What To Use As Filler In A Raised Garden Bed

First things first, let's set the record straight: "Dirt" is not soil! Soil is rich, full of nutrients, and is biologically active! In contrast, dirt is usually devoid of all these things. Soil improves with time and age, as the soil food web blossoms. It is a living, breathing, dynamic ecosystem of its own. Therefore, our goal here is not to simply fill our raised beds with soil, but to create an optimum living organic raised bed soil that plants love!

A diagram by Heidelberg Farms showing what the Soil Food Web looks like below ground. There are tree roots with compost and micro arthropods on the soil surface, with bacteria and fungi, mycorrhizae, and nematodes and protozoa below the soil surface, in and around the tree roots. — The Soil Food Web. Image Courtesy of Heidelberg Farms via Pinterest

I hate to say it… but no matter how much love, energy, or money you invest into your garden, if you have crummy soil, the result will be crummy plants. If you have gone through the effort to build or buy yourself some awesome raised garden beds, let's get them filled up with the right stuff! However, the answer isn't as simple as "go grab X brand of soil". In my experience, not one soil, be it in bulk or bagged, is going to be perfect for growing vegetables on its own straight out of the bag.

Another thing I hate to say is the word "perfect". I typically avoid using it because of the impossible expectations it implies. I'm going with it for this post, but know that the ideal soil can vary in its origins and composition, and yours may differ from ours! Think of creating the perfect soil as an art. There are many personal touches you can put on it. With that in mind, I am simply sharing the way we prefer to craft and build our organic living soil.

If you already have filled your raised garden beds with less-than-ideal soil, don't fret! There are ways to amend and improve it. We'll discuss that too.

Getting Started with Raised Beds

Looking for tips on how to build a raised bed ? Check out this post to learn and see how we build our beds!

THE PERFECT RAISED BED SOIL

"What kind of soil do you fill your raised garden beds with?" I get asked this question All. The Time! As I think we have already established, soil health and quality is everything when it comes to a bountiful, healthy, productive garden, so we don't mess around here! Using a combination of quality organic soil, compost, and an aeration addition will create the "perfect" soil. By perfect, I mean soil that is rich, fertile, holds moisture, but also has good drainage and what I like to call "fluff" to it.

Our target recipe is to fill raised garden beds with a mixture of about 40% soil, 40% compost, 20% aeration – plus a few other goodies that we'll discuss momentarily.

This is an approximate estimation, and doesn't need to be exact… Also, all of the same principles we'll cover in this post could easily be applied to container gardening, just scaled down. For example if you need to fill fabric grow bags, wine barrels, or pots.

Before you can go about choosing and purchasing soil, you'll need first to calculate how much volume is needed to fill the raised garden bed(s) you have.

Soil volume:

Bags of garden soil come in measurements of cubic feet, usually in a range of 1 to 3 cubic-foot bags. Bulk soil purchased from a local landscape supply company will be in cubic yards. First, calculate the volume of your bed in cubic feet. To do this, simply multiply the width by length by depth in feet (e.g. 4' x 6' x 1.5").

Now you have your total cubic feet, and can figure out how much bagged soil it would take to fill the bed! If you choose to get some bulk soil too, you'll need to calculate volume in cubic yards. To convert your amount to cubic yards, simply multiply your cubic foot number by 0.037037, or use this converter.

If math isn't your strong suit (or you're just feeling lazy…. no shame!) here is a raised bed soil calculator that will do it all for you, in both cubic feet or yards. Why didn't I just give this to you in the first place? Because I am mean, and wanted you to exercise that brain a little.

Ta da! Now you have your volume needed.

Tip: If you calculated that you need several yards of soil for your project, you are not going to want to fill all your beds using only bagged soil, trust me… Look up local landscape supply companies and see what they have to offer in bulk! On the other hand, if you need to fill just one modest raised bed or two, purchasing bagged soil could be the way to go.

40% SOIL

Bulk Soil

If we have several really deep, big beds to fill at once, we buy some organic soil and compost in bulk from a local landscape company and have it delivered. Bulk soil composition and type will vary depending on your location. Our local bulk soil is called "planters mix" and is comprised of 2 parts top soil, 1 part compost and 1 part soil conditioner. I have heard of other gardeners who enjoy using equal parts topsoil, composted manure, and sand purchased in bulk. In that recipe, the sand is serving as the "aeration" ingredient.

