Soil for Houseplants 101: Complete Guide

When I started my journey in growing plants, I had problems understanding two essential concepts: Soil and Lighting. This article will be about the former.

I believe that every beginner will face the same problems as I did in that regard. 

• Is there a difference between potting soil and potting mix?
• What are soil additives or amendments? 
• How do I choose the right potting mix?

All of these questions (and more) are a stepping stone in any beginner’s road. So, I started researching a lot about the subject to understand everything I needed.

Unfortunately, I didn’t find any comprehensive guide that includes the essentials I was looking for. So, it took me a long time before learning everything I wanted.

In this comprehensive guide, I will share with you everything I learned to get you started.

Let’s talk about soil.

For a plant to grow and thrive, it needs a suitable growing medium that will provide three essential components: Water, Nutrients, and Air. 

Consequently, if your chosen medium doesn’t help provide these components, the plant will fail to grow.

Moreover, to pick the best potting mix for your houseplant, you must understand central concepts such as drainage, water retention, and aeration.

However, before we get to those notions, let’s differentiate between the multiple names of growing mediums.

• Potting soil is a growing medium that contains both soil and other ingredients (organic and inorganic) that aim to provide the three components (water, nutrients, and air). It is often confused with potting mix.
• Potting mix is also called Indoor potting mix or container mix. It is a soil-less medium that contains strictly the organic and inorganic ingredients that we will discuss in a moment. Therefore, it is a sterile medium because it doesn’t carry pathogens present in soil (such as fungus).
• Garden soil is a dense soil-based medium to use in garden beds. You should avoid using it in containers since it will compact and cause drainage issues leading to root rot.
• Topsoil is soil taken from the top layer of the earth and isn’t suitable for growing plants in containers.

In short, we use potting soil and potting mix to grow indoor houseplants in containers.

Furthermore, we leave garden soil and topsoil for growing plants outdoor.

The three essential components.

We use potting soil (or potting mix) to provide those three components I have mentioned earlier (water, air, and nutrients).  Let’s delve into those.

Water and the role of drainage

Everyone knows that water is vital for a plant, but too much of it could be harmful.

Most houseplants need to absorb enough water from the growing medium while the excess of it exits. 

If the potting mix (or potting soil) retains too much water, it will get waterlogged.

This occurs when the soil particles are too small and compact when in contact with water.

Consequently, the roots will sit in water leading to fungal diseases causing root rot.

How can you avoid this problem? Well, this is where drainage comes into play! 

So, what is drainage?

Simply put: Drainage is the ability of water to easily flow through the soil. It means that a well-draining soil won’t retain much water and will allow it to exit freely.

However, it doesn’t mean that the soil should dry up too fast. On the contrary, water retention can be a primordial aspect in the case of some plants. Hence, we often add some materials to hold water.

We will see further below how to achieve a balance between drainage and water retention.

Air and the role of aeration

In addition to water, roots need oxygen. Therefore, the potting mix needs to allow water to exit and air to enter. 

There needs to be a perfect balance between water and oxygen for the healthy development of the roots.

If the soil is too compact, the roots will suffocate, and the plant will die.

Nutrients

All living organisms need to eat, and plants are no exception. Therefore, the growing medium must provide nutrients to the plant for its development, and that is what soil is normally there for. Still, you can improve those nutrients through additives and fertilizers.

The macro-nutrients are Nitrogen (N), Phosphorous (P), and Potassium (K).

• Nitrogen (N) is responsible for the growth of leaves.
• Phosphorous (P) plays a role in root growth and flower and fruit development.
• Potassium (K) helps with the overall functioning of the plant.

 Hence, this is why fertilizers are composed of these three elements (NPK).

Read also: 10 Succulents that look like corals

Soil additives

I have mentioned the main components a plant needs to grow. To achieve this purpose, we blend or mix several organic or inorganic amendments and (sometimes) add them to the soil to form an indoor potting soil.

On the other hand, soil additives or amendments are the sole ingredients of soil-less mediums we call potting mix.

Let us now learn what those are and explain their role and, therefore, when you should use them.

Inorganic soil additives

First, I will start with the inorganic amendments.

PERLITE

Perlite is probably the additive you will find most in any indoor potting mix. It’s those very light, white small rock pieces you notice in your mix.

Furthermore, it comes from a heated-up volcanic rock. But, you don’t really need to know more about its origins. The most important to keep in mind is its benefits.

We use this additive to improve aeration in the soil. Due to its porous nature, perlite will enhance the flow of air, drainage (the flow of water). Also, it will prevent soil compaction, which is beneficial to root growth.

Indeed, Its coarse pieces will get in between soil particles creating more space for air and water to circulate.

Although it holds a little bit of water inside of it (because the surface of each piece has small holes that will catch water), perlite is much better at holding oxygen.

