Potting Soils: Choose the Right Potting Soil
A good potting soil helps your container vegetable garden thrive!
By Steven Biggs
Choose the Best Potting Mix
Ever had potted plants that just seem to stall?
It could be the soil.
Good potting soil can be the difference between potted crops that grow like gangbusters...and those that don't seem to do anything.
This guide covers key soil ingredients, types of soil mixes, and tips to help you choose good soil. If you're a home gardener who wants to keep things simple but use the best soil mix, keep reading.
Key Takeaways
Potting soil is a soil blend made for the growing conditions in containers.
Despite the name, it often contains no garden soil.
Common potting soil ingredients include peat moss, compost, coir, perlite, and vermiculite.
There are specialty potting soils for specific plants and purposes.
You can buy ready-made potting soil or blend your own.
Understanding Potting Soil
Potting Mix vs. Potting Soil
Potting mix is made for the growing conditions in containers. Many have no soil. This mix has peat moss and perlite.
You might come across a few different names used for potting soil, including potting mix, growing medium (if you're reading technical sources), potting medium, soilless medium, and soilless mix. In British English, you'll also find term "compost" used for potting soil.
In a Nutshell
Potting soil is a soil mix made for plants in containers. Because the roots of plants in containers can't spread out to find water, moisture retention is important.
But even though we need the soil to hold moisture, we also need lots of little air pockets. That's called "aeration." Those little air pockets helps excess water move through the soil, while also leaving space for the soil to hold on to some of the water, until the plant roots can absorb it.
Some potting soil mixes—though not all—contain food for the plants. It could be in the form of a separate fertilizing product. But it might also come from one of the soil ingredients.
In short, a good potting mix retains moisture...yet drains well. It doesn't pack down with repeated watering.
Potting Soil vs. Garden Soil
Soil from the garden is rarely ideal for potted plants. That's because it often packs down with the frequent watering.
Garden soil can also contain weed seeds and diseases.
Key Ingredients in Potting Soil
Ask 10 gardeners for soil-mix recipes and you might come away with 10 different recipes. Like most things in life, there's more than one way to go about it.
But there are some ingredients that are frequently used to make commercial potting mixes. We'll look at some of them below.
Organic Ingredients
By "organic" we just mean something that was once living. Organic ingredients include peat moss, coir, compost, and composted bark chips.
Maybe before we even jump into these organic ingredients, let's not ignore the elephant in the room. There's an environmental footprint to potting soils. Peat moss is extracted from peat bogs, where carbon has been sequestered long-term. When we use peat moss for gardening, that carbon is quickly released back into the environment. Coir is touted an environmentally preferable substitute, though it's from a crop planted where there were once rain forests...and then it's shipped a long ways for a northern gardener like me.
If this environmental footprint is on your mind, here are some thoughts:
Don't waste potting soil—and certainly don't use it like a garden-soil amendment
Use your potting soil for more than a year where appropriate
Consider potting-soil ingredients such as home-made compost and composted forestry waste
Peat Moss
Peat moss is widely used because it holds water really well—like a sponge. Yet it still drains well. Lots of air pockets, yet still moisture that plant roots can take up.
When peat moss gets very dry, it repels water. (It becomes "hydrophobic" if you like the technical jargon.) So if you're mixing your own soil, moisten the peat moss beforehand. (Commercially prepared soil mixes often have an ingredient called a "wetting agent" that makes the peat less hydrophobic.)
Because peat moss is acidic, ground limestone is often added to peat-based potting soils. This helps to balance the pH. (And that's important because if the soil is too acidic, plants might not be able to take up the nutrients they need.)
Coir
Coconut coir is commonly used as an alternative to peat moss. We're talking about the fibre from coconut husks. As well as being used in place of peat moss, some gardeners blend coir with peat moss.
Coir-based soil mixes are sold in bags, as are peat-based soil mixes. But you can also find compressed bricks of coir. They're completely dry and very lightweight. These bricks are soaked in water before making a potting mix.
Compost
Here’s a close up of a potted bulb growing in a soilless coir mix. You can see the long fibres.
The nutrient content of a compost depends on what it's made from. For example, composted animal manures or composted seafood waste contain higher nutrient levels that something such as composted leaves (a.k.a. leaf mould.)
If you’re aiming to make your own peat-free soil mixes, leaf mould (composted leaves) is a traditional ingredient. Leaf mould breaks down more quickly than coir and peat.
Composted Bark Chips
Composted bark chips are sometimes added to soil mixes for larger plants. The bark bulks out the soil mix, while holding moisture.
