How to Fill Raised Garden Beds with Organic Soil

One of the most common gardening questions I get is: What kind of soil do you fill raised garden beds with? There are so many raised bed soil choices out there, it can feel confusing and overwhelming to figure out what’s best. It’s also pretty darn important to “get it right”, since soil health plays a HUGE and direct role in overall plant health and productivity.

Read along and learn how to fill raised garden beds with quality organic soil that is well-balanced, nutrient-rich, and microbially-active – so your plants will thrive! In this article I will share our raised bed soil “recipe” with bagged soil, bulk soil, compost, and amendment options along with tips on calculating soil volume and using hugelkultur to fill raised garden beds.

Getting Started with Raised Beds

Looking for tips on how to build a raised garden bed? Check out this post to learn how we build durable, attractive, long-lasting wood raised beds. The tutorial includes a step-by-step video and explores wood choices, bed sizing, location or placement, tips for gopher-proofing beds, non-toxic sealer, and other ways to make wood raised beds last longer.

Not feeling up to building your own raised beds? That’s okay! Gardener’s Supply Co has some really nice wood raised bed kits. We also love the sleek, high-quality, galvanized metal Birdies raised beds from Epic Gardening – and code “deannacat3” will save 5%!

Once you have your beds in place, it’s time to fill ’em up.

Investing in quality raised bed soil

Let’s set the record straight: “Dirt” is not soil! Soil is rich, full of nutrients, critters, microorganisms, 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.

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’ve gone through the effort to build or buy yourself some awesome raised garden beds, why skimp on the soil? However, the answer isn’t as simple as “go grab X brand of soil”. In my experience, not one soil is going to be perfect for growing vegetables on its own – bulk or bagged.

If you’re blessed with decent native soil, you can use some of that to fill your raised beds to offset cost (explored below). And if you already filled your raised garden beds with less-than-ideal soil, don’t fret! There are many ways to amend and improve existing soil, as discussed at the end of this post.

CREATING THE PERFECT RAISED BED SOIL

Within the confines of a large container, raised bed soil needs are unique and slightly different than in-ground gardens. The goal is to create a raised bed soil mix that is fertile, rich in organic matter, holds moisture, but also has good drainage and plenty of air pockets using a combination of quality organic soil (bagged and/or bulk), compost, and an aeration additive.

Our raised bed soil recipe:

• 40% soil (topsoil, loam, etc)
• 40% well-aged compost
• 20% aeration (e.g. sand, lava fines, perlite, pumice, cocoa coir, peat moss, and/or rice hulls)
• Other amendments such as worm castings and mellow, slow-release organic fertilizers

Next, we’ll talk about each of these components in detail below. Note that the ratios are approximate; they don’t need to be exact. Also, this raised bed soil recipe can be applied to other types of container gardening, just scaled down. For example if you need to fill fabric grow bags, wine barrel planters, or pots.

Before soil shopping shopping, you’ll need first to calculate the volume of soil is needed to fill the raised garden bed(s).

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. To calculate how much soil you need to fill a raised bed, begin by determining the volume of your bed in cubic feet. To do this, simply multiply the width by length by depth in feet (For example: 4’ x 8’ x 1.5′ = 48 cubic feet).

Now you have your total cubic feet, and can figure out how much bagged soil it would take to fill the bed. If you’re using bulk soil or compost too, you’ll need to calculate volume in cubic yards. To convert to cubic yards, simply multiply cubic feet by 0.037037 (or use this converter). Given the example of a 4×8′ by 1.5-foot deep bed used above, 48 x 0.037037 = 1.7 cubic yards of soil is needed to fill it.

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.

40% SOIL

Bulk vs bagged soil (or compost)

If you calculated that you need several yards of soil to fill your raised garden beds, you are not going to want to rely on bagged soil alone. The cost will add up quick! Look up local landscape supply companies and see what they offer in bulk – either for delivery or for pick-up, if you have a truck. For example, we often get bulk soil and compost delivered from Central Coast Landscaping or Mier Brothers here on the Central Coast. On the other hand, if you need to fill only one or two modest raised beds, purchasing bagged soil could be the way to go.

Bulk soil options

Bulk soil quality, composition and type will vary depending on your location. In our area, we’ve found varying grades of bulk soil including basic topsoil or “fill”, some premium container blends (similar to bagged soil), and a middle-of-the-road “planters mix” that is 2 parts top soil, 1 part compost and 1 part soil conditioner.

The composition of bulk soil will influence how much/what else to mix with it. For instance, if you all you can find is basic fill or topsoil, then you’ll want to follow the full raised bed soil recipe of 40% topsoil, 40% compost and 20% aeration to create a nice well-balanced soil. Or, use mediocre bulk soil as a “filler” at the bottom of deep raised beds, with a better soil on top.

Yet if you’re able to find premium raised bed soil in bulk, then it likely already contains a good amount of compost and aeration (sand, perlite, etc) so you can therefore scale back on those component of the recipe respectively. The landscape supplier should be able to provide details of the ingredients and make-up of the soils they offer!

Bagged soil options

Even if we use good bulk soil to fill the majority of our raised beds, we always add at least a few bags of high-quality organic bagged soil near the top. We’ve used a wide variety of bagged soils over the years, but tend to prefer the selections offered at our local garden centers and nurseries over big box stores. “Hydro” grow shops (those geared towards cannabis growers) usually have a great selection of premium soils too.

A few good options include Gardener & Bloom (G&B), E.B. Stone, Aurora/Roots Organics, Dr. Earth’s, and Fox Farms. Other cheaper or generic bagged soil can be used as filler towards the bottom of the bed, especially if you’re filling raised beds with bagged soil alone.

