Here's Your Complete Guide to Garden Soil
One summer, I led my kids out to the backyard, dug a few little holes with a hand shovel, scattered seeds I'd bought on a whim from the dollar section of Target, and turned on the water hose. We trooped back out every single day to soak the ground with water and check for signs of sprouting. A week passed. Still no sign of anything but dirt.
Feeling discouraged when nothing had happened by week two, I checked the back of the seed package, where it said we could expect to see germination within 5 to 10 days. Five to 10 days?!
Now that I've done my homework, I know that we could have waited 5 to 10 years—we were never going to see a single plant appear.
Those seeds had either rotted away in the thick clay of our Tennessee soil, or they'd been washed away from all that spraying. (Can you tell that I was a garden professional in the making?)
That's how I learned that the secret to any garden's success is often hidden underneath the lush vegetation.
It's the soil.
Here's all the dirt you need to know about garden soil.
Why Does Starting with Good Garden Soil in Raised Beds Matter?
When most of us start a garden, we head to the plant store first and focus on purchasing plants and seeds—the visible pieces of our gardens. It's all too easy to forget about soil when your head is filled with visions of cherry tomatoes ripening on the vine.
But soil is one of the most critical elements to get right when you're growing your own herbs, fruits, and vegetables. The raised bed and trellis provide a strong foundation for your garden, but it's the soil (and the water and the sunlight) that will fill your space with life. Good soil is the difference between having a bunch of boxes and having sprouts, plants, and of course, delicious fresh harvests.
If you go through the trouble of installing a raised-bed kitchen garden but skimp on the soil, you might as well just garden in the ground. The point of having raised beds is to fill them with the best soil possible so that you're not left standing there at the end of a growing season with a hose, wet dirt, and nothing to show for it.
Whenever I hear someone saying their plants aren't doing well (their leaves are changing color or the plants just aren't growing or they're having major pest issues), my first question is: "Have you checked your soil?" I probably sound like a broken record at this point.
Time and time again, in both my own garden and in those of my Kitchen Garden Academy students and clients, I've found that soil is the secret to success at any stage of garden growth... or the reason there's a lack of success.
If you can figure out what's going on inside the soil, then you can figure out what's going on above the soil too.
Now that you know a bit about why good dirt is so important, let's take a look at the different kinds of dirt and what makes up good soil for your kitchen garden.
Garden Soil vs Native Soil
Native soil is what your house is built on. It's basically the different layers of dirt you already have, and it differs broadly by region.
Those ornamental plants like boxwoods and the native trees and bushes you have growing in your landscape do well in your native soil because they have super strong, thick root systems that can push through soil that isn't very permeable or that is full of clay. My little seeds might have been having a tough time in my Nashville backyard, but there were plenty of other plants that were thriving there.
Most of the plants that you'll want to grow in your raised beds are annuals that finish their life cycles in 30 to 90 days. It's often best to start annuals from seed, but these plants can have a difficult time sprouting in certain types of soil because they don't have strong enough root systems to push through.
Peas, carrots, radishes, tomatoes, peppers, squash, eggplants, tomatillos, beets, cabbage, kale—these all have a fairly fragile root system. Their tender roots are not meant to push deep into the earth like those of an oak tree. Because of this, the roots for edible plants require a unique kind of soil, not the kind of soil that you're going to find in your landscape. And that's why it's not always a great idea to dig up a bunch of dirt from your backyard and use it to fill your raised beds.
Garden Soil vs Topsoil
When you hear the term topsoil, we're referring to that top layer of soil available to you in your landscape. Imagine taking a shovel and digging into the first three to six inches of your landscape soil. Generally, topsoil is fairly nutrient-rich and permeable, and it can be used in your soil mixture for your raised bed as long as it hasn't been degraded or sprayed with lots of chemicals or pesticides or something really nasty like Roundup.
Next time you step outside, pick up some soil from your landscape and see if you can separate out the different elements that I'll talk about below. The locations that I've gardened in have primarily had clay-heavy soils. Once you know what your topsoil situation is, you can decide whether you're going to amend your native topsoil to put into your raised beds or start from scratch.
Garden Soil vs Potting Soil
Potting soil is designed to provide support and nutrients for potted plants. Depending on the blend, potting soil may not contain any actual soil. There are soilless potting mixes that consist of various combinations of peat moss, compost, perlite, vermiculite, and coconut coir. The seed starting mix that you might use to start seeds indoors, for instance, might be nothing but coconut coir so that it's very light in texture for young roots. This same mix wouldn't do very much to support a mature plant's roots.
Now, let's look specifically at raised bed garden soil.
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The Two Key Traits of Garden Soil for Vegetables
The key to edible gardening success is a garden soil that's both nutrient-rich and permeable. Let's look at those two key traits individually.
