling them about quarter of the way with logs from a tree we had just cut down in our backyard. This turned out to be a more expensive investment over time due to the continual settling that occurs each year as the logs break down.

To maintain the proper soil height, I need to add 3-6 inches of soil between seasons. This makes caring for my perennial plants more challenging, as they settle along with the soil. It also requires more time, money, and energy than I'd like to spend. Additionally, the soil only stays level for a short period. Since logs vary in shape and size, they create pockets in the soil that can trap excess water, potentially drowning plants or washing away seeds.

Adding freshly cut logs can also deplete your beds of nitrogen if your plants don’t have at least 2ft of room in the soil to establish their roots. That’s because the microbes that are breaking down the soil are hungry for nitrogen. A lack of nitrogen will result in less-than-stellar plant growth and lame harvests.

If you feel passionate about filling your beds before adding soil, I recommend filling them with leaves. You’ll have to add water and really pack them in tightly, otherwise they will settle drastically in a matter of days. You’ll need a ton of leaves, so I would ask your neighbors and friends for bags in addition to whatever you can muster up in your yard.

How can I maintain healthy soil?

A great way to maintain healthy soil is by applying high-quality mushroom compost for heavy-feeding plants, like tomatoes, during the growing season and again to all your beds in between seasons.

If you have the time and energy to go further, consider adding a probiotic mix, organic matter (such as composted manure or shredded leaves), worm castings, compost tea, or fish emulsion.

I recommend avoiding the use of fertilizer (organic only) unless you conduct a soil test first to identify any deficiencies. Over-fertilizing can harm your plants and, if it runs off, may damage the environment. Fertilizer should only be applied when absolutely necessary.

Another excellent method for replenishing soil nutrients is by planting a cover crop. If you choose not to plant a fall garden, adding compost and sowing a cover crop will protect the soil through winter and enrich it for the next season. This is how I successfully revitalized my soil after mistakenly using low-quality soil during my first growing season.

At Sown in Thyme, we help Delawareans grow their own food by building the Kitchen Garden of their dreams. We offer additional services such as coaching, garden maintenance, and planting plans. If you'd like to work with us, book a free discovery call today!