Worm Castings
Worm castings—worm poo—add nutrients and microbial activity to a soil mix.
Inorganic Ingredients
With inorganic ingredient, we're mainly talking about "aggregates," things that add structure to the potting soil. These include perlite, vermiculite, sand, and grit. Light-weight materials such as perlite and vermiculite are common in commercially prepared soilless potting mixes.
Perlite
This popcorn-like, light, fluffy material is heat-expanded volcanic glass. It adds air pockets to the potting soil.
Vermiculite
Vermiculite is heat-expanded mica. It's like a sponge, able to hold water and nutrients. Because it’s so good at holding moisture, the soil is less likely to shrink when dry.
Sand
Common in home-made soil mixes to improve drainage and to add weight. Not so common in commercial mixes because it adds weight...and that makes shipping more expensive.
Vermiculite helps to retain moisture.
Perlite adds lots of air pockets to a mix.
Types of Potting Soils
Potting soil can be blended for specific types of plants (e.g. cactus), for certain plant needs (e.g. acid-loving plants, or "high porosity" for plants that don't like wet roots), and for specific uses, such as seed-starting.
Here are common types of ready-to-go potting soils:
All-Purpose Potting Mix
This high-porosity potting mix is made from peat and perlite.
As the name suggests, this sort of mix is intended for a wide range of uses and plants. Many commercial all-purpose blends (I've seen the name "general-purpose" used too) contain peat moss or coir, compost, vermiculite, and perlite.
High-Porosity Mixes
High porosity is just another way of saying it drains well.
This sort of mix is for plants that don't do well when the roots remain wet for too long. For example, if you're growing a potted lemon tree, good drainage is very important because the roots can quickly rot if the soil stays wet for too long.
(An interesting aside: I asked at my local garden centre why they're now stocking this sort of mix, which I only used to see in commercial horticulture. I was amused to learn that home cannabis growers favour it!)
"Organic" Potting Mix
It's worth mentioning that some companies market "organic" soils. These will contain the organic portion we talk about above (peat, coir, compost, etc.) meaning something that was once living.
But in this case, the word "organic" has an additional meaning: It means that the mix meets the standards of an organic certification organization. And that usually means that there's no wetting agents (something that makes the peat moss easier to wet) and that if there are fertilizers in the potting mix, they're approved by the certifying agency.
Seed-Starting Mix
The main difference with seed-starting mixes is that the texture is finer. Smaller vermiculite. No coarse bits in the peat moss. Perhaps no perlite. The idea is that a small, germinating seed isn't blocked by a hunk of something in your soil mix.
For what it's worth, I don't buy seedling mixes. They make sense for a commercial grower striving for a very high rate of success and uniformity. In my case, I always have ample seed for my smallish garden, so if the odd seed conks out because I'm using a general purpose mix and it's blocked by a big piece of perlite, it doesn't matter a bit.
Some people use an all-purpose mix, and simply screen out large bits, or break up the large bits while planting.
Homemade Potting Soil
What's the best potting soil? As I mention above, you're likely to find many different recipes for homemade potting mixes. The best mix depends on what you're growing, the growing conditions—and how you water! (I've met gardeners who know they're heavy handed with watering, so they blend especially well draining soilless potting mixes for their plants that don't tolerate wet roots.
If you don't need a lot of soil, you might find it easier just to use an off-the-shelf soil. But if you use quite a bit of soil, or if you have certain requirements, then making your own potting soil allows you to customize the ingredients and match them to the needs of your plants.
General-Purpose Soilless Mix Recipe
When it comes to general-purpose mixes, I keep my life simple, and just use an off-the-shelf product.
If you prefer to mix your own, here's a simple recipe you can start with:
2 parts peat moss (or peat moss substitute such as coir)
1 part vermiculite
1 part perlite
If you're using peat moss, add ground limestone so that the mix is less acidic. Add 30 ml of limestone for a 10 litre pail of soil.
One other tip: If the peat or coir is very dry, wet it first, before mixing with the other ingredients.
And one more tip: Using peat? Get "horticultural" or "blond" peat. Peat at the bottom of a peat bog has decomposed more, so it has shorter fibres. Because of the shorter fibres, it packs down more quickly. The peat that’s higher up in the bog has a lighter colour and longer fibres. It’s called “blond” peat. The blond peat is what you want because it gives a soil with more air pores, but at the same time, it holds water well.
It’s usually the dark peat that I see for sale at garden centres around here—because it’s less expensive and many people don't know the difference.