If possible, choose a few different bagged soils. Do not use “potting soil” only. It is light, fluffy, and will dry out quickly. 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 with or without added fertilizers.

40% COMPOST

Compost is organic matter that has been thoroughly broken down and decomposed into rich nutrient-dense plant food. It is a phenomenal soil conditioner, boosts the soil food web, 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 new raised garden beds with soil. So we do end up supplementing with organic bagged compost or bulk compost too.

Keep in mind that most bagged soils usually already have compost in them. So unless you’re using plain topsoil in your raised bed soil recipe, feel free to scale the “compost” ratio down to 10-30% (depending on the soil you’re using).

Compost options

Many bagged soil companies offer bagged compost products too. Again, these can be found at your local garden center or hydroponics grow shops. You should also be able to find compost locally in bulk, which is usually derived from green waste or possibly composted food waste. The bulk compost we purchase is made from local green waste and is OMRI-certified for organic gardening.

Our favorite bagged compost is Malibu Compost Biodynamic Blend, or just referred to as “Bu’s”. It is made from 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. However, please note that Bu’s compost is quite rich, so they recommend mixing only 25% of their compost to your soil blend.

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 compost 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 (antibiotics, pesticides, hormones, GMOs, ya know…) or have an otherwise unknown source.

Worm castings

Last but certainly 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”. Worm castings offer gentle, slow-release fertilizer as well as improve soil structure, microbe activity, water retention, and drainage.

I highly recommend keeping a worm bin at home! It is a terrific way to divert food waste from the landfill, and up-cycle that “waste” into an incredibly valuable product for your garden or house plants. Worm bins are inexpensive, easy to maintain, and no, they do not smell bad. Learn how to set up and maintain a super simple worm bin here!

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

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.

Note: Like compost, a lot of bagged soil mixes already contain some perlite, pumice, peat moss, 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).

Benefits of lava rock, pumice or other aeration in raised bed soil

Why add aeration material to raised bed soil? Well, as we talked about, soil is full of living things – including beneficial microbes, nematodes, worms, protozoa, fungi, and more – and those friends need air to survive. Even more, plant roots need air to thrive too! Did you know plants breathe through their roots just as much as they do through leaves? That’s why it’s important to not overwater, and always provide drainage holes in pots. Otherwise, the plants will drown!

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 increases moisture retention at the same time. They create air pockets for drainage, but also hold water within themselves like a sponge, helping to maintain evenly moist raised bed soil for a longer period of time between watering. Like many things in gardening, it’s all about balance.

Lava Rock

For the aeration portion of our raised bed 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 larger 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, use small perlite or pumice. Availability of all these products will vary depending on your location.

Worms

Another thing that will help with aeration is… my favorite: 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 around, they break up the soil. This 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!

Please note: 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 abut a woodland?) Within the confines of urban or suburban gardens, their presence is generally welcome and helpful! The bottom of our raised beds are blocked off with landscape fabric, so they can’t easily get into the native soil.

HOW TO FILL A 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.

Fill garden beds all the way up – 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 or soil. Filling shallow raised garden beds (e.g. 1 foot deep or less) as full as possible is especially important. Empty space in the left bed is wasted space for happy plant roots

What About Hugelkultur?

To offset cost and take up some space at the bottom of a deep empty bed (e.g. two feet or deeper), you could choose to add some small logs, branches, leaves, mulch, pine needles, or other woody organic matter, and then add at least a foot of soil and compost on top. The woody debris eventually breaks down to feed the soil as a carbon source over time.

This practice is called Hugelkultur. Learn hugelkultur pros and cons, best practices and materials to use, and how to make a hugelkultur garden bed here. However, I do not recommend adding non-organic matter such as rocks, plastic bottles, sytrofoam, or other random materials to take up space in your bed. I also don’t recommend using hugelkultur in shallow raised garden beds (1 foot deep or less).

Raised Bed Soil Fertilizers

Most “virgin” soil (especially topsoil) will 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’s already amended. Though I don’t think that is common practice for most bulk material.  

Organic Soil Amendment Options

Every gardener has their own preferences for 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. Between the compost, worm castings, aerated compost tea, and other more natural, mild amendments and practices we feed our soil, we just don’t find the former to be necessary!

Let’s talk about what we DO use.

Mellow Meals

We prefer to add 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.

Rock Dust

One optional amendment we always add to our garden is basalt rock dust. We add rock dust when filling new raised beds with soil, or when planting trees or shrubs in our native soil. 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!

Mulch

Finally, don’t forget to cover the soil with a couple inches of mulch. Mulch protects and improves raised bed soil by reducing evaporation, suppressing weeds, and buffering the soil (and plant roots) from temperature swings. We typically use a woody compost as mulch. Learn more about mulching best practices and pros and cons of 8 popular mulch options here, including bark or wood chips, straw, compost, and more.

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.

And that’s how we fill new raised garden beds with soil!

I hope that was helpful! Going forward, we do plenty of other things to feed our soil too – such as using mycorrhizae during transplanting. Or, routinely watering with actively aerated compost tea (AACT). Check out this article all about how we routinely amend and prepare our raised bed soil between planting seasons. Otherwise, thanks so much for reading. Feel free to ask questions in the comments below. Also spread the soil love by sharing this post if you found it useful!

You may also like:

• How to Install Drip Irrigation in Raised Beds (Drip Tape)
• 7 Ways to Make Wood Raised Beds Last Longer: Non-toxic sealer and more
• How to Make a Sturdy and Inexpensive DIY Trellis
• 9 Ways to Add Whimsy and Interest to Your Garden