How to Choose the Right Soil for Your Garden

If all those bags at the big box store confuse you, here's how to determine what kind of soil you need for your garden project.

Before I became a gardener, the differences among all the soil mixes eluded me. When I needed to fill up a planter, pot, or patch of dirt, I just chose the most economical bag of whatever from the big box store. You know the aisle—stacks of colorful bags with minimal information on them to help you understand the difference. You could come to understand this information through trial and error, as I did, or you can avoid my mistakes by reading on. 

What is soil, anyway?

What we call soil is really just dirt: It's a combination of organic matter like broken-down leaves, twigs, branches and whatever else has rotted in the space, as well as organisms within the soil itself. Usually the soil contains clay and/or sand as well as rock particles. The best soil for plants will contain nutrients, enough space for plants to easily spread out their roots, and moisture—but not too much moisture. Those nutrients need to be in relationship to each other, or the plants have trouble receiving it. If your soil is too compacted from clay, roots have trouble growing, and the plant can become stunted. If there's too much sand, the soil will not hold onto enough moisture or nutrients. The conditions in a pot or planter are quite different from a raised bed or just rows in the ground. 

Topsoil (the stuff in your yard) probably isn’t great

When I started gardening, I dreamed of simply digging into the dirt in my yard and planting. I had romantic notions of putting a spade into the garden and discovering loamy soil with happy worms. It’s more likely that your yard has poor soil, as a result of both the nature of the local dirt—perhaps it’s too sandy or clay-filled—and not being fed nutrients over the years. Regardless, that dirt from your yard is considered topsoil, and generally speaking, topsoil is not great soil. It has lots of matter not broken down yet, like  leaves and twigs, so it can become waterlogged. Unless the soil was under years of leaves that were allowed to mulch in place, it likely doesn’t have a lot of organic matter to give it nutrients or good soil consistency. Topsoil serves mostly as mass—a simple way to fill up spaces when building. In most circumstances, you’ll need to augment it with nutrients and other matter.  

Find mixes for your garden beds at local rock yards

If you’re building an entirely new garden, you could consider a garden bed mix, which you coudl buy at your big box store. It's not the most economical choice, however. Bags are sold by the cubic foot, and you’ll need to think in cubic yards. Most local rock yards have a three-way or four-way mix, which means it includes topsoil, compost, sand, and other organic materials. You can order it to be delivered, which means you’re just moving it—by wheelbarrow, most likely—from the pile where it’s delivered to your beds.

In most cities, you can purchase expensive garden bed mixes that are organic and considered higher quality. My personal opinion is that they’re never worth it. I’ve never yielded better results from high-end mixes. The soil you get into your garden is never the end result—you’ll be amending your soil every single year, many times a year. This delivery is simply the base to begin with. 

Compost is not soil

As you garden, you will hear continuously how important nutrients are to your soil, and references to compost as a way to achieve some of those nutrients. You might find yourself asking, since it looks like soil, why not just build an entire bed out of compost? The answer lies in understanding what compost is. When organic matter breaks down, whether that’s leaves, wood, plants, kitchen scraps, animals and their waste, or compostable trash, it becomes a nutrient-rich substance that looks a lot like soil. While it is rich in nutrients, it is poor in structure—it's so loamy and full of humus that it needs sand and clay and well. So usually, soil is top-dressed with compost once or twice a year. You just add compost to the top of your beds, and the nutrients will work their way into the soil through watering and rain. Since you lose volume in your beds season to season anyway —due to erosion, compaction and the soil on plants you pull out—this compost helps replace that volume. 

In many cities, you can get compost cheaply or even for free. The city takes the leaves it collects or collected green bins and makes the resulting compost available for residents. You should ask the city if they have such a program. This is the compost I use, exclusively. 

Planters need planting mix

There are different potting mixes for different kinds of plants, from orchids to cacti. For the most part, they address different moisture levels, and usually add a pop of slow-release fertilizer. Most planters have holes in them, or are made of under-fired terracotta, which will leach moisture. To counteract this, potting mixes include vermiculite, perlite, coco coir or peat moss and other organic materials meant to hold onto moisture. (By the way, you should endeavor to avoid peat moss, a diminishing natural resource, in favor of coco coir.) You may notice colored granules in the mix as well—this is likely the slow release fertilizer, which will over time, feed the plant. Since it doesn’t last forever, potting mix needs to be refreshed yearly by mixing it up again, to break up the compaction, and add fertilizer, like Osmocote. 

Seed-starting mix is kind of like a baby blanket

When you start seeds, you want specific conditions. The soil should be super fluffy and airy, to allow roots to flourish. The soil also needs to be fine, so it will fill the cells of a seed-starting tray. Tender little seeds and seedlings can get burned by fertilizer, so seed-starting mix usually lacks any added nutrients. While you might experience success using potting mix instead, seed-starting mix really sets you up for optimal success, and is always bought by the bag. 

Choose the right soil to start

There is no single variable as important in gardening as healthy soil: Soil that has good drainage, the right nutrients, and can hold onto the right amount of moisture. Ideally it is free of weeds, and the kinds of pests that take your plants out, like slugs and snails. Believe it or not, much of this is under your control.