Container Veg Gardening Course
Soil-based Potting Mix
I use a soil-based potting mix for larger outdoor potted plants. Here's my mix for growing potted fig trees. It's good for all sorts of other potted plants too. With the garden-soil component, this mix holds more moisture. And the garden soil and sand both make it heavier, so that large plants are less likely to topple in the wind.
1 part garden loam
2 parts soilless potting mix (I prefer a commercial-grade of soilless potting mix, see my tips on soil-shopping, below)
1 part sand
Add 30 ml of limestone for a 10 litre pail of the mix
If you want to read more about potting soil for fig trees, here are my recommendations for potting soil for fig trees in pots.
Shopping for Potting Soil
It’s buyer beware when it comes to small, domestic-sized bags of potting mix. Some is great…some is terrible.
A simple approach that I recommend to all my students is to buy potting soil mixed for commercial producers. The quality is consistently good. Which makes sense, because commercial growers know good soil and won't settle for less.
Commercial potting mix is sold in "bales" that are 107 litres (3.8 cubic feet). And the soil within is dry and compressed. So it's a fair bit of soil, but I think it's worth it.
Summary
Good potting mix is a key to success with container gardening. There are many special-purpose mixes available. In many cases, a general purpose mix works quite well. If it's an option, buy a large, commercial-sized bag; the quality is more consistent. When mixing your own potting soil, remember that there are many recipes--and that what's the best for you depends on how you water and what you're growing.
If you’re interested in potting soil because you’re growing vegetables in containers, grab this container veggie guide.
Frequently Asked Questions
Pin this post!
What are the key ingredients in a good potting soil?
A potting soil has an organic component such as peat moss, coir, or compost. This is bulked out with an aggregate such as perlite, vermiculite, or sand.
Can I reuse old potting soil?
Absolutely. The structure of the soil breaks down over time as the organic part (sphagnum peat moss, coir, compost) breaks down. And when it does, the soil will pack down. But you can usually use soil for more than one year. Some gardeners reuse potting soil by refreshing old soil with new soil. (Don't reuse soil that had diseased plants in it, though.)
Does old soil go bad?
Old soil does not go bad. But when peat moss gets very dry, it can be difficult to rewet—it repels moisture. So when using old, very dry soil, allow time for moisture to spread around...and be prepared to get your hands dirty by mixing it.
What is the difference between potting soil and garden soil?
Potting soil is made for plants growing in containers. It drains well. Garden soil is more likely to pack down.
What is a good potting mix for lemon trees?
Root rot is a common problem with potted lemon trees because they don't tolerate wet soil. Look for a “high porosity” type mix. If you're mixing your own potting soil, make it well draining.
Here's a post that helps you know when to water your lemon tree.
What is the best potting soil for fig trees?
It depends on how you water, the size of your fig tree, and the type of container you're using.
Here's a whole article that explains potting soil for fig trees.
Do I need a special soil for wicking beds and sub-irrigated planters (SIPS)?
Use a soil that wicks moisture well. That's because with this type of container there's a reservoir at the base, and water wicks upwards.
If you're buying soil, you won't find "wicking" on the label. If in doubt, remember that the large, commercial bales of potting soil are usually a good bet.
Here's an article that explains sub-irrigated planters and talks about suitable soil.
What about sterile soil?
A heat treatment kills most organisms in the soil: Insects, weed seeds, and diseases.
In most cases, this is not necessary. Healthy soil is living, with microbial processes going on. But when it comes to starting seeds indoors, sterile soil can be beneficial because seedlings are more susceptible to damping off diseases than large plants.
I've heard of people wrapping moist potting soil in foil, and putting it in the oven at 200°F for 20 minutes. Never tried it myself...I know my family wouldn't approve of the smell. However, I have had students tell me they sterilize soil in the BBQ!
Peat-based soilless potting mix is acidic. While not sterile, it's less likely to have disease organisms than something like your home-made compost.
Find This Helpful?
Enjoy not being bombarded by annoying ads?
Appreciate the absence of junky affiliate links for products you don’t need?
It’s because we’re reader supported.
If we’ve helped in your food-gardening journey, we’re glad of support. You can high-five us below. Any amount welcome!
Interested in Adding Worm Castings to Your Soil?
Tune in to this interview to learn about how to use worm castings.
Articles
Growing veg in containers? Find out about vegetable container gardening.
Course on Container Gardening
Here is a self-paced masterclass to help you grow a great container vegetable garden.