More than picking a raised bed design, more than picking the right varieties to grow, soil makes the biggest difference between a lush, thriving garden and one that struggles to grow. If you’re taking the time, effort, and money to build a raised bed, then you also want to invest in great soil.
Most raised beds benefit from a mixture of 50% compost and 50% topsoil, or a mixture of 40% topsoil, 40% compost, and 20% aeration material, on top of a weed barrier made from cardboard, paper, or landscape fabric. Don’t use a potting mix or add a rock layer at the bottom. Buy the highest quality you can afford.
However, it’s important not to get too mixed up in finding the “perfect” raised bed soil recipe. Instead, invest your time in finding quality materials, as not all materials are equal – or even a contender.
There’s little regulation over quality. When possible, always examine what you’re getting before you buy it. If you’re buying bagged materials, then buy one bag first. Ask other gardeners who they buy from – they’d love an opportunity to gush or vent about a supplier.
The best materials usually aren’t cheap, and raised beds require a lot of material to fill them. But skimping on the cost upfront will only end up costing you a terrible growing season and more money to fix it for future seasons. There are other ways to cut costs without cutting quality.
What Do You Fill Your Raised Beds With?
Fill your raised beds with a mixture of high quality topsoil and organic matter. Most raised beds benefit from a mixture of 50% compost and 50% topsoil, but you may also wish to try a recipe of 40% topsoil, 40% compost, and 20% aeration (like perlite, pumice, or lava rock).
When looking to buy any of these, ask other local gardeners for their favorite sources – especially if you’re unable to see the product in person before you buy. You can avoid the pitfalls caused by poor quality topsoil and amendments, and maybe meet some new gardener friends.
You can buy topsoil and compost in bags or in bulk – in bulk is cheaper, but you also need a place to store it.
Topsoil is the top layer of soil, and depending on the area, can be as thin as a couple of inches or deeper than a foot. This is the layer where roots and beneficial microbes grow. While it contains the most organic matter, the topsoil that you’re buying is a mixture of sand, silt, and clay.
But not all topsoil is the same. Since topsoil is really only what’s on top, it could be anything from sand to brick material to heavy clay.
To test the quality of the topsoil, feel it in your hand. If it crumbles easily and feels slightly gritty, it’s high quality. If the topsoil forms a clump or is difficult to crumble, then it has too much clay.
Also, look for topsoil that’s been screened (rocks and debris removed). Topsoil that hasn’t been screened is cheaper, but you’ll need to screen it yourself.
All topsoil, regardless of quality, will contain weed seeds.
Compost is decomposing plant and animal matter, AKA organic matter. Organic matter provides plant nutrients as it decomposes, improves soil structure, feeds beneficial microbes that support your plants, and increases water retention.
“Finished” compost looks like soil – it’s black or dark brown, crumbles in your hand, and you can’t see any plant or animal parts. Even though we call it “finished” compost, it’s still decomposing, and as beneficial microbes continue to decompose and convert it to usable nutrients, it’ll continue to feed your plants for up to five years.
Only use manure from cows, chickens, goats, sheep, and horses. Make sure that it’s “finished.” Initially, it may be too “hot”, or have too high nitrogen, which will burn your plants’ roots. It can also contain bacteria or parasites that’s not food safe. When it’s finished, it’s safe to use with plants – even your vegetables.
To prevent contamination (and to kill weed seeds), manure compost piles must reach an internal temperature above 140F for several weeks. Bagged manure compost is ready to use. If you’re getting manure from your own animals or from a farmer, then you need to manage your compost pile to reach these temperatures before use.
If you can, keep your own green manure compost pile. You’ll save yourself money in the long run. If you already have one, you can definitely use that compost.
If you’re filling up a deep raised bed, you may wish to throw in other organic matter that hasn’t fully decomposed yet, like dried leaves, vegetable and garden waste, grass cuttings, or fish bones.
Pretty much anything you’d put in a compost pile, but you’ll save time and space by composting them in place. These will decompose over time, lowering the soil level, which you can refill with new compost.
Triple Mix is a mixture of topsoil, peat moss, and compost (definitions can vary, though, so double check before buying!). This is a great combination for raised beds, as it’s much more airy than regular soil, but quality varies between sources.
Garden Soil can be just that – the topsoil from your garden or where you put the raised bed. If your soil is already good enough, you can pile it on your raised bed and save a bit of money on topsoil.
If it’s too heavy or has too much sand, you can mix your existing garden soil with purchased topsoil and compost to improve it, and you’ll save money to boot.
You can also buy “garden soil” in stores, but since there’s no definition, you could be buying anything. Better to just skip that and go with a topsoil, compost, or triple mix.
Raised Bed Soil is like a potting mix, but formulated for raised beds and large containers. Some use topsoil, some don’t. The appeal is that you don’t need to mix it with anything, you just pour it onto your raised bed.
But like all potting and soil materials, quality varies from product to product. It will also be more expensive than buying in bulk and you won’t have as much control over the composition, but it’s still a great contender, especially if this is your only option where you live.
Look for reviews and at the ingredients. Some suggest you mix in a compost of your choice as well.
Can I Use Potting Mix In A Raised Bed?