So, it is better to use for plants that like a lot of drainage and light soil like cactuses and succulents (such as Tiger Jade). Also, it is ideal for plants that need to dry out between waterings.

Of course, you can add perlite to your soil mix even if it already contains some of it, just to improve the airflow a little bit more.

VERMICULITE

Vermiculite is another mineral used as an amendment. It is brownish, lightweight, softer than Perlite, and shiny.

The main property of this additive is to hold water. Indeed, vermiculite can absorb up to four times its volume in water.

This characteristic makes it an excellent amendment for water retention. It will keep a high moisture level in the soil by releasing the water retained when the plant feels thirsty (perfect for a plant such as Pilea Depressa).

Also, it is often used for seed-starting and propagation. Moreover, it has the side effect of helping ward off any fungus when seed-starting.

In addition to its water retention attribute, it also helps with aeration and prevents soil compaction, just like perlite.

Still, Perlite is better for aeration than vermiculite, while vermiculite is better in water retention than perlite.

In short, you will add vermiculite for plants that need a moist (but not too soggy) growing medium.

PUMICE

Pumice comes from a volcanic rock as well, is pale grey or pale yellow, and contains bubbles of air that make it lightweight. However, unlike perlite, it doesn’t get processed before use.

Aside from that tiny difference, pumice and perlite are basically interchangeable. You would use it to improve aeration, drainage, and prevent soil compaction. Moreover, its porosity allows microbial life to thrive while maintaining soil structure better than perlite.

Although it has some absorbing attributes and will absorb spilled oil or rainwater, pumice is primarily used for aeration and drainage.

Also, because pumice is heavier than perlite, it is better used in the case of tall plants to prevent the pot from toppling.

Furthermore, it lasts longer than perlite and hence is a better choice. But, it may be harder to find in stores and is more expensive.

Pro tip: You should wear a mask while dealing with perlite, pumice, or vermiculite to avoid breathing in their dust which could be harmful to your health.

SAND

For succulents and cactuses, sand is a common soil additive. It mimics the growing environment of those plants and improves aeration and drainage.

Succulents love to dry out between waterings. They basically soak up as much water as they can and then prefer a dry environment until the next supply of water. Consequently, you will find sand in the majority of succulent soil mixtures.

However, it may be hard to deal with it for two reasons.

First, you must add it in the right proportions, or else you’ll risk compaction.

Second, you cannot use just any type of sand (like play sand) for the same reason.

CHARCOAL

Horticultural charcoal or Biochar is made of carbonated organic wastes such as tree trimmings, scrap wood, and plant material left from agricultural harvests.

It is an additive that improves aeration and drainage but also reduces water in the soil. Also, it lowers soil acidity and contains many nutrients found in the organic material from which it was made.

I chose to classify it as inorganic even though it is made of burned organic matter because it is mostly carbon.

LECA

LECA is the acronym that stands for Lightweight Expanded Clay Aggregate. It is nothing more than heated clay balls that expand as they absorb water.

Mostly used in hydroponic gardening (that is growing plants without soil), LECA is great for water retention.

Then again, it doesn’t provide any nutrients so you’ll either have to mix it with soil or add another source of nutrients such as fertilizers.

Furthermore, It also improves drainage and aeration when mixed with soil.

In addition, being a sterile medium, LECA reduces the risk of pests. Be that as it may, a major con is that it’s an expensive medium.

Organic soil additives

Let us now discuss the organic amendments. Here we go!

SPHAGNUM MOSS

Sphagnum moss is a plant that grows in bogs. It is sold screened and dried for horticultural use.

The main attribute of this organic additive is water retention. So, you will find that in most cases, it is used to help start seeds or cuttings.

Although it is a common amendment, sphagnum moss has two disadvantages. First, it retains water for too long, which in turn keeps the soil moist. Second, it could dry up the soil around the root system.

Furthermore, sphagnum moss doesn’t contain nutrients.

PEAT MOSS

Also known as sphagnum peat moss, it is sometimes confused with sphagnum moss. In fact, the two amendments are different parts of the same plant.

While sphagnum moss grows above the surface and is sustainable, peat moss grows below the surface and is not sustainable. Indeed, it takes thousands of years for it to grow back.

Peat moss helps with water retention in sandy soil, drainage in clay soil, aeration, and prevents soil compaction. Hence, it is a handy amendment.

However, since peat moss is not sustainable, it would be better to replace it with the following amendment that serves the same purpose.

Also, peat moss is acidic and consequently increases soil acidity (we’ll talk about soil pH in a moment).

COCO COIR

Coco coir is a fiber that comes from the husks of coconuts. It is an inert matter that serves as a soil amendment. Normally, it comes in cubes that you have to hydrate before use.