Plants Need Nutrients in the Soil
You have to feed your plants before you can expect them to feed you. How can you expect your leafy greens and your tomatoes to give you all these nutrients if they weren't grown in a nutrient-rich situation? Plants growing in dense, dry, or sandy situations lack the resources to take up vitamins and then give them to you. Your soil instead should be full of all kinds of vitamins and minerals when you're growing a food garden.
Remember this: Nutrient-rich food comes from nutrient-rich soil.
Roots Need Permeability in the Soil
Imagine your garden soil is a sponge, one of those big sea sponges that can take up lots of water and hold it. As soon as it's hit its capacity, water will just run right through it. That sponge also has lots of pockets for air. Your soil similarly needs to be able to hold water and air, two essential things your plants are looking for in order to thrive.
We don't want soil that won't allow anything in or out. Impermeable soil will stop those tender little roots of your edible plants in their tracks. That's why, if you try to grow kitchen garden plants in your landscape, you might find that the plants rot from the inside out because their roots are stuck and holding too much water that should be absorbed in the soil instead. Or they might just grow to a certain point and then stop. Bad soil literally stunts growth.
Make Your Own Garden Soil for Your Raised Beds
Rather than just buying bags of garden soil at Home Depot or your local hardware store, I recommend mixing your own garden soil for raised beds using the following four essential elements.
The Four Essential Elements That Should Make Up Any Garden Soil Mix
We want to recreate the different soil elements that we find occurring naturally but in the right ratios for our edible plants. We need something to give our plants structure, something to hold nutrients in the soil, something to add porosity, and something to add nutrients.
We find that in the following four soil composition elements:
- Organic matter
Here's a little more on each of those elements for those of you who want to get technical.
Clay Gives Structure to Roots of Kitchen Garden Plants
The majority of the places I've lived and gardened in have a native soil that's very heavy in clay—that's how soil gets that vibrant red color.
Clay consists of tiny particles that are packed together in a small space. This density means the permeability of clay soil is not great. As you can probably imagine, there's not a lot of room for pockets of air or water to flow through when you have all these particles squished together. If you've ever tried to garden with clay soil, you probably know how hard it is to dig into the ground, much less pull your shovel back up!
Garden soil needs clay though for this very density. Clay gives a solid and secure structure to soil so that it can hold roots in place and allow them to then grow strong.
You know in action movies when someone tries to grab ahold of the side of a cliff but it keeps breaking away in their hands? Your plants want to grab onto the soil to settle each layer of their roots as they grow down. They need some clay in the soil to give them that structure.
Silt Holds Nutrients in the Soil
I learned about silt when I was studying history and learned how the first civilizations were settled alongside rivers, where the silt beds helped people start to grow their own food and become farmers.
While clay consists of small particles, silt particles are more intermediate. That means silt is not as dense as clay; it has more air between particles, and it also has way more nutrients. The dark, almost-black color of silt is evidence of its nutrients. Silt is filled with vitamins and minerals that have been deposited there by rocks and water.
I often judge the richness of my soil by its color, and if my soil turns lighter brown, that's a sign to me that I don't have enough nutrients in my soil.
Silt is not dense enough to give structure to roots on its own, and it falls apart much faster than clay.
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Sand Adds Porosity and Permeability
Sand, as you can probably guess, has bigger particles than clay and silt; it's also less dense and more permeable, thanks to those large particles spreading out. Adding sand to your kitchen garden soil, therefore, will help achieve that sponge-like quality.
It may be porous, but sand is very low in nutrients. Think about going to the beach. You might see some plants growing in the sand, but they're not going to be vegetables or anything that needs nutrients to support the growth of roots and fruits.
The final element is compost, basically things that have broken down over time. Organic matter comes from dead plants, fallen leaves, even decomposing insects and animals (that's how nature gets lots of nutrients into the soil). As you can imagine, a dead earthworm is full of all the vitamins and minerals that made up the living thing, and those nutrients will go on to support whatever grows in that space.
If you've ever held freshly made compost, you'll know that it's very permeable, full of air pockets, like a big sponge. Unlike sand, it's also full of nutrients. Compost does not, however, provide great structure. It's too loose and liable to fall apart to give plants anything to hold onto.
The Ideal Soil for Raised Beds
When you're creating the ideal raised bed soil blend, the goal is to recreate nature inside of your garden and give plants their optimal growing conditions. That means minimal man-made products. We don't need to trump the system and force a miracle to have a thriving garden. Instead, ask yourself what kind of soil this plant would naturally grow in.
The optimal soil for vegetable gardens is a mixture of the above four elements. Clay is often prevalent in topsoil, either from your own landscape or a topsoil product that you buy locally.
