Houseplant 101: The Indoor Gardener's Guide to Soil
Now that we have talked about lighting, in our Ultimate Lighting Guide, watering, in Plant Watering Explained, and fertilizing, in How to Feed Your Plants, the final big piece of the houseplant puzzle is the potting medium, often referred to as soil. Even though it is one of the last things we are covering for general indoor plant care, it is certainly not the least! It is not one soil that suits all when it comes to choosing what to grow our houseplants in as not all soil is good soil and not every plant enjoys the same soil.
Understanding your plants and creating an environment in which they can thrive is essential, dare we say crucial, to their long-term health, including selecting the correct soil in which they are planted. Our plants need the appropriate mediums with the right qualities - this means selecting the soil that will benefit your plant and its roots the most. Your general goals with soil are that we want water to move through the medium quickly, leaving water absorbed by the media, while at the same time leaving air pockets for oxygen supply to the roots. But let's get into what soil is and how to choose the best type for your plant babies!
What is Soil and What is its Purpose?
We're going to let you in on a dirty little secret: houseplant soil is not true soil. It might look like the stereotypical brown "dirt" but it is something else entirely. The thing you think of when you hear the word soil for plants is probably topsoil, a blend that includes dirt, is very dense, is designed for garden beds, compacts easily, and holds lots of moisture, easily becoming waterlogged - NOT meant for indoor plants. However, the media in which houseplants are potted is actually known as a "potting mix" or "artificial potting media," a lightweight mix without any actual dirt in it designed for plants that live in containers.
To make it easier, please note that whenever we use the term soil throughout our content, we are referring to the mixes that are best for houseplants, it still doesn't mean true dirt but it is terminology that everyone understands.
The Cornell Mix formula, founded at Cornell University in the '60s, is the basis of a wide variety of indoor potting mediums today. Developed by Professor James Boodley and scientist Raymond Sheldrake, the artificial soil was made up of peat moss and perlite (which we will talk more about later on) along with a few other ingredients. Nowadays, you can amend this lightweight mixture as you wish, but they all essentially provide good drainage, are free of weeds and disease, promote faster root growth, give quick anchorage to roots and reduce disease problems for growers.
We know that the majority of plants need a medium in which to live, not only is it where their roots exist and create structure, but it also is where they get their moisture from, especially houseplants! It provides a home base and can be tailored to the type of plant by adding other amendments, if necessary. Our plants need appropriate mediums with the proper qualities to encourage healthy growth and these qualities include:
All-in-all, plant roots need water. However, they need whatever potting medium they are in to hold the correct amount of water, not too much and not too little. The ideal media will drain well but hold adequate moisture.
Just like plants need enough water, they also need airflow, where there is sufficient gas exchange for the roots to breathe. This also ensures the soil doesn't stay wet for extended periods of time.
Plants extract their nutrients from the soil in which they live, therefore sustaining their growth. Knowing this, it is important that the potting medium that they exist in is balanced and full of healthy nutrients, which can come both in the form of fertilizers and the proper potting mixture.
Finally, plants also need ample support in their potting medium so that their roots can achieve the proper anchoring, providing a stable foundation on which the plant can rely.
Some people like to mix their own potting soil to ensure that the mixture is light enough and provides adequate space for air, water, and healthy root growth, especially for those plants that cannot handle compact soil.
So, what is the issue with using outdoor soil or soil that is meant for gardening on your indoor plants when your outdoor plants do just fine in it? Well, let's check out the following reasons:
The primary problem with using real soil, outdoor or purchased from your local garden centre, is because it is much too dense since it is composed of clay, sand, and silt. Prone to compacting and hardening when dry, this makes it virtually impossible for plant roots to spread and blocks the soil from absorbing water.
There is a high risk of bringing in soil-borne diseases if you pot your plants in soil that was outside. This, mixed with most likely weed-infested soil can cause a host of problems for your plants.
Finally, garden soil generally contains fungal spores and bugs that can thrive indoors in containers as well as outdoors. They will often infiltrate your other plants and most are herbivores, meaning they will consume the greenery and roots of your beautiful plants.
Although the potting mix that you choose, or in which your plants are planted, is not true soil, it is still the foundation of plant growth and is critical to the overall health of your plants. You cannot talk about plant care and never consider the medium from which plants receive their nutrients, which is why it is just as important as getting the lighting correct, providing proper watering measures, and fertilizing appropriately. Next up: a breakdown of the potting mix components.