We never rely on bulk soil alone, since it doesn't always seem to be the highest quality or our preferred texture. At least that is the case in our experience. Therefore, the bulk soil is added more as a space-filler at the bottom of very deep beds, like our 2-foot deep ones, up to about a quarter full.

Bagged Soil

For the remainder of the volume, we go for high-quality organic bagged soil blends, plus compost. We might continue to mix in a little bulk soil here and there to increase the volume, but not nearly as much as the other good stuff. For bagged soil, we usually turn to G&B Organics (by Kellogg) which is carried at our local Ace Hardware garden center and local Farm Supply. If we are doing a big shopping spree at Home Depot to buy lumber and other supplies, we'll pick up some of their Kellogg Organics line of bagged soils.

We aren't 100% loyal to Kellogg only, and this is not a sponsored post… I am simply sharing what works well in our garden! Their products are readily available, affordable, OMRI-certified for organic gardening, and create good results! Both G&B Organics and Kellogg offer big ole 3-cubic foot bags of raised bed planting mix and soil conditioner. Other great brand options are Dr. Earth's, Roots Organics, E.B. Stone, or Fox Farm, to name a few.

You do not want "potting soil" only, as it is too light and fluffy for raised beds. In the photo below, you can see that we mix various types of bagged soil and conditioners. By combining a few different things, you're getting a nice variety of composition and texture. Some are a little more woody, some more fluffy, some with perlite or pumice, some without.

An image of an almost empty wood raised bed. It is five foot by three feet, and 18 inches deep. Various bagged soils lay around the outside of it, waiting to be added.

Again, this bulk-bagged combo is mostly used for when we are filling many large beds at once. For instances where just one or two modest beds need to be filled, we'll often go with all bagged material. For a smaller volume, we've found it isn't worth the delivery fee or minimum order amount to get the bulk soil. Plus, the bagged stuff is often times superior in quality anyways.

40% COMPOST

Compost is organic matter that has been thoroughly broken down and decomposed into rich nutrient-dense plant food. It is a killer soil conditioner and will make your garden shine! We try to add as much homemade compost from our worm bin or our large compost pile as possible. (To learn about 6 different ways to compost at home, plus some general compost 101 do's and don'ts, see this article!) Unfortunately, we usually cannot make quite enough homemade compost to meet all of our needs – especially for large projects like filling raised beds. So we do end up supplementing with organic bagged compost or bulk compost too. The bulk compost option that we purchase is made from local green waste.

Our favorite bagged compost is Malibu Compost Biodynamic Blend, or just referred to as "Bu's". It is aged, composted cow manure from certified organic dairy farms. They use biodynamic practices. The compost ingredients include organic aged manure, straw, vineyard wood chips, plus yarrow, chamomile, valerian, stinging nettle, dandelion, and oak bark! It is not uncommon to rip open a bag to dozens of happy worms squirming around in there.

A photo of a bag of compost, by Malibu Compost. It is sitting on the gravel, leaning up against a half-filled raised bed behind it. — We love Bu's. People hear me talking about this on Instagram and often think I am talking about how much we love "booze"… 😂

Unfortunately, Bu's is primarily available on the West Coast. If you live in the Northeast, Coast of Maine makes a similar product. I'm sure there are many more companies out there, all over the U.S. Leave a comment if you are aware of a good local product like this in your area!

Manure versus Compost

Note that fresh manure is very different from compost. Yes, Bu's starts with manure, but it aged and composted over time to become a more mild, balanced product. Fresh manure that hasn't been properly aged is very high in nitrogen and can burn your plants. Therefore, avoid adding fresh animal manure to your garden. I am also a little leery of animal manures that are not from certified organic operations, or have an otherwise unknown source.

Compost Options

Aside from our homemade compost and Bu's, we are able to get an "organic compost" product from our local landscape supply company in bulk for big projects. Check to see if your local companies sell something similar! We also consider some of the bagged products like G&B Organics "Harvest Supreme" as more of a compost amendment than a soil, so we take those into account when shooting for that 40/40/20 ratio.

