The Ideal Soil Mix for a Raised Bed

If you just added a new raised bed to your garden and are wondering how to fill it, this is the guide for you—it’s all about soil, and how to choose the ideal soil blend that’ll help your vegetables, herbs, and flowers thrive.

Let me start by saying that I’ve built many raised beds over the years and filled them with soil in a variety of ways. I’ve experimented with bagged soil and bulk soil, and even made my own planting medium. The soil I have now is the best soil I’ve ever had—loamy and full of worms—and it only continues to get better each year!

But it wasn’t always that way in the beginning. I’ve had my share of mishaps and I’ve certainly spent more money than was necessary to build good soil. But that’s the key here: you build good soil over time, and I’ll help you do it right from the start.

The kind of soil you want should be teeming with fungi, bacteria, protozoa, nematodes, arthropods, earthworms, and decaying organic matter. The higher the microbial activity, the healthier your soil is.

Building up this biodiverse ecosystem takes a bit more effort than just making a trip to the garden center and buying lots of bags of “garden soil” to fill up your raised bed. Sure, you can go that route, but it’s expensive and won’t provide you with a robust growing medium. Good healthy soil is made, not purchased.

The ideal soil should have a good balance of clay, silt, sand, and organic matter. These are the four essential elements of soil that give structure, hold nutrients, add porosity, and provide nutrients. When you have all four elements in relatively equal amounts, you have loam—an ideal soil type that holds plenty of moisture but also drains well so air can reach the roots.

So how can you achieve that?

While the list is short, this is just the bare minimum of what should be present in your soil.

Topsoil is the topmost layer of native soil in a garden, yard, or field. It’s typically the upper 3 to 8 inches with the highest concentration of microorganisms and it’s fairly nutrient-rich and permeable.

You can dig up the topsoil from your yard, buy it in bags from a home improvement store, or buy it in bulk from a landscaping company. Not all topsoil is created equal, so if you’re buying topsoil, ask questions as to where it’s from and what goes into making it.

Inspect the topsoil and do the “squeeze test,” where you simply take a handful of topsoil and squeeze it. It should hold together but crumble easily when you run your finger through it. If the topsoil is hard to break apart, it’s too heavy. If it doesn’t hold together at all, it’s too sandy.

Good topsoil should be neither sticky nor sandy, and it should lean toward the darker end of brown (instead of gray or light tan). It should also smell fresh and earthy, not sour or rancid.

Compost is fully decayed organic matter, such as fallen leaves, plant debris, fruit and vegetable scraps, and even decomposing insects and animal waste (which is how nature gets a lot of nutrients into the soil).

Not all compost is created equal, so you definitely want to do your homework here. If you make your own compost at home, keep in mind that unless you’ve mastered the art of hot composting, there will likely be seeds from your vegetable scraps that’ll sprout. For this reason, it’s a good idea to put homemade compost toward the bottom of your raised bed so that any seeds won’t germinate.

If you buy bagged compost, read the label to make sure you’re getting a high-quality product. A lot of the cheap bagged compost mixes are just bark with some manure in them.

Some municipalities also offer their own compost, made from the materials that people toss in their green cans (yard waste bins). You can find municipal compost at your local recycling or waste center, and it’s often free or very inexpensive. While this is usually better than bagged compost, be aware that it might have other waste products in it. (The municipal compost in my town sometimes has bottle caps and other random items that made it through processing, so it’s not the cleanest.)

My favorite type of compost comes from my local landscaping company. I buy it in bulk every year (by the cubic yard) and have been using the exact same compost for the last several years, so I know it’s an excellent product and works in my garden.

Get recommendations from your local nursery or gardening group if you’re unsure where to start, and don’t be afraid to ask the landscaping company where they get their compost and what ingredients are in it. Once you find your go-to source, you’re set!

The percentages below (which reference volume) are approximate and they’ve worked well for me, but they don’t need to be exact.

In fact, my favorite bulk soil from my local landscaping company is called the “50/50 mix,” as it’s made with 50 percent clean screened topsoil and 50 percent screened Central Oregon compost.

This is a decent all-around soil blend that will work for almost anything you’re growing, and it will improve over time with a few simple practices (which I cover below).

When would you use this instead of a 50/50 mix? When your topsoil is a bit on the denser side, or when you’re growing plants that prefer sandy loam or very well-draining soil. My strawberry beds are filled with soil like this, as strawberries are prone to root rot.

(You can see what that looks like in my guide to planting bare-root strawberries.)

