It’s safe to say that a successful garden begins with good soil. And while you may think that dirt is just dirt, all soils aren’t created equal. What are the differences between the various types of soil and which is the best?
The truth is that there isn’t really one best soil – in fact, your best choice for the job depends on quite a bit on what you plan to do with it.
So what is the difference between potting soil and topsoil? And which should you choose for your project?
What is potting soil?
The name potting soil is a bit misleading – there’s actually no “soil” in the best-quality potting mixes. We call them soil because they’re a growing medium but they’re typically a mix of organic materials like peat moss, pine bark, perlite or vermiculite and other ingredients.
It’s designed to be well-aerated and lightweight, to retain moisture yet still promote drainage – allowing for good root development and helping container plants to thrive.
What is topsoil?
Topsoil is defined as the top layer of the earth’s crust – the natural layer that has formed after millions of years of erosion and rock breakdown. There are no specific ingredients required in order for dirt to be called topsoil.
This soil will typically be rich in nutrients because plant matter has lived and died in it over many years – topsoil in wooded areas will contain leaf matter and decomposed wood pulp whereas the soil from farming fields is turned often and can be exhausted due to repeated plantings.
This is why rotation is an important part of crop farming – it allows the soil to rest between successive plantings as the nutrients are replenished by the remains of each crop.
Topsoils can also contain clay or composted manure as well as less-desirable elements such as weed seeds, fungi or soil-borne bacteria.
Enriched topsoil – what you usually find in garden centers – is a mix of regular local topsoil with organic matter. This improves the soil quality and allows for better plant growth.
What is the difference between garden soil and potting soil?
Garden soil is simply that – soil from the garden. It doesn’t necessarily contain any nutrients and often requires quite a bit of amending before it becomes ideal for planting. This type of soil is heavy and compacts easily after only a few waterings which means that your roots won’t have enough space to spread out and thrive, and moisture can’t penetrate the soil as easily.
Diseases, bacteria and weed seeds can easily hitch a ride with your garden soil, and suddenly you have lots of problems attacking your new plants.
Potting soils, on the other hand, are precisely mixed using strict formulas and recipes. As mentioned above, most potting soils are based on peat moss, with other ingredients added to optimize them for certain uses.
While a seed starter mix is typically very fine and fluffy – allowing fragile, fine roots to spread easily, perennial mixes tend to have larger pieces and to contain more bark pieces.
Some potting soils include vermiculite or perlite. These fluffy featherweight rock pieces help to aerate the soil and control the moisture content. And what’s more: high-quality potting soils are guaranteed to be sterile, meaning that you can be confident that they’ll have no weed seeds or diseases in them.
Can garden soil be used for container gardening?
While it may be tempting to use soil from your existing garden to fill your plant containers, it’s not usually the best idea. Because garden soil tends to be denser than potting soil, it doesn’t drain as well, and it will require the additions of worms or grubs to aerate the dirt. As mentioned earlier, it can also contain weed seeds or fungus spores that can grow in your container and seriously impact the health of your plants.
Though it’s not a good idea to use only garden soil in your containers, you can add a small amount to other components in your container to help the mixture retain moisture and nutrients. If you do, be sure to choose garden soil dug from several inches below the surface.
That’s where you’re more likely to find the cleanest soil available, free of the weed seeds, insects, and other pathogens that reside on the soil surface.
Can potting mix be used as topsoil?
It’s not ideal to use potting soil as a topsoil. While potting soil may not contain weed seeds like topsoil might, it often doesn’t contain any real soil either.
This is often why potting soil is referred to as soil-less. And different potting mixes are designed for different container uses – they are formulated to be lightweight and provide the appropriate nutrient mix for the plants they support. That’s why potting soil is best used for when your plants are still in containers.
When combined with soil outdoors it can cause the soil in your garden to dry out because it can often drain too well. The best choice for traditional gardens is to add 2”-3” of enriched topsoil to your planting beds, mixing it lightly through the soil without disturbing any existing plants or dormant root systems.
This will give your plants a good cover and a transitional layer of soil where organic matter can begin to decompose and nourish their roots. If your chosen topsoil isn’t enriched – if it’s not loamy and doesn’t contain added organic matter – you’ll also want to add some compost to your beds.
Can I make potting soil?
Not only is it possible to make your own potting soil, many gardeners find that it saves a significant amount of money when compared to purchase pre-made potting mix. It will also allow you to specially formulate your soil for your plants.
Though a general mix of 1/3 organic matter, 1/3 vermiculite or perlite and 1/3 peat moss is a fantastic base for most plants, you could create a lighter, fine-textured mix for cuttings or to use as a seed starter. Succulents and cacti prefer a sandy, gravelly soil and potted trees like a coarse sand interspersed with lots of pine bark.
Remember to base your soil choice on the vegetation that you’ll be planting and where it will be. Potting mix is always the best choice for container gardens. But enriching your existing soil with compost, topsoil and fertilizers is the only way to create a traditional garden bed outdoors. The only question now is: what are you planting next?