There is a ton of advice out there on soil for raised beds, but not all of it is good advice.
If you want to grow healthy plants that resist disease and carry high nutrition, it is critical that you have great soil, whether you're growing in a raised bed or in the ground.
But in a raised bed, you have to fill it with something - it's not just a matter of adding amendments to what you've got. Since this site is dedicated to helping everyone grow the most nutritious vegetables possible, I'll cover all the important points of what to use and what to avoid, and why.
Not just any old mix will do (nor will layers of old wood, straw and compost).
This is a long article so I have broken it up into sections. I recommend you read all the way through because for optimal health and nutrition it's important to understand the why as well as the how of soil for raised beds.
You can also click on any of the links below to scoot straight to that section.
You can't use regular garden soil for raised beds because it will compact down into a concrete-like mass that won't provide enough air or drainage for growing healthy plants. It will also be so heavy that it will exert a lot of outward force on the sides of the bed, possibly blowing them out.
In nature, undisturbed soil doesn't generally compact for two reasons.
The first is that it is already at "ground level", surrounded by soil of
the same mass so it has nowhere to "go". Above ground (in a raised bed)
the soil is surrounded by only a bit of wood and air, which has a
harder time resisting the downward and outward pressure of the soil mass
being pulled out and down under the pull of gravity.
The second reason is that when left undisturbed, there are lots of earthworms, microbes and other creatures that move up and down throughout the soil, loosening it as they go. When you (or a manufacturer) disturbs soil by digging it up and putting it in a raised bed, it damages the soil structure and ecology that would otherwise keep it loose and friable in nature. This is the same reason that rototilling or plowing eventually creates more compaction.
Organic matter consists of composted and decayed things like leaves, manure, straw, grass clippings, bodies of live or dead animals/microbes/worms - anything that is or was alive.
The top 6-8 inches of healthy, in-the-ground topsoil is ideally about 3-6% organic matter by weight, which translates into about 25-30% by volume. The remaining 94-97% of soil in nature is made up of inorganic bits of rock of different sizes ranging from microscopic clay particles to sand-sized grains. (And in a natural system, soil microbes excrete acids which dissolve minerals from these bits of rock to feed your plants.)
But soil for raised beds needs to be 40-50% organic matter by volume in order to keep the soil weight (mass) down, prevent compaction, provide adequate aeration and drainage, and generously feed the microbes that are necessary for healthy plant growth. Healthy soil teems with a wide diversity of fungi and microbes, who literally transport mineral and organic nutrients to plant roots and exchange them for sugars and other substances that are secreted by plant roots. Microbes function like waiters in a restaurant, serving food to patrons, who then tip them for their service.
Organic matter is broken down and digested in stages by soil microbes until in it eventually becomes a stable substance known as humus. Humus binds certain nutrients in the soil, making them available to plants and preventing them from washing away. It also creates a loose, friable, rich soil structure that plant roots can easily penetrate.
We are now learning microbes are literally in a direct symbiotic relationship with plants, surrounding their roots and in some cases living inside the roots themselves.
So not just any organic matter will do. If you want to work with nature, the soil for raised beds, like the soil in the ground, must be free of toxins (including artificial fertilizers) in order to support the microbes that literally feed your vegetables.
In vegetable gardening, both definitions are used, depending on what you are talking about. This can lead to confusion, which manufacturers may exploit when labeling soil for raised beds.
In chemistry, "organic" means any compound that contains carbon. All life forms on Earth are carbon-based, so anything that is or was alive can be considered "organic". "Organic matter" in the soil is all the stuff that was once alive and contains carbon.
The other definition of "organic" is a legal one. The USDA has a strict, defining legal code for what can be labeled "organically grown". It applies only to food, which must be grown without pesticides, herbicides, artificial fertilizers/feeds, or genetically-modified organisms. Only food can be labeled "certified organic" and then only if it is tested by an independent certification agency and found to be compliant with USDA standards.
The recipes for "potting mix", "potting soil", "soil for raised beds" and "raised bed mix" actually have no defining difference and the ingredients can vary widely according to manufacturer. Manufacturers can name their mixes whatever they want depending on their target markets, but the terms all mean some blend of ingredients intended for growing plants in containers of one size or another, from a small plant pot to a raised bed.