Last but not least, worm castings (aka worm poop!) are compost too. Worm castings are so good for your plants, they're referred to as "black gold". I highly suggest you try starting a worm bin at home! It is a terrific way to divert food waste from the landfill, and up-cycle that "waste" into a incredibly valuable product for your garden or house plants! Worm bins are inexpensive, pretty easy to maintain, and no, they do not smell bad. Learn how to set up and maintain a super simple worm bin here!

A hand holding a handful of rich looking soil, which is actually worm castings. A few red worms are in with the worm castings. In the background, there is a whole tub of worm castings, — A little tub of screened worm castings or "black gold" being added to the new raised bed, from our worm bin. As you can see, a few friends came along for the party!

If you aren't up keeping your own worm bin, you could purchase finished worm castings and add a little to each bed. We can usually find bags of G&B Organics Worm Gro locally, or you could pick up some worm castings from Uncle Jim's Worm Farm online!

To find premium bagged compost and soil, I suggest looking not only at your local garden center, but also your local hydroponic store or "grow shop" too! Those shops are usually geared towards growing cannabis, and carry some high quality products.

20% AERATION

Nearly as important as the compost component, an aeration addition is key to healthy soil. This could include lava rock, pumice, or perlite. Coarse sand also promotes aeration and drainage, but not quite as effectively as the others.

Why add material for aeration? Well, as we talked about, soil is full of living things, and they need air to thrive! Those living things include beneficial microbes, nematodes, worms, protozoa, fungi, and more. Last but certainly not least, your plant's roots need air to thrive too! They breathe through their roots! You'll often hear me stress to not over-love your plants with too much water, effectively drowning them.

An aeration additive doesn't only provide air. Their presence promotes drainage and prevents the soil from over-compacting. It may seem counter-intuitive, but absorbent materials like lava rock and pumice also increase moisture retention at the same time as providing good drainage. They'll hold water within themselves, helping to maintain an evenly moist raised bed for a longer period of time between watering. Like many things in gardening, it's all about balance.

Keep in Mind:

A lot of bagged soil mixes already contain some perlite, pumice, or rice hulls. Those are all forms of "aeration", and are adding to our target total. So if you use primarily bagged mixes, read the ingredient list. If they contain those things, you can go lighter on adding additional aeration (e.g. perhaps only 10% extra, if any). However, if you buy only bulk soil and compost products that do not contain any aeration additive, you'll want to add more. Make sense? Like I said, this doesn't all need to be an exact science.

Lava Rock

For the aeration portion of our soil recipe, we have come to love volcanic rock. It is also called lava cinders and is frequently used in aquaponics. We get the small 3/8 inch to quarter-inch size. Don't use huge chunks! Lava rock is full of pores, that not only promote aeration and drainage, but are also the perfect habitat for beneficial microbes to grow. They don't float to the soil surface like white bits of perlite do, and are generally more affordable than pumice.

Our local landscape supply company carries volcanic rock both in bulk and in half cubic-foot bags. For any Central Coast locals, I am talking about AirVol Block in San Luis Obispo. This is where we get a lot of our hardscaping materials like stone blocks, green rock gravel, cobblestones, and pathway pavers too.

If you can't find volcanic rock, pumice or perlite can totally be used! Availability of all these products will vary depending on your location.

Another thing that will help with aeration is… my favorite, worms!

Worms

Did you know that Cornell University refers to worms as "living soil amendments"? Worms play a vital role in the overall health of organic soil, as part of the soil food web. By simply being present and doing their wormy thing, they continually aerate, nourish, and improve soil structure! Furthermore, as they cruise through soil, they break it up – which in turn improves drainage, increases moisture retention and oxygen flow to plant root systems – all very good things!

"Worms feed on plant debris (dead roots, leaves, grasses, manure) and soil. Their digestive system concentrates the organic and mineral constituents in the food they eat, so their casts are richer in available nutrients than the soil around them. Nitrogen in the casts is readily available to plants. Worm bodies decompose rapidly, further contributing to the nitrogen content of soil."
NSW Government Agriculture

Adding worms to raised garden beds

As worm castings are added into each of our raised beds, a few red wiggler compost worms (and their cocoons or babies) usually hitch a ride too! Now they've been introduced into the bed, and will continue to populate. Red wigglers are fairly small worms, reproduce quickly, and break down food matter fast, thus creating castings faster. They're perfect in a worm bin, and good in the garden too! However, red wigglers generally like to stay near the soil surface. On the other hand, earthworms like European Nightcrawlers like to dive deeper in the soil to do all their good work. We add some of those big bad boys to our garden too!

