Growth Medium 101: The Best Potting Soil For Indoor Plants - Part 1

If you're new to the indoor gardening world, the concept of a special curated potting mix might seem a little overwhelming. Or, maybe you hear words like "airy, well draining soil" or "houseplant potting mix" and you're not really sure what that means or what best soils for indoor plants look like. Well let's take a deep dive into the world of soil and what soil mix is perfect for you plant and plant parenting style.

Getting Started – Types Of Potting Soil Mixes

Potting soil isn't one size fix all, meaning, you can't just grab dirt from outside or garden soil and use it for your indoor plant. Outdoor soil is typically very dense and made of water retaining materials that will quickly root rot any regular potted indoor plant.

Soil for indoor plants is made from peat moss or coco coir, worm castings, and perlite or vermiculite. Comparing the two soils side by side, you'll notice how much lighter the indoor potting soil will feel compared to an outdoor mix.

If you want to grow big, beautiful indoor plants, getting the right type of potting mix is key. Many common indoor plants, like Monstera, Fiddle leaf fig, and Hoya, require certain amendments for the roots to truly thrive and healthy plants to not succumb to root rot.

Soil for Indoor Plants: Potting Soil Ingredients

Potting soil or potting mix are made out of a number of ingredients. Below we describe a few of these ingredients to make the best potting soil for indoor plants.

Organic potting soil

Organic potting soil, like Espoma organic potting mix, is a great brand of potting soil you can find in many garden centers. However, with most potting mixes, soil straight out of the bag is usually a key for disaster. While these potting mixes are made for indoor plants, they still hold onto a lot of moisture. Even the under-watering plant parent can run into problems with root rot if the soil isn't properly aerated.

Coco coir

Coco coir is a by product of coconut husk and a great soilless mix option! Coco coir is used for many different things like floor mats, brushes, mattresses and a great alternative for peat moss and regular potting soil. Personally, I prefer coco coir over regular potting soil because of fungus gnats. Fungus gnats love damp soil and organic potting soil holds onto water a lot longer than coco coir. If you tend to over water your plants or deal with fungus gnats a lot, consider switching to coco coir. Read more about how to get rid of fungus gnats here. But note, it can become compact over time if you're not regularly watering the plant.

Coco coir is also a great alternative for peat moss. Peat moss is obtained through a very disruptive process from the peat bogs. Peat moss and coco coir have the same components; they both are great with water retention, perfect for seedlings, helps increase air flow in your potting mix, and make a great alternative to soil. However, coconut coir has the added benefit of being the more sustainable option.

What should I add to my potting mix?

As mentioned earlier, high quality soil isn't always enough for our indoor plants. Before adding amendments to your soil mix, there's a few things to consider. First, what type of plant are you potting up? Do you have tropical plants? Or do you need a more succulent mix made from sandy soil for cacti and succulents? Are you a person who tends to over water your plants or under water your plants?

Generally speaking, plants with thicker roots (like Monstera, most Philodendron, and Anthurium), like soil with more additives in it. This allows the root to receive proper air flow which helps with soil drainage and prevents wet feet, or the roots staying wet with too much moisture too long.

It's also important to keep in mind, that if you tend to under water your plants, it's best to not add too many amendments to your soil. The more airy your soil mix is, the quicker it'll dry out, especially if you have drier conditions or if your plants are in terra-cotta. However, if you find yourself on the other end of the spectrum and over water your plants, then adding more potting mix ingredients can save your potted plants. I'd also recommend potting your indoor plants in terra-cotta to help the plant's roots not stay moist.

Aeration And Drainage Materials

Perlite and vermiculite

Perlite and vermiculite are both naturally occurring minerals that are commonly used to add drainage in soil for most indoor plants.

Perlite is a type of volcanic mineral and in its final state, its a light weight white rock. Perlite is a great cost effective way to add some drainage to your soil. It's commonly found in local garden centers, big box stores and easily found online.

Vermiculite is type of mineral that is similar to perlite, but more dense and porous. The biggest difference you'll notice between the two in your soil blends is that perlite will slowly start to float to the top of the soil of your potted plants, leaving the bottom half of the soil compacted and without proper drainage. Vermiculite, on the other hand, will stay in its place and continue to give your plants the drainage it needs. However, vermiculite is harder to find in stores and can become expensive when ordering online. If vermiculite is inaccessible to you, don't hesitate to reach for perlite! Any drainage is better than no drainage. Just make sure to repot your plant once a year and always plant it in a pot with drainage holes.

Orchid bark and fir bark

Orchid bark and fir bark are my favorite additives for my soil blends. They add the perfect amount of drainage, help aid in a healthy root system, and also help with water retention. Both of these barks are found easily at garden centers or even at your local pet store.

In my experience, fir bark is usually found in smaller pieces which is great for smaller potted plants, like african violets. Orchid bark is usually found in bigger pieces which is ideal for bigger plants with a more established root system.

Sphagnum moss

Sphagnum moss is perfect for many tropical plants but specifically ideal for moisture loving plants like Anthurium and Philodendron. Sphagnum moss has great water retention qualities and helps with movement in your own mix. It comes in bigger pieces that I like to cut into pieces before adding to the soil mix. Additionally, sphagnum moss is great for seedlings, moss poles and propagation.

Click here if you want to learn more about propagating indoor plants.

Worm castings

Worm castings is another great additive for any potting soil mixes. Earthworm castings add valuable nutrients to general potting mix or soil in a passive way. Simply add some earthworm castings to your soil mix and over time vital nutrients will slowly be released to your plants. This is perfect for container gardening and can still be used if you use synthetic fertilizers or other organic nutrients.

Epiphyte Mixes

Epiphytes are types of plants that grow on other plants in a non parasitic way. Epiphytes aren't used to growing in heavy, dense soil so when we have them in our house as potted plants, they need a well draining potting mix. It's always fun to experiment with different soil mixes and see what works best for you, but if you need a starting point, here's my go to mix:

Base: Coco coir, fir bark, earthworm castings, and perlite (if I have any extra laying around).

Sometimes, I'll add in horticultural charcoal or sphagnum moss if I have any extra I need to use up. This is my starting potting mix most plants, and I will add more fir bark or perlite to different genuses of plants, if needed. For example, for Hoya I will make sure to add more bark or shredded coconut husks since Hoya roots love to cling to bark and love a particularly chunky mix.

Succulent Mixes

Succulent and cactus soil mixture are a bit different because normal indoor soil is typically too dense for succulent and cacti. When making a succulent mix, buy a soil that speciality made for them. Next, add coarse sand (different than standard play sand), this will add proper aeration to the soil. Lastly, add perlite or pumice. This is a pretty standard method, feel free to experiment to your plant parenting style and the type of cacti you're potting up.

Cacti and succulent garden
Photo by Brandon Cormier on Unsplash