Houseplant Soil Ingredients 101

This article shares a breakdown of the different houseplant soil ingredients you might encounter, including what each of them might be used for.

Houseplant soil 101—let’s talk ingredients!

Hey all, this is going to be a monster of an article. And it will be all about houseplant soil! How long can I talk about soil for? A long time. But I promise I’ll try to keep it to the basics you need to know.

One of the things that overwhelmed me when I started learning about plants and gardening was all of the different types of soil. Did I need to add perlite to my soil? Was is moss good for? Can you use garden soil for houseplants? Why shouldn’t I use peat moss? Oh God, there’s bark, too? And what the hell is vermiculite?

Often store bought indoor potting soils are great right out of the bag. But sometimes you might want to switch things up for certain plants. Or you might just want to know what is in the bag you’re using! I’ll break down some of the most common things I use for houseplant soil and what they are good for.

Outdoor soil vs. indoor soil

First let’s start with the basics. I’m going to separate soil into two groups: outdoor soil and indoor soil. Yes, that is a gross generalization and yes, I know it is a spectrum. But this will help us cover the basics.

Outdoor soil is, for lack of a better word, dense. It will often be labeled as “garden soil.” Garden soil is often designed to be mixed with soil in the ground to improve it. Outdoor soil can also work as indoor soil if it’s designed for outdoor container gardening—but not always.

That’s why I generally buy a potting mix labeled “indoor potting soil” or “well-draining houseplant soil.” Look for those words. These are usually good to go out of the bag for houseplants because they have additives to help prevent soil compaction and facilitate aeration and drainage. 

But what makes outdoor/garden soil and indoor/container/houseplant soil different? The ingredients and the additives mixed in. Outdoor/garden soil is way too heavy for houseplants. It retains too much moisture and is very dense. This can prevent oxygen flow and lead to root rot, among other issues.

Why is indoor or container soil important?

Indoor plants generally take much longer to dry out between waterings and require more air flow to their roots. Therefore, indoor soil mixtures designed for houseplants have certain additives in them that help address these needs. You will start seeing words like “sphagnum moss” and “perlite” mentioned when reading plant care guides and while browsing on houseplant groups.

To help keep costs down for my spring houseplant repotting this year, I bought a big bag of soil designed for container gardening. I chose a high-quality soil from a local nursery. It is actually a super light blend out of the bag with plenty of perlite already mixed in.

However, for some of my plants, I added additional perlite and scoops of coco coir, worm castings, and occasionally orchid bark. If this sounds overwhelming—it isn’t. They are all just additives that help to facilitate things like drainage and air flow.

Big disclaimer here: I know houseplant people can have a lot of opinions on things. This is just what I use and what I’ve found works. Experimentation is great. If I’ve omitted something you love, it just means I haven’t personally used it so I can’t speak to it. No shade. Now let’s walk through each soil additive.

1. Sphagnum moss

Sphagnum is a genus of mosses with hundreds of species. But I’m not going to get too far into sphagnum as a genus because as a soil additive, you’re dealing with the dead kind. 🙂 Alive, it grows in wet climates like the surface of swamps or bogs.

It can be sustainably harvested because it will regrow without problems. Dried, it makes a wonderful additive or substrate for growing plants because it is excellent for lightweight moisture retention. I have a whole tutorial on how to root plants in sphagnum moss.

Sphagnum moss vs. peat moss

But what about peat moss? Is it the same or different? Well, it’s actually from the same plant, it’s just a different part. It is the layer of decaying, water-saturated sphagnum below the surface. It has a finer texture. However, despite its popularity, harvesting it isn’t sustainable.

That’s because you have to dig up the swamp to get to it, and it can take hundreds of years for the habitat to regrow, regenerating at the agonizingly slow rate of 1 mm per year. I didn’t realize peat moss was environmentally problematic until a couple of years ago, and I am trying to move away from using it, especially as an additive. 

But it can be hard to find mixed that don’t include peat moss. So why is it so popular? Well, it retains a lot of moisture while remaining light. That means that it helps facilitate good drainage and is great for plants that like damp but not soggy soil.

Peat moss is also physically light and a bit fluffy when it is dry. It helps prevent soil compaction and encourages good aeration and air flow to a potted plant’s roots. So it’s kind of a bummer to have to stop using it, right? Well, not necessarily! Let me tell you about its cooler and more sustainable friend: coco coir. 

2. Coco coir

Coco coir (affiliate link) is fiber from the outer husk of coconuts. It’s what your doormat is probably made out of! It has tons of uses and is sustainable because it comes from coconuts. Which don’t take hundreds of years to regenerate like peat moss does.

Coco coir is a good alternative to peat moss because it provides many of the same benefits. It’s lightweight, even used in upholstery padding and mattresses. Because of this, it also increases air flow in the soil. Also like peat moss, coco coir retains water without becoming too heavy.

As a soil additive, it generally is harvested, dried, and packed into bricks. You can then crumble the coco coir off of the bricks and mix it in with your soil. So you get a lot for your dollar, too. In my entire history of caring for houseplants, I have ordered only two bricks. A little goes a long way!

Bonus points for you if you can find an indoor potting soil that uses coco coir instead of peat moss. I know most of the pre-mixed soils I see around where I live use peat moss. (Which is fine for your plants! Just iffy on the sustainability issues.)

3. Perlite

Perlite is probably already in almost any indoor potting soil mix you’ll buy. It’s the little white bits that look like rocks but are really lightweight. And that’s why perlite is so fab!

