Soil substrates – what is used in a potting mix?

In my recent post, I touched on the potting mix that I make and use for my indoor plants. Knowing about soil substrates and making your own mix is the best way to ensure that your plants get what they need to thrive!

I thought it would be a good idea to work through the list of soil substrates that are most commonly used in potting mixes and what they do for your plants.

Houseplants require a good balance of water, air, structure and food, and the following substrates provide different elements to help with this:

Coconut Coir Peat

Down to earth

Coconut coir is a sustainable substrate made from coconut husk. It’s a great addition to any potting mix because it has good drainage capabilities and absorbs water well. Coconut coir is sold as a solid block which needs to be flushed before use, but it’s easy to do and will last you a long time. Coconut lacks nutrients so your plants will need to fed regularly to make up for this.

Peat moss

Soil mix

Peat moss is made up of decomposed moss from peat bogs that take a long time to break down. Like other soil substrates, peat provides the plant with good drainage and aeration. It is quite acidic, so might need to add other parts to counteract it.


Down to earth

Perlite is made from mined volcanic rock. It’s a very permeable soil substrate and is great for aerating your soil. Happy, healthy roots love perlite! I also use perlite to propagate plant cuttings too!


Vermiculite is similar to perlite in that it traps water, but it actually absorbs water and retains it for longer. It looks pretty shiny and cool too!


Down to earth

Bark is a very useful substrate because it provides plants’ rooting systems with good aeration and drainage. It’s a great addition to any potting mix and is particularly useful if you have a habit of overwatering your plants… (We all have those moments!)


Essential soil

Compost is made up of decomposed organic matter. It’s great to use in any potting mix because it acts as a natural plant fertilizer. You can make your own compost, but I don’t have the space or the time, so I just use a store-bought mix.

Worm castings

Essential soil

Worm castings, aka worm poop, provide plants with good nutrients and microorganisms which help them thrive. Strange thought, but it really works!


Sand is used in to help break up heavy soil. It’s relatively cheap to source and is used in potting mixes for its drainage capabilities. When using sand, it’s good to combine it with perlite because it can prevent aeration.


Charcoal is a new addition to potting mixes, but it is used because it absorbs excess water and helps prevent bacteria, fungus and root rot. I haven’t been able to find it in any stores lately (hence no picture), but I’m hoping to experiment with it more in future.