Houseplants and Potting Soil

Which potting soil should I use? This is a question that many gardeners ask themselves when they start to take an interest in indoor plants. And the answer for the laidback is very simple: potting soil straight out of the houseplant potting soil bag, bought in a hardware store, will be suitable for the vast majority of plants. (And the amateur gardener should start with a common and simple plant – yes, I know that the Venus flytrap is very cool, but wouldn’t it be better to have a plant that survives more than two weeks after it is acquired?)

Then the budding gardener becomes more interested in growing houseplants and wants to move on to more and more complex plants (again, we’re not at the dionaea stage yet, but maybe the lipstick plant or croton?). At this point, he starts to buy plants compulsively, filling every vaguely bright corner of his house with green friends, devouring books and videos on houseplant care, and he asks himself the question: what would be the best potting soil tailored to the needs of each of my plants?

What Is Potting Mix?

In fact, the word potting soil or mix does not refer to a specific composition: you only need to compare several brands of “potting mix for houseplants” to see that they differ considerably from one another. Indeed, a wide variety of ingredients can be used in the composition of potting soils, here are the main ones.

The foundation of potting soil

Coconut fiber

Coconut fiber comes from the outer fibrous husk of coconuts. It is crushed and used as a base for various types of potting soil. It is fairly homogeneous and relatively airy. Its properties with water are unusual: coconut fiber retains water well when it is already wet, but becomes hydrophobic (i.e. it repels water) when it dries out completely. It must then be completely immersed for a few minutes to rehydrate it.

Sphagnum Moss

This green moss must be immersed in water to be moistened; it then retains the water well, while allowing unparalleled aeration of the roots. It is therefore very practical for epiphytic plants, which do not grow in the ground.

Blonde Peat and Brown Peat

Blonde peat comes from the decomposition of sphagnum moss. It is useful because it is very airy, but still retains water well. It decomposes quickly and therefore needs to be renewed. The more advanced the decomposition, the darker the peat; blonde peat eventually becomes brown peat, although it is more of a gradient than a clear division. The darker it is, the lumpier it is. The main advantage of peat is that it is acidic, making it possible to adapt overly alkaline soils for plants that need acidic soil. That being said, few houseplants absolutely need acidic soil.

Draining Elements

Perlite

Perlite is often used in the composition of indoor potting compost. It is white and tends to rise to the surface. It is a light material, derived from volcanic rock. It is mainly used to aerate the potting mix, but it also retains some water: it can therefore be used to help cuttings take root, since it is sterile.

Styrofoam Balls

These balls are similar to perlite and act in essentially the same way, except that they do not retain water. They are used to aerate the soil for plants that do not like perlite. Be careful when using them: they should remain strictly in the inner pots and not be mixed in with the garden, as they are not readily biodegradable.

Vermiculite

Vermiculite comes from heat-expanded mica. It retains water well, while allowing for relative soil aeration. It promotes the absorption of nutrients by plants, but it can be a bit expensive.

Sand and Other Small Stones

Sand is another draining element that reduces water retention in potting soil. There are several types of sand, depending on the fineness of the grain. Heavy by nature, it is especially useful for plants whose restricted root system makes it difficult to anchor them in their pot.

Although sand is fine and mixes well with potting soil, a similar effect can also be obtained with small stones. That said, it is sometimes difficult to obtain uniform potting soil, especially if the stones are a little large.

Bark

More or less crushed pieces of bark can be added to the potting soil to reduce its water retention. Bark is not expensive, but it can tend to deprive the soil of nitrogen as it slowly decomposes. Bark is also used to aerate potting soil, particularly for epiphytic plants that cling to it in nature.

Pumice

Pumice is another light volcanic stone that adds drainage to the soil and does not retain water. It also adds nutrients to the soil, but it tends to be relatively expensive. It is another element that aerates the soil.

Optional Materials

Fertilizer

There are many ways to fertilize houseplants. In most commercial mixes, the soils are amended with slow-release fertilizers, which most of the time look like round “pearls” or perlite, sometimes green in color.

These ingredients are not strictly necessary, but welcome. You can also add compost, homemade or purchased, or vermicompost, made from earthworm droppings, which is also very popular.

Obviously, well-fertilized soil means we don’t have to fertilize every time we water, but we mustn’t forget to renew the soil, because the fertilizer is eventually absorbed over time.

Horticultural Charcoal/Activated Charcoal

These charcoals, which are different from ordinary charcoal, can be added to the soil. They increase drainage and also have fertilizer-binding properties, i.e. it absorbs and slowly releases it. It reduces soil acidity and can help the soil not to subside too much. It is also an element that needs to be replaced frequently. Activated charcoal is often presented as a means of eliminating bacteria and odors, but its effectiveness is controversial.

Mycorrhizal Fungi

It often happens that when you open a bag of potting soil, you see paler spots on the surface that look like insect eggs. Panic! Horror and damnation! In fact, more often than not, these are mycorrhizal fungi for which the gardener has actually paid. These are beneficial fungi that improve the quality of the potting soil.

For more information, read this article.

Fungicides

Several products can be used as fungicides, which are used in case of an accident or as a preventive measure. They prevent the unintentional growth of harmful fungi, especially when potting fragile plants or fresh cuttings.

Diatomaceous Earth

Diatomaceous earth is not necessarily an ingredient that is mixed with the soil, but it can be used to cover it. It has a strong insecticidal effect. It is best to use it only in case of infestation, because diatomaceous earth attacks all insects, even beneficial ones.