Often used as a replacement for peat moss, it helps with water retention, drainage, and aeration. On the other hand, it doesn’t provide any nutrients, unlike many other organic amendments.

RICE HULLS

Rice hulls can also replace peat moss or coco coir. It is a byproduct of agriculture (just like coco coir) that would otherwise be considered waste. Therefore, they are more sustainable than either.

These hulls are the husks removed from each grain of rice after harvest. They are then parboiled at high heat to sterilize them.

You would use rice hulls to increase water retention, drainage, and aeration. But, they are a little bit less effective than peat moss or coco coir. Furthermore, they are biodegradable, so they feed the soil as they break down.

Also, it is neutral pH and lasts a whole growing season.

COMPOST

Compost is an organic amendment made of both decomposed animal and vegetal matter.

Elements such as decomposing plants (dried leaves), food waste, manure, and other nitrogen-rich materials make up this soil additive.

Unlike additives we have seen so far, compost’s primary purpose is to provide nutrients. Although it does hold some water (like peat moss, for example), compost serves mainly as a food provider.

Still, with all its benefits, compost has the drawback of attracting pests and sheltering fungus. Furthermore, its smell may be unbearable for some.

ORCHID BARK

Orchid bark is dried-up tree bark that comes in chunks. You would use this soil additive to improve drainage, aeration and prevent soil compaction.

Similar to compost, orchid bark has the downside of attracting pests and sheltering fungus. However, when it decomposes, it provides nutrients to the growing medium.

Note that it usually comes in different sizes and that the bigger the size, the longer it takes for it to decompose.

Also, orchid bark can dry up the soil too fast hence may not be adequate for many plants. Still, some plants (such as Aroids or other epiphytic plants) will benefit immensely from this organic additive.

EARTHWORM CASTINGS

Earthworm castings are worm droppings (poop.. yeah). They are perfect for pumping nutrients into your soil.

Moreover, it could be a good replacement for compost. Besides that, there isn’t much we could say about this additive.

So, if you want to naturally fertilize, earthworm castings are the way to go.

That was a long list, wasn’t it? Fortunately, you won’t be needing any more than that to start.

Now that you have a general idea of soil additives and their roles in any potting mix, I will explain two other notions that you should be familiar with.

Fertilizers

Sometimes, you will have to deal with the depletion of soil nutrients. Other times, the potting mix won’t provide enough nutrients for your plant to grow.

So, both times, you will need something more to feed your green friends. This is where fertilizers come into play.

Fertilizers contain macro and micronutrients for a plant’s healthy growth. I have talked before about the macronutrients (NPK), while micronutrients are Zinc, Iron, and Boron to name a few.

Also, there are several types of fertilizers.

First, you have those made of naturally derived sources such as kelp or tea compost.

Second, you have the synthetic ones (chemical). Most people prefer the former for obvious reasons.

Another distinction is between liquid, granular, and slow-release fertilizers. Since this post isn’t about fertilizers, I won’t get more into the subject for now.

More importantly, note that fertilizers come in handy to compensate for the lack of nutrients, especially in the growing seasons.

Soil acidity

I have just explained that we use fertilizers in case of nutrient depletion. One way this depletion would occur is in too acidic or too alkaline soil.

Indeed, extreme pH values reduce the availability of some nutrients to the plant. Whereas low pH decreases the availability of macro and secondary nutrients, high pH reduces the availability of most micronutrients.

Also, some additives such as peat moss are acidic and contribute to lowering the pH of soil.

Therefore, you might face the necessity to adjust the pH levels of your soil. This can be done either by adding lime or dolomite to raise the pH or adding sulfur or aluminum sulfate to lower it.

Cheat sheet

Here is a table that will serve as a cheat sheet. You can use it to decide which amendment to add to your potting soil (or potting mix).

FAQ

Conclusion

To sum up, whether you decide to buy a potting mix/soil or to make your own, it is necessary to know what soil additives are and what they are used for.

Moreover, you will sometimes have to improve your growing medium by adding the appropriate amendments.

Inorganic additives are generally there to improve drainage, aeration and prevent compaction.

On the other hand, organic additives tend to be there to improve water retention and provide nutrients. Note that compost and earthworm castings are by far the most potent when it comes to nutrients.

Remember that the potting mix you choose will vary depending on the plant you’re growing. It is therefore essential to understand the needs of your houseplant to meet them.

One last piece of advice I would give you is to only buy brand-name potting mix since low quality could harm your plants. Also, you should keep the bag tightly closed and kept in a dry and cool place to avoid attracting insects.

So, there it is, the complete guide on potting soil for beginners. I hope I have helped you understand the basics. Please don’t hesitate if you notice anything missing. I would be happy to enrich it more.

Here is the second article on the basics series on how to choose a pot for your houseplant.