It is important to realize that when it comes to soil for raised beds, manufacturers often use the term "organic" in its chemical definition, which makes it both legal and routine for manufacturers to label their product "organic" even when it contains toxic ingredients that will end up in your food.
But all is not lost.
While you can't find "USDA Certified Organic" potting mix, you can find mixes labeled "Approved for Organic Production" by OMRI (Organic Materials Review Institute) or WSDA (Washington State Department of Agriculture). If you plan on buying commercial soil for your raised beds I highly recommend looking for one that carries one of these labels. Here's why:
That old adage, "One man's meat is another man's poison", could be rephrased as "One company's toxic waste is another company's cheap raw ingredient for raised bed soil". Toxins in your soil will go into your food.
There are no laws prohibiting the use of sewage sludge (aka "biosolids" or "compost"), feedlot manure, and many other toxin-containing wastes in commercial soil for raised beds. Here's what you may be putting into your vegetables if you do not use carefully-sourced homemade or OMRI-certified mixes:
Biosolids is a marketing name for sewage sludge. Sewage sludge consists of composted human sewage. One-hundred percent of the samples tested by the EPA in 2009 contained one or more of the following: dioxins, endocrine-disrupting hormones, chemotherapy drugs, steroids, flame retardants, pesticides, herbicides, furans and… well… more stuff you don't want in the vegetables you're going to eat. There are no laws prohibiting or governing the use of sewage sludge in potting mix, or its being labeled as "organic" or "compost", or regulating its use on farms and gardens.
Feedlot Manure Cows in feedlots are fed steroid hormones, antibiotics and corn heavily sprayed with glyphosate, much of which shows up in their manure (and the rest in their meat).
Commercial "Compost" is usually a mix of ground bark and a nitrogen source. It is legally allowed to contain toxic ingredients including sewage sludge and feedlot manure as nitrogen sources.
Free Municipal Compost Many municipalities now have yard waste composting facilities, and offer compost and mulch for free or for a nominal charge. Some cities have had to halt these programs because the levels of certain herbicides and pesticides in the municipal waste (from lawn clippings, etc.) was so high that it was killing trees, flowers, and people's vegetable gardens.1
Peat Peat is actually a wonderful addition to soil for raised beds. It stabilizes the mix and helps hold it together, holds air, adds a bit of acidity, retains moisture but also gives it up to plant roots, and is natural. The problem with peat is that it is a non-renewable resource, regenerating itself at a rate of only about 1 mm (0.04") per year. Harvesting removes 16" and destroys the fragile and precious wetland ecosystems known as peat bogs. Peat bogs act as planetary cooling systems and are home to endangered plants and animals. Peat use is often listed as "sustainably harvested", but if it grows at 4/100s of an inch a year and they peel off 16", I'm not sure how they are defining "sustainable".
"Composted" Shredded Bark or Wood Chips would all be good choices if they were fully composted and broken down to the point where you couldn't tell what they used to be. There are two reasons why they must be fully broken-down:
1) We tend to think of "forest products" as coming from a wild natural forest. But many come from intensively-managed plantations sprayed with pesticides and herbicides that are poorly regulated because they aren't destined to be food. The toxins are generally degraded to less-toxic elements if they are completely broken down, but if you can tell it was wood or bark, they are not yet fully decomposed.
2) to liberate the nutrients within the cell walls of these substances, microbes have to eat them and fully digest them, using those raw materials to build their own bodies.
In the process of body-building (arrrgh.... me tough!), just like you or I, the microbes need nitrogen (protein precursor), which they will take from your soil and away from your plants. Only when the microbes themselves die are all those nutrients released back into the soil to become "bioavailable" to plants.
In nature the microbial breakdown process can take from weeks to years to fully complete, depending on temperature, moisture and other factors. Manufacturers need to move these waste products along before they are fully composted, so they usually add a nitrogen source to their mix so that the breakdown process can continue in your raised bed without the microbes stealing nitrogen from your garden. But there may be a problem with this, too.
The nitrogen sources they most often use, are (you guessed it), the same toxic sewer sludge and manures already discussed.
So what's left? What the heck should I use?
1) Bagged Commercial Mix
Buy only bagged products that carry the OMRI or WSDA "Approved for Organic Production" label, such as Dr. Earth, Happy Frog, Black Gold, KIS Organics or even Miracle Gro Organic with the OMRI label.