Even though plants should be repotted at points throughout their life, it is best to not do it too soon. On the flip side, if your plant is left in the same soil for years and years, many of the nutrients and natural microbes that exist will be gone, leaving old, stale soil. Therefore, if you aren't going to repot, it is wise to refresh the top couple of inches every couple of years, and make sure you are fertilizing appropriately!
The Breakdown of Potting Media
A decent indoor potting mix is generally composed of two main ingredients: peat moss and perlite. Most pre-mixed tropical soils absorb moisture, resist compaction, and tend to dry out rather quickly (so that the soil doesn't stay wet for long periods). Sometimes, certain brands can contain additional nutrients, or different plants you purchase from a greenhouse may come with slow-release fertilizer pellets already blended in with the soil (ever notice any blue, teal, or grey balls in your plants?).
If you purchase a mixed bag of soil, you can rest easy knowing that it is sterile, absent of any disease or pests. If you are using old soil, make sure to sterilize it first or throw it away if you know there have been pests or root rot that have occurred in it.
Let's go over what peat moss and perlite actually are:
Over thousands of years, organic matter submerged under the water in bogs has broken down to form a type of soil, known as 'peat'. This peat is harvested after the bog has dried out temporarily, it is then dried further, screened, and compressed into bales of peat moss, or used as the basis of most potting mixtures. It is spongy, retains water well, provides a great basis for potting mixtures, and is quite affordable.
Potting Tip: Peat moss can sometimes resist the initial wetting but if you dampen it before using it, water will absorb just fine.
Cons - Dry peat moss will shrink inward and pull away from the edges of the container it is in, compacting so that any water poured on it will run off and not absorb - you should consider using an aerating tool or bottom watering at this point, or, worst case, repot into fresh soil.
This is the most common amendment used in the houseplant world. Those styrofoam-like balls that you see in the soil are actually white, lightweight pebbles with micro-pores formed from superheated volcanic glass. It is a great option for those that use hydroponics, or for rooting cuttings. It is sterile, with a neutral pH, aiding in aeration, water control, and keeping the soil loose. Since it decomposes very slowly, it can be reused and is non-toxic.
Cons - It doesn't hold water very tightly and is very lightweight, so it can float to the top when watered or blow away in the wind. Be sure to avoid exposure to any dust that perlite produces as it can be an irritant.
Besides the primary parts, there are plenty of additional amendments that can be mixed in with soil to enrich it, depending on what you wish the soil to achieve or what your plant needs!
For absorbency and structure, the following amendments are recommended:
A natural fibre extracted from the outer husk of coconuts, coconut coir has become very popular in recent years. With even greater water-retaining capabilities than peat moss, it has a close to neutral pH, making it less acidic but similarly lightweight.
Cons - Infertile and lacks a microbial base, not adding any beneficial organisms to your soil. It is also high in potassium (which can interfere with calcium uptake in some plants) and may contain salt due to its harvesting environment (wash before use).
Often used on its own, or mixed with orchid bark, to root cuttings and to house epiphytes, sphagnum moss is a slightly acidic, natural plant material that is harvested live and then dried. It offers structure and aeration while also holding lots of moisture and remaining quite light.
Cons - It is not overly fertile and does decay slowly over time, compacting as it does so.
Along with moisture retention and support, aeration and drainage are also necessary, the following amendments, for which, are recommended (excluding perlite since that was discussed above):
A light, naturally occurring mineral, vermiculite looks like tiny, accordion-shaped pellets and is composed of multiple layers of shiny plates. Including it in potting mixes creates air channels and it is quite water-absorbent with a neutral pH, great for those thirsty plants! It provides a slow leak of micronutrients and places for fungi/microbes to aid in the plant's growth.
Cons - Since it is more moisture-retentive than perlite, it can keep the soil wet for too long.
Organic matter that can provide a slow release of nutrients as it decays, wood chips also improve the drainage, aeration, and absorption capacity of soil.
Cons - Due to its organic nature, it decomposes rapidly and uses up soil nitrogen in the process (pine is more desirable than hardwoods as it takes away less nitrogen)
Often used in place of wood chips, bark is a preferable amendment because it is not as absorbent as the chips, providing aeration but repelling water (bark contains a wax that repels water to protect its tree). It also doesn't break down as quickly as the chips!