Two hands held together, cupping a large pile of european nightcrawler earth worms. The hands are poised over a garden space — European Nightcrawlers from Uncle Jim's Worm Farm, about to to be added to the soil around our fruit trees – where the soil is compacted. A few got added to each bed too!

Please note: As strange as it sounds, most earthworms are not native species to North America. They are excellent additions to your garden, but some can disrupt native ecosystems (such as forests) if they're introduced elsewhere. This is particularly a concern with a dense population of European nightcrawlers. Thus, please make responsible decisions when adding worms to your soil, depending on your garden (e.g. does it abutt a woodland?) Within the confines of urban or suburban gardens, their presence is welcome and helpful!

FILLING YOUR RAISED GARDEN BED

Now that we have a better idea of the types of materials we want to add to our raised beds, it is time to fill them up!Yep, our beds are 100% full of all this good stuff. Aside from sometimes adding bulk soil to the bottom of our deepest beds, there is no other "fillers" at the bottom. Just how the roots like it!

The goal is to get all of these materials evenly mixed, as much as possible. We'll generally add them in "lasagna layers", mixing as we go. For example, add several inches of bulk soil or bagged soil, a good layer of compost, a couple inches of volcanic rock, and mix. With that, the bed may only be about quarter full. Repeat with more layers of each, mix. Continue this process until the bed is full.

An image of a newly built redwood raised bed, only about half full at this time. In the bed, there is a combination of soil and compost, with some red volcanic rock on top, about to be mixed and then continued to be filled. — A "lasagna layer" of soil, compost, and volcanic rock. We'll mix this up, and then add more of the same until it's full!

On that note: try to fill your beds all the way up! They don't need to be overflowing necessarily, but at least up to within a couple inches of the top. When you first water the bed, it will compact and sink down a little. Depending on how much it sinks, you may want to top it off with another layer of compost. We use compost on the top of our soil as mulch !

A post will be added later about mulching, but in a nutshell: Do it! Materials such as compost, leaves, straw, or pine needles can be used to top off a bed and increase moisture retention. Use what is most readily available and appealing to you!

A Note About Hugelkultur

To take up *some* space at the bottom of a deep empty bed, you could choose to add a few inches of small branches, leaves, mulch, pine needles, or other woody organic matter, and then add the other recommended raised bed soil and compost on top. The woody debris eventually break down and feed the soil as a carbon source over time. This follows a practice called Hugelkultur. However, I do not recommend adding non-organic matter such as rocks, plastic bottles or other random materials to take up space in your bed.

What about Fertilizer?

Most "virgin" soil will probably need some amending with mild, balanced, slow-release fertilizers to keep your plants healthy, happy, and productive! This is true whether it comes trucked in bulk or from a bag.

If you filled your raised garden bed primarily with high-quality organic bagged soils and compost, you can go pretty light on the fertilizer for the first growing season. Those bags usually contain pre-amended soil with light fertilizer and some compost added. But as the next growing season comes around, you'll want to start implementing a fertilizer routine for your raised garden beds. As your plants grow, they will use up a lot of the available nutrients in the soil.If you fail to amend the beds in preparation for the new crops, you set them up to flounder and starve!

On the flip side, if you started primarily with bulk soil and compost from a local landscape company, you'll most definitely want to add fertilizer from the start. That is, unless they say that it is already amended. Though I don't think that is common practice for most bulk material.

Amendment Choices

Every gardener is going to have their own methods and opinions about fertilizers and amendments. Let me tell you about ours, and then you can make you own decisions and research further from there!