If you’re using a potting mix in an elevated raised garden bed (basically a raised garden on stilts, or a very large container garden), then yes, use a potting mix. The lighter components and increased water retention will be a benefit. They’re mixed especially for container gardening.
But if you have a raised bed on the ground, then you’re better off using a topsoil/compost mixture. The particle sizes are larger, and if you’ve set it directly on the ground, you can mix the existing soil with the adding topsoil for better drainage.
Is Straw Good For Raised Garden Beds?
Straw can be an amazing mulch or even raised bed (like in straw-bale gardening), but if it’s been contaminated with persistent/systemic herbicides, then the herbicides will cause deformed leaves and flowers in broad-leaf plants, or even prevent seeds from germinating. In high enough concentrations, the herbicide will kill the whole plant.
Also called plant growth regular (PGR) herbicides or system herbicides, persistent herbicides include aminopyralid, aminocyclopyrachlor, clopyralid, and picloram. These herbicides take up to 3 years to break down in the soil – and if you’re using straw as a mulch, it definitely hasn’t broken down that much.
These herbicides also affect manure and green composts. While other herbicides break down quicker, are digested in the animal’s stomach, or degrade in the compost heat, persistent herbicides persist into the finished compost.
If you wish to avoid this altogether, you could use pea straw, alfalfa meal, or even brown craft paper as a mulch.
If you can buy straw (or compost) directly from the farmer, ask them if they use any of these systemic herbicides on their straw.
You can also test for persistent herbicides by growing peas, beans, or tomatoes in two pots – one filled with “safe soil” and the other with a mixture of the safe soil and the amendment you’re testing. Plant the seeds to test the germination, and if they both germinate, compare the plants after 30 days. If one has stunted leaves, then you know it’s contaminated.
If your soil becomes contaminated, you’re going to have to wait 2 – 5 years for the herbicide to degrade. In the meantime, you can plant corn, brassica, or grasses, grow cover crops to improve the soil, or grow in containers or raised beds that don’t have contact with the ground.
Should I Put Cardboard In Raised Beds?
Cardboard makes an excellent weed barrier and much, providing several advantages since it:
- Is cheap and easy to acquire, especially since everyone is ordering more things online. The thicker the cardboard, the better, and shipping boxes are perfect.
- Is non-toxic and organic gardening friendly. Regular brown cardboard is pretty much just paper. Avoid using anything glossy or with a lot of ink (like the Amazon boxes with Prime TV ads). Remove all the tape, as that won’t decompose.
- Lasts for a season before decomposing and adding carbon to the soil.
- Kills existing grass and weeds. It forces seeds to germinate by trapping warmth and moisture (the ideal germination conditions), but blocks out all light – killing existing plants and newly germinated plants too. Given enough time, it can also kill grasses that invade your garden via rhizomes. Those plants then decompose, adding nutrients into your soil.
- Saves time and labor. Again, because it kills the existing grass and weeds, you don’t need to spend the time and labor digging them up. Just spread out the cardboard and build your raised bed on top.
For a weed barrier or mulch, you can also use:
- Newspaper, so long as the ink is soy-based. Other inks are toxic. Avoid using glossy materials. It’ll decompose over time and add carbon to your soil.
- Brown craft, packing paper, and brown paper bags. Like newspapers, it’ll decompose and add carbon. If you’re covering a large area, you may need to buy a large roll, but you can also get it for free in arriving packages.
- Landscape fabric. Landscape fabric is costly, but it will last the longest. However, it won’t decompose, so at the end, you may just end up with ragged plastic.
- Jute burlap. Burlap will last longer than cardboard and shorter than landscape fabric, but unlike landscape fabric, it’s made of natural materials (jute) and will fully decompose. Like landscape fabric, it’s costly.
- Dried leaves. If you’re bagging up your dead leaves to send to the garbage dump, you’re giving up a pretty powerful and free compost amendment. Add them to your compost, use them as a mulch, or add a layer of composted leaves at the bottom of your raised bed. It’ll decompose pretty quickly.
Should I Put Rocks In The Bottom Of My Raised Bed?
No, you should not put rocks in the bottom of your raised bed. If you add a layer of rocks, you’ll cause drainage problems and end up with a layer of soggy soil.
When water moves between layers of finely textured material (like topsoil) to coarser textured material (like rocks), the water slows down, hovering in the layer just above the coarser textured material.
The coarser the material, the more water will remain. And rocks are probably the coarsest material you could add (except pop cans and golf balls – don’t use those either!).
If you have a raised bed with finer textured material sitting on top coarser textured material (like a beautiful loam soil on top of clay-based soil), mix up the two layers for at least six inches. By mixing the materials, you don’t have a strict difference in texture.
Instead, you have a more gradual change that will allow water to move more quickly. You can still have your beautiful loam topsoil above that mixed layer – you just won’t have the drainage problems.
Likewise, if you want to use small rocks to add aeration, mix them into the soil. You might have heard gardeners grumbling about picking rocks out of their soil (or you might be building raised beds to avoid your very rocky soil), but a small amount mixed in is beneficial to your soil.
Filling up your raised bed can be expensive. But remember, you’re not just investing in one summer – you’re investing in many growing summers to come. Pick as high-quality materials as you can afford.
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