If you live in a state where pot-growing is legal, the pot-grower's supply stores usually carry these mixes at a much lower cost than hardware or home-improvement stores. (While I don't "use", if I run out of homemade mix in the middle of winter, I buy seed-starting mix there for half of what it costs at the local Ace.)
A 4' wide by 8' long by 1' high raised bed takes about 22 - 1 1/2 cu ft bags of potting mix. (See formula .)
2) Bulk Commercial Mix
A cheaper option (usually) is to buy soil for raised beds in bulk for delivery or by pickup load from a reputable nursery, or from a landscape materials yard. Don't buy "topsoil" because just like the soil from your backyard it will be too heavy. I suggest visiting the nursery or yard before buying to check out the texture and feel of the mix, and also to ask what is in it. Be wary, however, as most bulk mixes carry no label and may contain the same toxic ingredients as non-OMRI bagged soil.
3) Homemade Mix
In my opinion this is the best option because you have the most control over what goes into it. Make your own soil for raised beds from ingredients sourced locally that are free of the yukkies listed above. (Recipe a bit further down...)
Bagged soil for raised beds is usually labeled in cubic feet, whereas bulk soil for raised beds is usually sold by the cubic yard. One cubic yard equals 27 cubic feet.
To calculate how much you need, use this formula:
If you measure the outside dimensions of your bed rather than the inside dimensions, you'll give yourself a bit extra to compensate for the fact that the soil will settle somewhat.
And if you don't have your own homemade compost yet it might be a good idea to buy about 10% more soil than you need so that you have a bit extra to add in later when as it gets digested down by soil microbes. Many beginning gardeners don't realize that whether in raised beds or in the ground, adding more organic matter year after year is necessary. The earthworms, fungi and microbes are continuously digesting the organic matter down and feeding its nutrients to your plants.
Recipe for Homemade Organic Soil for Raised Beds
|about 5 parts||good quality finished compost|
|about 2-3 parts||native garden soil|
|about 2 parts||carbon source: one or a combination of coconut coir (rehydrated), aged (composted) ground bark, shredded leaves, peat moss*, straw|
|about 1 part||composted cow or poultry manure*|
|about 1 part||worm castings|
|rate appropriate for your square footage||complete organic fertilizer* (Steve Solomon recipe, Dr. Earth, or Happy Frog)|
|at startup and ongoing||microbial tea*|
Recipe Chart Notes:
Finished compost The absolute best thing to start with for the 40-50% organic matter requirement is homemade compost made from your own organic yard waste, kitchen scraps, your own or neighbors' organic chicken or duck manure, fall leaves, etc. Most people starting raised beds don't have much (or any) homemade compost yet, so they will have to try to find clean compost locally. Call your local nursery.
Peat moss works wonderfully, but remember it is not sustainable in the long run.
Manures Try to source these locally from an organic producer rather than using bagged mixes from big box stores that are sourced from feedlots or commercial poultry operations. And avoid horse manure. It is hard to believe but a lot of hay is so heavily sprayed with herbicides (of the "pyridine carboxylic acid" class) that even after it comes out of the back end of the poor horse, is still deadly to many vegetable crops.2
Complete organic fertilizer application rates here.
Microbial tea recipe here.
Mixing and Filling
If you are using bagged raised bed mix, just dump it in. Well that was easy…
If you're mixing your own ingredients, make sure you have accurately calculated how much your total mix needs to be and then figure the right proportion of each ingredient needed to create that total. Might be useful to know that a standard 5 gallon bucket holds about 0.668 cubic feet (just under ¾ of a cubic foot).
How you mix and use these ingredients depends somewhat on how deep your raised beds are. If they are over about 8" high, it will be easier to mix your ingredients together on the ground in a pile than in your raised bed. If you're on bare dirt or a driveway, no problem, but if you're on grass you might want to put down a tarp first. Mix all the ingredients up as evenly as you can using a shovel.
If you have a really high raised bed, two feet or higher, some folks put old rotted logs or firewood on the bottom to fill space and provide more carbon (aka "hugelcultur"). If you do this you'll need to greatly increase your available nitrogen so the breakdown organisms don't rob nitrogen from your plants while doing the hard work of breaking down all that carbon. And it will also settle more as the raised bed soil works its way down into the spaces between the logs.