We don't ever use the "heavy-hitter" fertilizers, like blood meal, bone meal, or feather meal. I call them heavy-hitters because they're usually really high in one macronutrient or another – nitrogen, phosphorus, or potassium (NPK). While all those nutrients are essential, we don't feel the need to dose our plants heavily with animal byproducts. Due to their strength, there is an increased risk for shocking and "burning" your plants. With all of the compost, worm castings, aerated compost tea, and other more natural, mild amendments and practices we use, we just don't find the former to be necessary!

Let's talk about what we DO use.

Mellow Meals

Instead of those "heavy hitters", we prefer to add more mellow, balanced, slow-release, plant-based fertilizers to our raised bed soil. Down-To-Earth brand has been our go-to for the past several years, though we use other brands at times too. The main products we use are kelp meal, neem meal, alfalfa meal, or this vegan all-purpose fertilizer. They're all OMRI-certified for organic gardening.

We sprinkle in these mellow meals on top of the soil, lightly scratching and working them in to the top few inches. We do this a couple times per year, usually between switching out crops in beds. Keep in mind we garden year-round here, so your fertilizing schedule may vary. I always suggest going a little lighter than the instructions on the box. You don't want to accidentally "burn" the plants! Even these gentler options do pose some risk of that, if you overdo it. They all have their different benefits so we usually mix a few of them. If you were to start with just one, go for the all-purpose stuff.

Sprinkling in a combination of kelp, alfalfa and neem meals to the top of the soil once the raised bed is full. This will be scratched into the top few inches of the soil, and watered in. We also added some rock dust here, and earlier when the bed was only half-full.

Rock Dust

One thing we always add to our garden is basalt rock dust. For brand-new raised beds, the wild areas of our yard with native soil, or when we are planting a tree. We consider this more of an amendment than a fertilizer. It is slow-release and cannot burn or harm plants like fertilizer can. Rock dust comes from volcanic ash. It is low in macronutrients, but absolutely loaded with trace minerals.It helps increase nutrient uptake by plants, increases crop yields, improves plant immunity, boosts pest resistance, and generally enhances soil and plant health. Check out Remineralize.org for more information and research about rock dust!

Another amendment we use in our raised bed soil is biochar. If you are curious to read more about biochar, see this post from Regeneration International.

There you have it!

Those are the kinds of things we add to our garden beds. When planting seedlings, we sprinkle mycorrhizae in their planting holes. Once the plants are established and growing, we routinely water with actively aerated compost tea (AACT) and seaweed extract. However, this post is about FILLING a raised bed, not maintaining one.

Therefore, check out this article all about how we routinely amend and prepare our raised bed soil between planting seasons.

IMPROVING EXISTING SOIL

Do you have raised beds that are already full of soil that you aren't very happy with? Or more like, that your plants don't seem very happy with? It's okay! In fact, we have been in this boat before too. Do not feel the need to go dig out all your soil and start over. There is hope!

Before you consider replacing your raised bed soil, try amending it first using some of the materials we've discussed already. If your plants are sad and small, have you been fertilizing them adequately? Is your soil too compact? Try to mix in some aeration additive. Does it seem like the soil drains too quickly, or dries out to quickly? Add compost and worm castings! That, along with watering with aerated compost tea, will increase the microbial life in your soil. For better moisture retention, drainage, and "fluff", peat moss or coco coir could also be mixed in. Note that coco coir is the more sustainable option of the two.

First, try those measures for a season and see if there is noticeable improvement. I really think there should be. If not? Next, consider taking out at least a portion of the old soil and replacing with the types of things discussed in this post.

The final raised garden bed for this example, now completely full of soil, compost, and aeration additions. It is being watered, with drips of water running down the side of the bed, and a rainbow has appeared in the water spray. — The final full bed. Make sure to give it some water to get everything happy, and to get those microbes kicking!

Two hands cupping rich brown organic soil with a few red worms, hovering over a garden bed with leafy greens in the background below.

And that is how we create our "perfect" organic raised bed soil.

I realize that if you are new to this world, this all may sound completely overwhelming! I get it, and I don't mean to make you feel intimidated! The goal is simply to set you and your garden up for success!

Above all, if you had to take away just one message from this, it would be this: Compost compost compost, and worms worms worms! If you get those life-givers into your beds, the rest is simply an extra boost of kick-ass.

Thanks for reading. Feel free to ask questions, and spread the soil love by passing this along!