What is Soilless Potting Mix?

close-up of a pile of peat moss

Can plants grow without soil? Yes! Indoor plants love soilless potting mixes. If you’re new to container gardening or want to know why you keep seeing “soilless mix” on bags of potting “soil,” we’ve got the dirt on this strange mixture.

What is soilless potting mix?

A soilless potting mix contains primarily organic materials that provide a home in which your plant roots can grow and receive nutrients. The end result? You end up with a healthy, strong plant.

“Organic materials?” you may ask. “I thought soil was an organic material?” Actually, organic materials come from living things, while inorganic materials usually come from the ground (like rocks) and do not consist of plant or animal (living) materials. 

Therefore, soil is primarily inorganic because it is composed of about 90% inorganic materials, such as silt, clay, sand, and gravel. The other 10% or so is organic material, usually in the form of decomposed matter like leaves, feces, and other plant debris. Conversely, soilless potting mixes are primarily composed of organic matter with a few inorganic components, namely perlite or vermiculite.

As you can see, a soilless potting mix will be quite different from the soil outside your front door.

Here’s a quick breakdown of ingredients you may see in store-bought soilless potting media:

Soilless Mixes

Peat moss
✓ Coconut coir (AKA coco peat)
✓ Vermiculite
✓ Perlite
✓ Synthetic fertilizer
✓ Natural fertilizer (such as worm castings)
✓ Wetting agents (to help the peat moss absorb water)

Ingredient List

Whether you’re new to gardening or have been planting for years, you may not know exactly where these components come from. Here’s the inside story on the ingredient list for your favorite soilless potting mix:

Peat moss: Peat moss is a natural product that is extracted from peat bogs (usually from Canada). It holds moisture well but also repels water (hydrophobic). Experts recommend wetting this material on its own before you add in other elements of your mix. (Some peat moss comes with a wetting agent to help it absorb water more easily.) Peat moss is used to make the soilless mix lighter in weight or to increase its water retention ability.

Coco coir: Also called coco peat, coco coir is made from the outer hulls of coconuts (coconut husks). It is usually sold compressed in a “brick” form that you rehydrate in water before adding to your mix. It is often used as a replacement for peat moss in soilless mixes. It holds water and aerates the soil at the same time. Some say it’s more environmentally friendly than peat moss.  

Bark: A by-product of the forest industry, bark adds aeration to soilless mixes. If you buy (or make) a soilless mix specifically for orchids, you’ll notice that it includes more bark than standard soilless mixes to provide the extra aeration orchids require. Large pieces of bark increase aeration but decrease water retention.

Vermiculite: Vermiculite is a silicate mineral that is related to mica. When heated, it expands into an accordion shape and can hold air, water, and nutrients in between these layers.

Perlite: When you see perlite, think “pop rocks” (like the candy), because that is essentially what perlite is. Perlite is formed when volcanic rock is ground and heated to a high temperature. The result is that it expands from four to 20 times its original size. Perlite helps the mix to drain well and helps air to circulate throughout.

Worm castings: Worm castings (AKA worm poop) are used as a fertilizer in some soilless mixes (mostly potting mixes rather than seed starting mixes). These castings add nutrients and good bacteria that strengthen container plants.

Why you need a soilless potting mix

Soilless mixes and topsoil are not interchangeable. Here are a few of the differences between the two:

Soilless Potting MixTopsoil
Designed for container plantsDesigned to enrich in-ground plantings
Lightweight Heavy
Promotes water retention and good drainageToo dense for container plants; promotes root rot
Some mixes are sterile
Higher risk for weed seeds, diseases, or herbicide residue
May include slow-release fertilizerDoesn’t usually include fertilizer

Since water management, aeration, and a disease-free medium are crucial for potting mixes, most potting enthusiasts go with a soilless growing medium to ensure plant health from the get-go. 

Pros and cons of soilless potting mixes

If you’re a novice gardener, take a cue from professional growers: Don’t use regular garden soil to plant indoor plants. Here are a few reasons soilless potting mixes are popular with indoor gardening enthusiasts and gardeners:

Pros of soilless mixes

✓ Most mixes are sterile, reducing the chance for disease
✓ Better drainage than topsoil or garden soil
✓ Good aeration and porosity
✓ Control over ingredients
✓ Customizable for different plant species
✓ Easy DIY project
✓ Affordable to buy if you’re not a DIYer
✓ High success rate for starting seeds and planting house plants

Cons of soilless mixes

X Can take trial and error to land on just the right mix
X Buying individual components can be risky. Purchase from reputable suppliers to avoid diseases.

Make your own soilless potting mix

Recipes for potting mixes are as prevalent as cake recipes: Every gardener has a favorite. Although there are almost as many variations on soilless mixes as there are for the perfect chocolate cake, here’s a simple one from Michigan State University to get you started:

  • One-third coco coir or sphagnum peat moss
  • One-third vermiculite
  • One-third fine compost
  • Add 1-2 cups of worm castings per 5-gallons of mix

Mix well, and start planting.

FAQ about soilless potting mix

1. Should I use plastic or terra cotta pots for my indoor plants?

For indoor plants, most will use plastic pots for a few reasons:
Less frequent watering
Not as heavy
Less expensive
Less likely to crack if you drop them

However, terra cotta (unglazed) pots may be better in some indoor environments:
If you tend to overwater, terra cotta dries the soil more quickly
If you have cacti, succulents, or plants that must always have dry soil

Both have their advantages and disadvantages, so consider these things to select the one that best suits your plants and watering habits.

2. Should I make my own soilless mix or purchase it from a retailer?

Most beginners start with a purchased mix, but as you gain experience, you may want to start making your own. Even if you’re a novice, you may want to buy different mixes for specialty plants.

You can buy special mixes for orchids and cacti, for example. Orchid mixes usually include a higher percentage of bark while cacti mixes include more inorganic ingredients to increase drainage.

3. What is the difference between soilless potting mix and soilless seed starting mix?

According to Michigan State University Extension, soilless seed starting mixes have a finer texture and don’t contain compost or manure like some soilless potting mixes. If you’re buying a soilless mix to start seeds, know that the two are not interchangeable. Go for the soilless seed starting mix (or make your own) for a better chance of success. 

If you’re too busy potting your indoor plants to worry about the lawn, let one of our local lawn care pros take mowing off your to-do list.

Main Photo Credit: Ragesoss | Wikimedia Commons | CC BY-SA 3.0

Sarah Bahr

Sarah is a writer who has previously worked in the lawn care industry. In her spare time, she likes to garden, raise chickens, and mow the grass with her battery-powered lawn mower.