When I first started growing plants in pots I wondered if I really needed to buy “potting soil” (called “potting compost” in the UK), or if I could use something cheaper, like topsoil or regular garden soil.
I read a few articles that said you really should use it, but aside from potting soil being lighter, and therefore the pots easier to move around/less likely to collapse a balcony, I wasn’t really sure what the benefit was.
I did see that potting soil was a lot more expensive than topsoil, which was often on special for $1 a bag. And I suspected that special “potting mix” might be just another unnecessary upgrade designed to part a newbie gardener from the cash in her wallet.
Then I started taking night courses in horticultural science at a local college and learned, to my surprise, that there are actually good science-based reasons to use potting soil in your pots! I still remember the night in class when we covered this material and I finally understood (yes, this was such a momentous occasion that I vividly remember it more than a decade later—some things are just important 😉 ). So if you want to know the science (well, a simplified version of it) keep reading as I love sharing this.
If you just clicked on this article to find out whether you really had to use potting soil for container gardening—the answer is yes—you can head off to the store now while the rest of us talk soil.
The functions of soil
As far as plants are concerned, the soil they grow in needs to fulfill four functions:
- Anchor roots
- Supply water
- Supply oxygen
- Supply nutrients
Because a pot is a completely contained environment, unlike a farmer’s field, the small amount of soil in the pot has to meet all four of these requirements. In a farmer’s field the plant’s roots can spread to get water or get away from too much water, seek out nutrients and oxygen, and anchor themselves sufficiently. In a pot, they’re limited to just the soil you provide. That soil has to hold some water, but not too much, and provide space for air, water and roots to permeate it. It also has to contain nutrients to feed the plant.
Water is the key
In the great garden soil vs. potting soil debate there are two main factors to consider, and they both relate to water:
- Regular garden soil is usually too dense (i.e. too tightly packed together, because it’s made up of a lot of small clay and silt particles) to allow water to drain well within the confines of a container. There’s just not enough room from the top of the pot to the bottom of it for this process to take place.
- The water table (i.e. the point at which soil is saturated with water) in a pot is essentially at the bottom of the pot. In a farmer’s field it’s usually several feet or more deep in the ground (this is the super-abbreviated explanation, but in a field there are different layers of soil, with different textures, and the water table is way down deep). Because of this, the soil at the bottom of a pot can easily become, and stay, saturated if you fill the pot with regular (i.e. dense) garden soil.
Most plants can’t grow if their roots are constantly sitting in water. Therefore, because the soil in a pot is not nearly as deep as a farm field or garden, you need a looser, better draining soil mixture in a pot than in a garden so that the roots have room to grow in soil that’s still adequately moist, but is above the water table.
That’s why you need to use potting soil in a pot.
Oxygen and nutrients are also factors
Supporting reasons include that garden soil is likely to be too dense (i.e. the particles too fine and stuck together) to allow oxygen to get to plant roots. It also may or may not have enough nutrients (especially if it’s very sandy soil or is from an area with nutrient depleted soil), although you can certainly always add fertilizer to a pot later.
Potting soil provides the right structure and nutrients for plants to grow—ideal mixes are usually made up of about 50% solid materials (45% mineral particles like sand, silt and clay and 5% organic matter/compost), 25% water and 25% air (oxygen).
So that’s why you should use potting soil in pots. Potting soil allows your potted plants to access the right amount of moisture, oxygen, nutrients and moisture, while providing a structure in which to anchor their roots.
Woohoo! If you’re as excited about that explanation as I am we should go for coffee, or go shopping for potting soil together—I think we could be friends! Seriously, it’s important stuff.
But I get it if you don’t really want to know all that. I’m here to help you know what to buy so that you can grow nice plants and have a fabulous garden. I appreciate your patience.
A couple other questions I often hear about potting soil are:
Is there a difference between brands/types of potting soil?
Yes. There are different formulations for specific needs—e.g. general container gardening vs. for particular plants like cacti and succulents. For general outdoor use you just want to look for a label that says potting soil or container gardening mix. Some brands have fertilizer added and/or a product that helps retain moisture—I often buy this kind as our summers can be very dry and I can use all the help I can get.
Note that you do get what you pay for in most cases. I’ve had friends buy “potting soil” at the dollar store and, well, it wasn’t as advertised. You don’t have to buy the most expensive kind around, but just make sure it’s of decent quality. If it costs $1 it’s likely not what you’re looking for.
Can you make your own potting soil mix?
Absolutely. If you need a large quantity of potting soil or have concerns about buying bagged soil you can alter garden soil to make its texture coarser and improve its structure. There are plenty of DIY potting soil recipes on the internet, This one looks pretty good.
I hope you found this article helpful. If you did, please leave a comment below or share this article, and your new knowledge, with your friends.