Things to Avoid

Black Earth/Black Peat

Black soil is very rarely used for houseplants… and maybe it’s better that way. Real black soil comes from former swamps, it is too expensive; what you find on sale on the market is often black peat, which is low in minerals and excessively acidic.

The Laidback Gardener was also rather critical of it.

Garden Soil

We do not recommend using garden soil for indoor plants. Its composition is too variable to recommend its use. It is difficult to protect oneself from unwanted insects without negatively affecting the microcosm naturally present in the soil by trying to sterilize it. In addition, its composition, which is more or less rich in clay, can asphyxiate plant roots.

Where Can I Buy the Ingredients for My Potting Soil Mix?

Most of the ingredients listed above can be easily found in the houseplant section of hardware stores. Some of the rarer ones are sold in garden centers. If there is an ingredient that you can’t find, you can often substitute it with another one from the same category. Obviously, it is not necessary to use each of these ingredients: personally, I only use three or four and my plants are doing very well.

Mix Your Own Potting Mix

The advantage of mixing your own potting compost is that you can adapt it to the needs of the plant and to your own gardening habits. Start by taking a base of potting compost: for example, coconut fiber or a ready-mixed commercial potting compost.

The next step is to add drainage elements. The proportion of these really depends on the plant… and the diligence of the gardener. For plants that require less water, adding a lot of drainage elements reduces the risk of overwatering (especially for gardeners, like me, who tend to be generous with the watering can). An absent-minded gardener might, however, want soil that drains a little less well to protect his plants from his forgetfulness.

Finally, most of the optional elements are not strictly necessary, but are still welcome, such as a source of fertilizer, an antifungal agent, etc.

My Personal Technique

As a laidback gardener, I don’t go overboard making my own potting soil. I start by buying a commercial potting soil on sale that week at the hardware store (I prefer one that includes mycorrhizae when possible). Then I add more or less drainage material (especially perlite, but sometimes also sand if the potting soil needs to be heavier); the amount of drainage material really depends on the plant. Finally, I add a handful of slow-release fertilizer and sprinkle cinnamon on top, mainly because I think it looks nice and smells good.

Some Basic Recipes

The idea here is not to give you recipes with measurements, rather, the idea is to talk about what should be in your mixture depending on the type of plant you are caring for.

Cactus

Almost all cacti require a very well-drained soil. Additions of bark, perlite and sand are quite normal, giving the soil an uneven appearance. Sand is particularly useful: most cacti appreciate narrow pots and therefore become a little too heavy for their support, which makes them lose their balance. Sand, adding a lot of weight, reduces this risk.

Succulents

This category of plants is vast, so their needs are just as varied. The higher the degree of succulence, the more they can be treated like cacti. When the plants are only “slightly” succulent, the number of drainage elements to be added to the potting soil is also limited.

Commercial potting soils, known as “for cacti and succulents”, often already contain drainage elements… but that’s enough for my taste! However, they are a good base to which we can add our favorite ingredients.

Epiphytes

Epiphytic plants can also greatly benefit from the addition of draining elements, unless they are kept in very narrow pots, as is often the case. Pieces of bark, in particular, which are reminiscent of the natural environment of epiphytic plants, are suitable for use.

Orchids

Orchids, epiphytic plants par excellence, require a mixture composed mainly of pieces of bark, coconut fiber and drainage elements. This allows their roots to breathe properly and stay healthy. We also see orchids, often produced on a large scale, planted in various mosses (especially sphagnum moss): this type of potting soil can increase the risk of root rot, especially if it is packed too tightly in the pots.

Commercial “orchid” potting compost is often suitable for epiphytic plants. However, for my orchids, I prefer to add bark, because I still find them too heavy – when walking in the garden, you often find pieces of dried bark that you can crush by hand or with a mortar.

Araceae

Aroids are generally prone to root rot when kept in soil that remains moist for too long. A little addition of draining elements helps them, especially when gardeners tend to overwater. This is much less necessary for “forgetful” gardeners.

Bromeliads

Like the Araceae, Bromeliads appreciate the addition of draining elements in the soil. Having a relatively small root system, the addition of a little sand helps to keep them in balance.

The Thirsty

Some plants, such as peace lilies (Spathiphyllum), coleus (Coleus blumei) or zebra plants (Aphelandra squarrosa), always seem to be thirsty. They require constantly moist soil, which is sometimes allowed to dry out only very slightly. A coconut fiber-based potting soil is easy to keep moist and still allows the roots to breathe properly.

African Violets

Saintpaulias require a special type of soil: light, airy, but which retains moisture well. This soil is composed of three elements: brown peat (at least 50%), vermiculite and perlite. This is because violets are one of the few plants that require slightly acidic soil.

Other indoor plants that need acidic soil: azaleas, begonias (optional) and blue hydrangeas (which are not… exactly long-lived indoor plants).

Conclusion

In short, it is not difficult to mix your own potting soil and adapt it to each of your houseplants. You just have to take into account the type of plant, its water needs and its other characteristics. Recipes also vary according to the gardener: those who water too much will want to add more draining elements to their soil and vice versa. There are other elements, such as fertilizer or fungicide, which can also be beneficial, but they are not absolutely necessary. Once the plant is growing in the appropriate substrate, it is much easier to care for.