Cons - This is a great mix to use for rooting cuttings but it does use nitrogen as it decays, adding acidity to the soil.
A porous volcanic leftover, pumice is known as a natural soil conditioner, maintaining soil stability as well as aeration and drainage. It is heavy enough not to float and doesn't compact or decompose over time. Fertilizer and water can adhere to its irregular pores, storing them as the plant needs them.
Cons - Since it has a neutral pH, it adds no nutrition to the soil mix.
Known as a traditional modification for breaking up heavy soils, sand is most common in succulent mixes (as long as it is coarse). It aids in water drainage, is inexpensive, reusable, and has zero absorption capacity.
Cons - If you use the wrong type of sand, such as those with small particles, this will compact and reduce aeration, rather than increasing it.
Generally placed in the bottom of pots, or mixed in with the soil, they can provide some extra drainage. The excellent porosity allows for absorption, retention, and slow release of water and fertilizers, increasing the nutritional capacity of the soil. They can also help reduce odours!
Cons - Despite the fact that they can provide aeration, it is quite limited and you don't want to add too much charcoal to your potting mix.
Tiny stones are a cheap filler found in not-so-great mixes. They are often placed in the bottom of pots, underneath the soil, to improve drainage when there is no actual drainage hole (keep reading to find out why this is actually a myth).
Cons - Although they could provide a trickle of micronutrients and some aeration, it is not enough to justify their weight and necessity.
The only real benefit of using pebbles or stones in your pots is that it keeps them weighed down and not falling over. Obviously, gravel drains faster than soil, but soil holds water much longer than gravel does, so water won't run out of the soil onto the gravel until it is fully saturated (gravel doesn't make soil drain faster). Just like usual, water settles toward the bottom of the soil layer. Since the pot has been partially filled with stones, this raises the soggy soil layer higher, closer to the root ball. When the saturated zone is closer to the roots, this can result in them sitting in wet soil longer than they would've been if the rocks weren't there.
Now that we have covered absorbency, structure, aeration, and drainage, next up is nutrition or the fertility amendments. We cover this much more extensively in our Fertilizer Guide, but here is a short summary:
These are generally water-soluble, liquid fertilizers, or slow-release pellets that can be added to your soil for some long-term nutrition.
Organic options will often be more of an amendment or additive that can be incorporated with your soil mixture as you make it. The most common of which are worm castings, compost, leaf mould, fish emulsion, manure, blood meal, bonemeal, wood ash, greensand, kelp meal, and so much more. Just make sure you are including the right option for your plants!
The number of amendments that you wish to provide to your plants will completely depend on your resources as well as the type of plants you are potting. Some may require more than others, but you can generally be happy with the basic mixture of peat moss and perlite if you don't want to make it too complicated. We find the breakdown of the different potting mediums quite interesting and figured you would also be able to learn a little something if you have never thought about it before.
What is Soil pH?
Although not a physical amendment to add, understanding soil pH and how it affects our plants will help us understand their overall health as well. Maintaining proper soil pH, which is the soil's degree of acidity or alkalinity, is vital for healthy plant growth. When pH is too low (0 or most acidic) or too high (14 or most alkaline) in the soil, nutrients can’t release to the plants. Improper soil pH can also present itself in other ways, via nutrient deficiencies (again, this is discussed more thoroughly in our Fertilizer Guide) and more. Soil pH is different than fertilizer, but it is definitely something to keep in mind when you are considering the health of your soil!
Amendments alter soil pH and physical characteristics, such as texture and porosity, which will affect plant growth indirectly. Fertilizers, on the other hand, change the chemical nutrient makeup of the soil, influencing plant growth directly. Most houseplants thrive in slightly acidic to neutral soil (roughly 6.0 - 7.0), which you can always check using a soil test kit of soil pH meter.
Choosing the Best Media for Your Plants
Now for the fun part: choosing the best potting mix for your plants! Like most plant care, you want to mimic the conditions of the natural environment in which each plant exists as much as possible, therefore ensuring a long and healthy life. How much you do this is entirely up to you, most plants will be just fine in regular potting mix with peat moss and perlite, like our JOMO Potting Mix, but some plants, such as Aroids, would love a chunkier soil if they had the choice. Certain potting mediums will be much more beneficial than others when it comes to certain plants, especially aroids and epiphytes.
