You should keep in mind that the best soil mixture for any raised bed can and sometimes should vary from one gardening situation to another.
Different Soil Mixtures for Different Situations
For example, certain plants require a different level of soil pH from that preferred by other plants (blueberry shrubs need an acidic soil mix, for example).
Also, your local climate may dictate a different mix than that required by someone in a very different climate (if you live in rainy Seattle, you will want a mix tailored to drain well…if you live in arid Arizona, you will probably want a mix that retains moisture as much as possible).
One of the great benefits of raised beds is that you can easily create your own perfect soil mix from scratch for your given situation, instead of trying to figure out how to amend your native soil (which involves sending off samples to be tested and then determining what it needs).
All Purpose Raised Bed Soil Mixtures
In this article, I want to highlight two very different recommendations from two prominent gardening experts concerning a good all-purpose soil mixture to fill your raised bed with.
Mike McGrath of NPR’s You Bet Your Garden recommends a raised bed mixture consisting of 50% screened topsoil and 50% high-quality compost. These two ingredients should be mixed together well (not layered), and once in place will never need to be tilled. I have tried this mixture myself, with good results.
Another prominent raised bed gardening expert, Mel Bartholemew (author of the popular gardening classic Square Foot Gardening), recommends that you fill your raised beds with something he calls Mel’s Mix.
Mel’s Mix is (by volume) 1/3 coarse horticultural vermiculite, 1/3 peat moss, and 1/3 blended compost.
He recommends mixing the ingredients thoroughly and using at least 5 different sources of compost to provide a variety of nutrients (bagged compost often comes from a single source, such as cotton burrs or chicken manure, so it is a good idea to get a variety of sources and of course make your own compost at home to serve as one of your five sources).
Mr. Bartholemew says that the compost in his mix provides all of the fertilizer or nutrients that the plants will ever need, and that the only thing you will need to add to his mix over time is a little more compost each season (as the original compost breaks down and is used up by your plants).
He also says that his mix only needs to be 6 inches deep, never needs to be tilled, and that your raised bed bottom can be lined with landscaping fabric or cardboard so that the roots of the plants don’t even reach into your native soil.
As an aside, I might note here that some sources would call Mel’s Mix “soilless” because it does not contain “field soil”, like that found in your yard or garden. Other sources say that if a mix contains compost (as Mel’s does), then it cannot be considered soilless.
Regardless, Mel’s Mix is essentially a high-quality form of potting soil, much like what any professional grower would use to grow container plants in.
A Good Rule Of Thumb About Good Garden Soil
Also, I might add a tip here about garden soil that I heard recently and thought was very interesting: one should always be able to sink their forefinger all the way to the knuckle into their garden soil.
This simple test will tell you if your soil is as soft, airy and “friable” as it needs to be, and illustrates the difference between proper garden soil and, say, your lawn, which is probably so packed and dense that you can’t sink your finger into it at all.
“Big” Problems With Big Tall Raised Beds
I want to talk about something that I think is hugely critical to think about before you start building and filling your raised beds.
First, I love TALL raised beds that place the level of the soil somewhere around waist high. The convenience of such a raised bed over a structure that is literally 6 to 8 inches off of the ground is just monumentally important to me from the standpoint of making the raised bed gardening experience so much more enjoyable.
But there are a couple of really big problems that arise when you make a tall raised bed:
it costs a lot more to fill a waist-high raised bed with an expensive premium soil mixture than it does to fill a 6 to 8 inch high raised bed
a tall raised bed filled with soil requires substantial cross-bracing to keep the sides from bulging outwards (or totally failing) from the pressure of the heavy soil mixture inside it (this issue is exacerbated when the soil is really wet and from “frost heaving” – the expansion of wet soil when frozen)
Now I want to mention a couple of options that address these issues concerning tall raised beds.
Option One: Lasagna Gardening
You could fill most of your tall raised bed with a mix of organic compostable materials (2 parts shredded leaves to 1 part grass clippings would be ideal) and then top that off with 6 to 12 inches of your preferred soil mixture. You will want to put a few layers of newspaper or a single layer of cardboard in between the compostable materials and the soil mix to keep the mix from filtering down into the compostable materials in your first season.
This is referred to as “lasagna gardening” and achieves three key objectives at once – piling up material to be composted, raising the level of your soil to a very convenient height, and saving you money by not having to fill the entire container with expensive mix.
It also negates to a large degree the outward pressure on the sides of your raised bed because their is much less heavy soil inside it. However, I would still recommend some cross bracing of your raised bed.
Over time, the compostable materials at the bottom of your bed will shrink in volume as they break down into compost. Then you can simply add more soil mix or compost to the top to keep the height you desire, and you will eventually have great compost below your mix to feed your plants in future seasons.