The ideal garden soil blend will be dark in color and have some moisture in it. It won't fall apart in your hand the minute you pick it up; it'll hold together in some shape or form but still have a bit of looseness. Instead of letting water run right through, the ideal soil will hold water just long enough for the plants to take it up.
In my book, Kitchen Garden Revival, I explain my perfect organic sandy loam mix of compost, topsoil, and sand to use in my own raised beds and the ones my kitchen garden design company installs for clients. It's called the 103 blend. It's basically equal parts topsoil, coarse sand, and compost, plus a little secret I save for readers and students. In the picture below, you can see an example of what you could buy at a big box store like Home Depot, though ideally the topsoil and compost would be organic.
How to Maintain Garden Soil Over Time
I wish I could tell you that once you fill your beds with the ideal soil, you're done. But, as I write in my book:
“Even though your 103 blend is 103% perfect and your watering system is on point, there is a keyword that I need you to remember with your kitchen garden, c h a n g e. That's what happens every single day in your kitchen garden. I'm sorry to say that even though your new garden started close to perfect, that doesn't mean it will stay that way.
Every single minute, the soil and the water levels are changing. On the upside, nutrients and moisture are moving to the roots of your plants, but on the downside, there is not an endless supply of these things. You can count on this: your garden will be counting on you for help. This is what makes you a gardener, after all.
I think someone somewhere said that the only thing that's certain is change, and this is certainly true for your kitchen garden. Begin with certainty that the simple soil mix and your preferred watering system are going to work in your garden, but do not assume it's going to stay perfect. Take care of the soil and water, and your garden will, for the most part, take care of your plants."
Eventually, the plants will take all the nutrients from the soil—there is not an endless supply, even if you start with the best soil. That means you need to be adding compost into the mix frequently. Before planting something new, add a 1- to 2-inch layer of fresh compost to renew the nutrient base for your plants.
After a couple years, you might notice that your plants are falling over or not staying strong after they've grown. The main problem there is probably support missing from the soil. It's time to bring in more topsoil or a little bit of extra clay to work into the soil and give plants the structure they need.
If you notice that plants get to a certain point and then their leaves turn yellow or the entire plant seems to be rotting, there's probably not enough permeability in your soil. The roots are sitting in too much water or their growth is being stunted when they're unable to push through soil that's too dense. Add more permeability with sand. This will create those air pockets for roots to push through.
Change is not something to be afraid of. It's a good sign. Your plants are growing and taking nutrients from the soil so that they can feed you and give those nutrients back to you.
As a parent to four teenagers, I sometimes feel like my only job is to order groceries and feed them. Plants are like teenagers. When your plants experience a big growth spurt or do something special like fruiting or flowering, they are taking a lot of food out of the pantry. Your job as the gardener is to pay attention to the soil and make sure the pantry (the soil) is well stocked.
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Organic Garden Soil Gives Back to Us
Listen, soil is so magical. Just think about the compost in a good soil mix. It's organic matter we can reuse, then let the things that it grew break down and become compost for next season. Using compost instead of fertilizers is one of the ways we can change our environment for the better. Did you know compost can actually sequester carbon? It's amazing to think we can do a little good for the world by taking care of our soil.
One of the great things about soil is that it gives back to us. You can never be more generous to soil than it is to you. You can't out-give soil. It gives back to you three to four times the nutrients that you gave it.
Remember that story, The Giving Tree? I think we need a book called The Giving Soil because it is so generous. I am always adding new things to my garden soil, new compost layers, new little treats. I treat it like it's a kid that needs to be fed constantly. The more I view it that way, the more it gives back to me with beautiful, overflowing plants and lots of harvests. I am certain that you will find the same is true for you.
Start with the Best Garden Soil for Vegetables
Prioritize having good kitchen garden soil from the outset, even if that means you have to wait to purchase those perfect trellises. Get the soil right. Don't look for a quick fix like MiracleGro or some synthetic fertilizer. If you want my specific ratio of topsoil, sand, and organic matter to use in your raised bed soil mix, check out my online garden setup course, Kitchen Garden Academy, or grab a copy of my first book, Kitchen Garden Revival.
Use this garden soil calculator to figure out how much soil you need to fill your raised beds and whether bags or a truck delivery is the more economical option for your garden.
And remember, whenever you're feeling discouraged in the garden, when you're not sure what's going on, or if you just feel like this year has been a big disappointment, don't look at the plants. Don't look at your raised beds or your trellises or your design.
Look at your dirt. Pick it up in your hand, feel its texture, try to pick it apart and determine whether it's got clay or sand or silt or compost in it. Decide if it's permeable, if it's got enough nutrients, if it's got enough structure—and then give it what it needs. Listen to your soil. Feed it well, and it will feed your plants, which will feed you back 10 times.
That's the dirt on dirt!
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