An aroid is any plant from the Araceae family and they can usually be identified by their unique "flower", which is actually a colourful leaf, or spathe, curved around a spadix that holds numerous tiny flowers. Some well-known aroids are Monsteras, Philodendrons, ZZ Plants, Peace Lilies, Pothos, and Chinese Evergreens (Aglaonemas). They do well in a loose, well-draining soil that is high in organic matter.
The ideal potting media varies from plant to plant because each plant's original habitat determines its preference, and should be tailored to this when possible. Is your plant from a sandy desert? Was it originally found in a moist swamp, forest floor, or rocky crevice? Think about where your plants are found naturally. For a lot of plants, this soil mix will be the classic peat moss, perlite, and some compost, it holds moisture, drains decently, is fertile, and has a fairly neutral pH. However, for those plants that love airflow and great drainage, you can often do better, starting with a basis of peat moss and then adding more, based on what your plant would prefer - orchid bark, perlite, and sphagnum moss are the most common additions.
An epiphytic plant is any plant that grows upon another plant (or structure) merely for support, living in a harmonious state with the other plant and deriving its nutrients and moisture from the air. This is why loose, chunky soil is important! Albeit, this is a little trickier when plants are kept indoors because they don't have the regular moisture and nutrients in their surrounding air as they do outdoors. Air plants, Bromeliads, Rhipsalis, Orchids, and most Hoyas are some common epiphytes. Keep in mind that the degree of epiphytic nature does depend on the plant.
If a plant is kept in the wrong potting media, its soil can become too compact - month after month of overhead watering, without the benefit of worms and weather for aeration, is a recipe for compaction. We know by now that the environment in which the roots exist is just as important as the environment in which the foliage exists. So, feel free to start with a decent retail mix, saving you time, effort, and money, and then mix in your own additives depending on the plant, or check out our Aroid Potting Mix, which will keep your aroids and epiphytes very happy.
Soil Tip: Make sure to aerate soil periodically, using a chopstick or wooden spoon to gently poke holes in the soil or even squeezing the plastic nursery pot every once in a while to break up the soil a bit (without disturbing the roots too much)
Let's go over some plant-specific mixtures that we would recommend:
Succulents & Cacti:
These little guys prefer coarse, well-draining soil, usually something that includes about a third of coarse sand grains.
African Violets & Ferns:
Choose a soil with high humus content (dark organic matter that forms when dead plant and animal matter decomposes), keeping it loose and well-draining. Coconut coir or peat, vermiculite, perlite, sand, and orchid bark would be some great amendments to consider.
Now that we know they are epiphytes, it would make sense that certain species are happiest growing in nothing but fir bark (commonly sold as Orchid Bark) or sphagnum moss. If the roots are left in excessive moisture for long lengths of time, they will rot, especially Phalaenopsis Orchids.
These plants will prefer a more porous potting media, which should also contain structure and aeration amendments. Peat moss, bark and perlite mixtures would be ideal, but make sure to do some additional research from plant to plant.
If you want a trailing, bushier aroid, stick to using a general purpose potting mix, but if you have an aroid that you want to push out mature growth a little quicker, consider using a chunkier mix to signal to the plant that it needs to latch and climb.
All-in-all, there are special mixes that have been developed for the fussier plants that have specific requirements (such as Succulents and Phalaenopsis Orchids), and then there are the classic, general potting mixes that will suit plenty of other plants. The best way to change the potting mix of your plants is to 1) wait until that plant needs to be repotted, and 2) mix in some of the old soil with the new mix as much as possible to help the plant adjust. Topping off the old soil with a new soil mix, creating layers, will only cause issues.
Repotting Tip: Make sure you are not reusing your soil between plants as this can potentially transfer disease or pests.
These are just all some tips and tricks if you wish to spoil your plant babies further. Adjust based on your plants and make sure to do any additional, plant-specific research before making your final decision!
We know that was a lot of information but, seeing as choosing the correct potting medium is one of the top elements of plant care, it is important to get it right! Certain plants will have a higher maintenance soil routine than others so, although lots of plants will enjoy the classic mix, it is important to tailor your decisions as much as possible. Make sure your chosen mixture achieves the main elements of what each plant needs in regards to drainage, structure, and airflow, balancing it appropriately.
It may be easier to simply select the general potting mix from your local garden centre, but it is often best in the long run to accommodate your plants a little bit more, encouraging them to produce lots of beautiful foliage for you to enjoy. Trust us, you will appreciate the payoff.