Option Two: A Raised Bed/Compost Bin Combination
An alternative (but similar) raised bed design to the lasagna gardening idea would be to build a tall raised bed structure, fill it mostly with compostable materials, and then have 6 inch deep plastic or wooden containers filled with something like Mel’s Mix (or some other premium potting container mix) placed across the top of the raised bed structure (so the raised bed structure is really a compost bin designed with an open top into which you can fit large planting containers).
With this idea you don’t have to worry about your compostable materials mixing with your soil mixture – they are kept totally separate.
But, you still achieve the raising of your planting surface to a very convenient height, and you have created a place for storing your compostable materials underneath!
You would also be able to remove your planting containers easily from the raised bed structure to:
aerate, remove, or add to the compostable materials underneath
simply move the planting containers to a sheltered spot to protect them from oncoming inclement weather (a late or early freeze, violent storm, etc.) or a shady area where you are more comfortable working
So I hope you can see that raised beds can take on a number of forms, each of which brings with it a significant change in functionality!
Preparing The Raised Bed Site Before You Fill It
Before you begin filling a raised bed, many sources recommend that you remove any existing vegetation and till the native soil in the area where your raised bed will be.
This will improve the ability of the bed to drain. It will also allow deep-rooted plants to sink their roots deeper than the medium with which you fill the bed.
You could even improve the native soil with compost when you till it. It might be your only chance to work this area since it will be covered with your raised bed.
Protect Your Raised Bed From Critters!
It would be a good idea to invest in some galvanized hardware cloth (with no larger than 1/2 inch square openings in the mesh) to place under your raised bed and on top of your native soil. You will want to extend the hardware cloth beyond the outer perimeter of your bed a foot or so.
This will keep rodents and other creatures from burrowing under the sides of your raised bed to make a cozy home inside of it.
I intentionally neglected this step when I built my first raised bed and (wouldn’t you know) I had something (probably a mouse) make a small burrow under the side almost immediately after I filled the bed with soil.
The two mixes recommended by the two experts in this article each could be said to contain the basic “building blocks” of a good soil mixture. There are certainly a number of other popular soil amendments that could be added to your mix, depending on your needs.
For example, most experts would recommend adding an organic, slow-release, balanced fertilizer of some kind to the soil once or twice a year (early spring is ideal but you could also fertilize in the fall), either lightly worked into the surface or covered with compost or mulch.
Other possible amendments (and their uses) include the following:
greensand – a slow-release source of potassium and micronutrients
perlite – an alternative to vermiculite which does not hold moisture as well
soybean meal – a slow-release source of nitrogen
gypsum – improves drainage
rock phosphate – a slow-release form of phosphorous and micronutrients
alfalfa meal – primarily a source of nitrogen but also phosphorous and potassium and micronutirents
sulfur – increases soil acidity and makes calcium available to plants
dolomite lime – increases soil alkalinity and adds calcium and magnesium
wood ashes – decrease soil acidity and are a source of potassium and other nutrients
epsom salts – a source of magnesium and sulfur
blood meal – a source of nitrogen
bone meal – a source of phosphorous and some nitrogen
aged (at least 6 months) manure – a source of nitrogen and minerals
seaweed – a source of all three major nutrients but especially potassium
coffe and tea grounds – a source of all three nutrients and also great for the compost pile
shredded bark – improves soil structure and break down slowly
wood chips and sawdust – improves soil structure and break down slowly
I should note that if you have free access to any of these amendments then of course you should think about adding them!
A Special Soil Mixture For Starting Seeds
I should add that if you will be direct sowing seeds outside in your raised beds, you might want to consider topping your soil mix with something more suitable for sprouting seeds. Such a medium should be finer textured than your soil mix, moist and spongy.
A perfect recipe would be Mel’s Mix without the compost (just the vermiculite and the peat moss), an inch or so deep, on top of your raised bed. Once your seedlings are up, you could then layer some compost on top of your bed to complement the seed-sprouting medium.
Or you could simply carefully screen the compost and leave it in the Mel’s Mix (again, the idea is to make your seed starting medium a finer texture, and some bagged composts can be pretty rough textured with bits of bark and plant stems).
Don’t Forget The Mulch!
Finally, remember when you are filling your beds to leave some room for an inch or two of mulch.
This mulch doesn’t have to be added early in the season when the weather is mild and you are trying to warm the soil, but you will surely want to add it later to combat the summer heat and weeds, retain moisture, and as way to minimize nutrient loss from your soil (which can occur from the exposure of bare soil to the elements).
Also, remember that many experts now say that the very best mulch of all is an inch or two of pure compost. I was skeptical of this at first, thinking that the compost would be much more susceptible to weed formation versus something like a bark mulch. I was shocked to see almost no weeds at all in my compost mulch through the growing season.
If you like the look of a bark mulch, at the least you could cut back on the cost of this mulch by using much less of it, simply sprinkled on top of a compost “base”. I used to think that I needed an inch or two of this bark mulch everywhere (many gardening sources really preach the use of woody mulches), but again the compost is a superior mulch by itself so a sprinkling of the pretty bark mulch would suffice to beautify the bed.