Coarse sand is not the same thing as play sand (that super fine, beach-like sand sold in bags in home improvement stores). You need sand that has some grit or pebbles in it, and it goes by different names: paver sand, builders sand, all-purpose sand, or silica sand.

Disclosure: If you shop from my article or make a purchase through one of my links, I may receive commissions on some of the products I recommend.

Organic matter is the special sauce in all my garden soils, and depending on the season, it can be any number of things:

Fallen leaves – They’re free (which I love) and they’re plentiful in fall. I don’t have a lot of deciduous trees in my yard, so I just ask my neighbors if I can pick up their bags! (Bags that would often just go to the landfill, so it’s a win for everybody.) Larger, thicker leaves should be shredded first before adding them to your soil, but smaller leaves can be mixed in whole.

Pine needles – What I have plenty of in my yard, however, are pine needles, so those go in my soil mix too. (Contrary to popular belief, pine needles do not acidify soil—so go ahead and use them if you have them.)

Worm castings – I buy worm castings (also known as vermicompost, or worm manure) in bags, and a little goes a long way. Worm castings are biologically active and nutrient-rich, and they help improve soil structure by adding aeration and enhancing moisture retention.

Poultry manure – I raise chickens, so I have a neverending supply of well-aged poultry manure. That said, you can often buy bagged poultry manure or source from local farmers or backyard chicken-keepers.

Straw – I let all the straw I use for mulch break down each year into my soil, and simply layer compost on top of it. Straw bales are inexpensive and readily available at feed stores; just make sure yours aren’t sprayed with herbicides.

There are many other things you can add to your soil, so use what’s widely available in your area.

However, I also have a short list of what I never use in my raised beds:

❌ Horse and livestock manure – This was a hard lesson learned many years ago in my old garden. Horses and livestock may consume hay that comes from fields sprayed (or oversprayed) with herbicides. These herbicides (which target broadleaf plants) pass through the animals’ digestive tracts and remain active in manure, even after it’s been composted. The herbicides then leach into soil with rainfall or irrigation, where they can persist in the soil for up to four years.

A lot of common garden vegetables are sensitive to herbicides, including tomatoes, peppers, spinach, lettuce, carrots, and peas, as well as most flowers. If you try to grow them in herbicide-contaminated soil, you’ll get stunted or deformed plants with poor to no production.

If you still want to use horse or livestock manure in your garden, always ask what the animals were fed, the origin of the hay, and if any herbicides were used on the hay or pasture. Some livestock owners might not know this (or they believe their manure is safe because their animals are healthy), so be sure to do a plant bioassay before the compost comes in contact with your soil.

❌ Peat moss – Peat is sometimes used in raised bed soil to help retain moisture, but I don’t recommend it. Sustainability questions aside, peat is very hard to rehydrate once it dries out, and it can actually repel water. If you want to improve soil structure, add more (or a different kind of) compost, such as leaf mold or mushroom compost.

❌ Fill dirt – Fill dirt is the material below the topsoil, and it’s the kind of dirt you use to fill holes in your landscape or change the ground elevation. Because it’s usually taken from the subsoil, it has little to no organic matter. Don’t be tempted by people offering free fill dirt from their home construction projects—good topsoil is always worth the investment for your garden.

How to maintain healthy soil

As they grow, your crops will take the nutrients they need from the soil, so it’s important to replenish that organic matter each year if you want to maximize production.

I keep it simple in my own garden:

1. In early spring (before I plant my warm-season crops) and in late summer (before I plant my cool-season crops), I topdress with a couple inches of compost over my raised beds—this is just garden speak for adding a layer right on top of the soil. There’s no need to work it in with a rake or shovel.
2. I always use mulch to protect the soil. As the mulch breaks down, it also adds organic matter to the soil. (These are my favorite types of mulch for an organic garden.)
3. When I replace plants, I don’t pull up the whole plant, roots and all—I cut it down at the soil level and leave the roots in the ground to decompose naturally. The roots add lots of good organic matter and feed soil microbes, while the spent plant is usually laid right on top of the soil and used as mulch. (I call this the “chop and drop,” and it works especially well in winter since rain and snow help break down the materials faster.)

If you follow this easy three-step process, you’ll have better soil in year two, really great soil in year three, and amazingly rich soil in years four and on.

Never leave your raised beds empty at the end of the year, even if you’re not growing anything in them. Plant a nitrogen-fixing cover crop like fava beans in fall or a frost-tolerant cover crop like Austrian winter peas in winter, grow one of these edible cover crops that can feed you and the soil, or do this to recharge the soil over winter. Like fine wine, your soil will age and get better with time!