Perlite itself is an amorphous volcanic glass that occurs naturally, which sounds very much like something you wouldn’t put in soil. But it is! And it’s great. It’s also used for a ton of things including construction, biotechnology, and beer making. 

Perlite’s lightweight nature helps prevent soil compaction and generally loosens things up. It also has low water retention, helping water flow freely through soil and preventing overwatering. I like mixing perlite and damp sphagnum moss to root cuttings in my DIY propagation box. Perlite is also excellent for succulents and cacti.

4. Vermiculite

Vermiculite is a hydrous phyllosilicate mineral. Basically it’s a naturally occurring mineral used as a moisture-retentive medium for growing plants. Like peat moss and coco coir, it retains water without being too heavy. This is a great option for plants that like to be moist but not sopping wet.

I do want to flag that some people have some health and safety concerns over vermiculite, but it’s generally not an issue at all as a soil amendment.

5. Orchid bard

Orchid bark (affiliate link) isn’t just for orchids—though orchids do love it! It’s just tree bark that comes in dried shavings and chunks. It’s a nice soil amendment for many plants.

Orchid bark makes soil very chunky, creating larger pockets of air in the soil and helping with aeration, soil compaction, and water flow. I personally do not use bark often, but I do throw some into my bigger potted plants to help with aeration.

Aroids (plants in the araceae family) generally like it. When I recently repotted my monstera deliciosa with a jute pole, I threw in a couple handfuls of orchid bark. I find that using too much orchid bark can make the soil dry out too fast.

When I repotted my heart-leaf philodendrons this spring, I included orchid bark. That was a mistake. They dried out too fast and actually ended up looking rather sad. Once I repotted with fresh soil and no orchid bark added in, they perked right up.

6. Sand

Sand is another soil additive that is commonly found in succulent soil mixtures. It’s great for cactuses, succulents, and some other drought-tolerant plants that do well in hardy soils like hoyas. (See my article on how to make your own succulent soil.)

Sand helps with aeration and drainage, and it helps mimic the natural desert environment and its sandy soil. However, make sure to avoid things like play sand, which can actually compact in the soil and defeat the purpose. Look for something like horticultural sand or horticultural grit when shopping for it.

7. Worm castings

Worm castings (affiliate link) are worm poop. Yum. Here’s how it works: Worms eat waste, worms digest and poop out glorious castings that are chocked full of good bacteria and nutrients. You can buy it by the bag if you don’t want to set up your own worm tower.

I have used worm castings for my houseplants as an alternative to fertilizer. I just throw some in to my soil mixture before I pot the plant up. I don’t measure. If I am not repotting, something I just mix some fresh worm castings into the top few inches of the soil with a fork.

Diluted houseplant fertilizers are pretty easy to use, but I don’t like having to worry about. Worm castings also won’t burn your plant’s roots and lead to damage like some fertilizers can. And the good stuff they bring with them helps the soil too, not just the plant.

Soil issues you might encounter

I’ve gone over 5 of my favorite soil additives to create the best soil for houseplants, including what they are and why I like to use them. Now I want to touch on a few other houseplant soil issues that I have experiences.

White fungus in houseplant soil

If you notice a white powdery looking mold growing along the soil line of your plant, it’s almost certainly saprophytic fungus. This will not harm the plant, but it’s possible that the underlying cause of the white fungus could be problematic.

Saprophytic fungus feeds on decomposing stuff in damp soil. There is so much stuff living in your soil that it’s entirely possible the fungus exists but only shows itself when you overwater your plant. That’s because saprophytic fungus feeds only on wet soil.

If you’d like to get rid of it, simply scrap it off the top of the soil and aerate the top layer of soil a bit using a fork or chopsticks. This will help increase air flow. Let the soil dry out before watering it again if you can. If the problem is persistent, repot the plant using fresh, high-quality soil. Clean the pot thoroughly before repotting, too. (That’s always a good rule though!)

Mushrooms

Occasionally you might even get cute little mushrooms growing in your houseplant soil. It might be startling the first time you see it, but they probably aren’t harmful. I saw a bunch of little cuties in some rhaphidophora hayi plants at Lowe’s recently (see picture below).

Simply pluck the mushrooms out and throw them away. DON’T EAT THEM! Since the mushrooms need damper-than-normal conditions to sprout, aerate the top of the soil using a fork or something similar. Then let it dry out. Consider reassessing your watering routine to ensure you don’t start a mushroom village again. It’s usually a sign that the soil is remaining too damp.

Yellow mold

Yellow mold bothers me way more than the white mold does. This kind of mold has kind of a slimy look to it with small bright yellow “balls” in it. It’s also a kind of fungus. While it looks gross, it isn’t harmful to the plant.

However, much like the white fungus, it could indicate you are overwatering or not letting the soil dry out enough between waterings. I have experienced this type of yellow mold in some of my lower light plants, specifically a snake plant I had in my daughter’s room.

I simply scooped the spot out, trashed it, aerated the top layer of the soil with a fork, and let the soil dry out. When I eventually repotted the plant, I threw the old soil out and repotted with fresh soil.

In conclusion…

The key to thriving houseplants lies in understanding and choosing the right soil. By incorporating the right additives like sphagnum moss, coco coir, and perlite, you can create the most ideal environment for your plants. It’s also important to recognize and address common soil issues such as mold and mushroom growth.

Do you have any soil tips or experiences to share? Drop them in the comments below! Keep experimenting and happy planting 🙂

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