Something that I've never seen the famous YouTubers (is that some kind of potato?) talk about is how deep roots actually grow when planted in healthy soil, unrestricted, in the ground.
If we want our vegetables to be able to draw water up from down deep (meaning have drought resistance) and gather nutrients from wide-ranging feeder roots in conjunction with mycorrhizal fungi, we need to give them s p a c e.
A University of Nebraska researcher named John Weaver did a painstaking study in 1927 where he and his team dug trenches next to growing vegetables and mapped out their root systems. This is an example from his paper, showing the depth and breadth of a beet root at 3 ½ months of age. Its roots spread 8 feet wide and go ten feet deep:
It's possible to grow all kinds of vegetables in all kinds of raised beds, on all kinds of surfaces with all kinds of soil and natural or artificial fertilizers. And there's a YouTube for every possible combination.
But this beautiful drawing puts me in awe of nature and inspires me to care for my native soil in all its wonderful complexity and intelligence.
So now, at the end, I am going to pose an uncomfortable question:
Why garden in a frame made of a dwindling resource (wood), or worse, toxic resource (cinder blocks from mining slag/fly ash) that must be filled with a bunch of inputs (that are hard to cleanly source) rather just garden in the ground as nature and our ancestors have done for 10,000 years?
Why?... Because raised beds are beautiful, tidy, easy to reach, and drain well, which can be a plus if it rains often where you live. They are great for people who have back issues, are in a wheelchair, or are gardening on a driveway. And the number one reason for most people is that they don't have to dig.
Why not?... They are far more expensive and resource-intensive in the long game when compared to growing vegetables in the ground, and some designs make it difficult to maintain healthy soil ecology.
To use fewer resources, you can saw up pallets to use as lumber for your raised beds, shop at your local ReStore for used materials, or salvage stuff by "shopping" the free section on NextDoor. (I got two good raised beds this way). These are all great ways to build a raised bed on the cheap.
But all wooden raised beds have moisture on one side and are dry on the other, which over time will make the wood swell on the wet side and warp, decay and eventually need to be rebuilt or replaced.
Steel stock tanks have also recently become popular for raised beds, but consider the poor beet root pictured above. Think how hot the soil will get under that steel, how the microbes will fare, how roots cannot grow out the bottom, and how the tank won't drain. The earthworms cannot come up. You'll have a really hard time creating healthy soil ecology in one of these.
As our planet warms we might all benefit by taking a step back and taking in a wider view. I like to use "Google Earth" as a verb. When we Google-Earth what sustainability means in our rapidly-changing environment we should consider where our inputs come from, what effects their use will have, and where they will end up when we're done with them. How is the life that sustains our own life, faring under our care?
If we're going for high nutrition, above and beyond just "homegrown", we need to care for our micro-critter partners.
There are many places left on Earth where stuff still grows in undisturbed natural systems, sustainably, indefinitely, as it has for millions of years. Nature had it all beautifully balanced before we plowed up the prairies (etc).
It was only after we plowed, drove off all the carbon into the atmosphere, killed the microbes, watched ten feet of topsoil blow away and started pouring billions of tons of toxic chemicals on what was left, that we began to develop the chronic "diseases of civilization" and created climate change.
Now I'd rather see us all garden in framed raised beds than not at all, and I honor and will completely support everyone in their choice, whatever that is.
I just wanted to give a shout out to nature's way. There are pros and cons to raised beds, and if you haven't built your bed yet, you may want to consider raised beds from more than one angle. If you're into sustainability, your energy might be better spent healing the soil that you have. It wants to be revived and to live and if we love it back to life it will reward us with the most nutritious food you have ever eaten. Granted it'll take a bit of work at first.
I LOVE Joe Gardener, and Justin Rhodes, and many other great people on YouTube. Joe's beds are gorgeous.
But I estimate that for the average Joe like you or I, at non-wholesale prices, one of Joe's 4' x 8' x 1 ½' raised beds would cost (at retail from a Home Depotey-kinda place) about:
Let's assume the purchased raised bed mix has enough nutrients for one season so you wouldn't need to remineralize your first year. So that's $1374. X 1.07 (tax) =
About $1500. per bed... (Fall 2020)
I know most folks won't be building beds like Joe's. But for my money, I'd rather buy quality tools and heal my native soil.