Welcome back to the spring edition of this blog series. In the fall we covered outdoor gardening, mostly focusing on edible crops. This semester will still be plant related, but with a more specific focus on indoor plants (aka houseplants).
Just like in the outdoor garden, the first steps to successful houseplants come with the soil you use. However, the soil used for potting houseplants is not the same as the soil you’d find outside (And its usually referred to as “potting media” rather that “soil”). Even a houseplant grown in the very best outdoor soil would struggle to survive. This is because outdoors the wind, sun, and exposure to the elements makes soil dry out faster than it would indoors. Since plants don’t usually like to sit in soil that stays soaking wet, we need to make up for the lack of these environmental factors by adding materials to the potting media in order to allow it to drain water better and dry out faster.
Materials:
We break potting media materials into two categories:
Base media: Materials used to create the base of the potting media. They usually contain mostly organic material, and have a high water holding capacity. Two of the most common are peat moss and coco coir, and commercial potting mixes often have a combination of the two. They are functionally very similar, so it doesn’t particularly matter which one you use.
Drainage/Porosity: Sometimes also called mineral components, these are used to improve drainage by creating gaps through which water can move and also allows air to move through the potting mix
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- Perlite: Perlite is a naturally occurring siliceous rock that, when heated, expands from four to 20 times its original volume. These are the little white pieces you find in potting mix which resemble Styrofoam. It is extremely light and porous, and when pressed will crush down into a powder. It is the most common additive for improving drainage because it is extremely cheap and works quite well. The one downside is that it can float up to the top of the pot overtime because it floats in water.
- Pumice: Another naturally occurring rock with tons of tiny holes that is also very lightweight and porous (Although not quite as light as perlite), it is used straight as it is mined, not heated like perlite. This is a better alternative for perlite in most cases, because it has all the benefits and will not float to the top of a potting mix even when watered frequently. However, it is more expensive than perlite and usually a little harder to find.
- Orchid bark: This name is misleading, as the bark sold as “orchid bark” is intended to be used for orchids, but it is not from orchids. It is usually pieces of fir bark/coconut husk pieces. It makes a good additive for many tropical plants which come from soils that may have a large amount of decomposing plant material mixed in. It is not quite as porous as the above two, and holds a little water.
- Vermiculite: another naturally occurring mineral which is expanded through heating. It appears similar to perlite, and acts much the same way except unlike perlite it absorbs water so it cannot be used as the primary drainage material for most mixes because the mix would stay too wet.
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Can’t I just use a commercial bagged potting soil?
The answer to this is a little tricky. For some plants, a commercial mix may be good enough, but not ideal. For others, using a cheap commercial mix will almost certainly result in the eventual death of the plant. Most commercial mixes contain some perlite or other drainage material, but usually they don’t have enough of it. For this reason, it is always a good idea to use a commercial mix more like a base and add drainage material as needed.
*The exception to this would be high end specialty mixes or those made by local nurseries/greenhouses because these mixes may already be mixed correctly. But if you’re working with a bag of miracle grow (or similar) from a big box store, the above advice is important.
Recipes for soil success
The right ratios of base material and drainage for your plants will depend on the species of plants being grown, and it is always a good idea to look up a specific plant before making a soil mix for it, but here are some general guidelines:
Catci/Succulents:
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- 20-40% organic material/base mix
- 60-80% coarse mineral/drainage (No orchid bark)
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Aroids (Like monstera, philodendron, pothos, etc.):
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- 40-60% organic material/base mix
- 20-40% perlite
- 20-40% orchid bark
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Ferns:
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- 50-70% organic material/base mix
- 30-50% coarse mineral/drainage
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Most Others:
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- ~50% organic material/base mix
- ~50% coarse mineral/drainage
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That’s about it for potting mixes, or at least to depth that we’ll be covering in this blog. As always, these are general guidelines and I suggest doing a little bit of your own research before making a specific potting mix.
Your technical expertise really shines through here– who knew there was so much involved in keeping a plant alive! It was honestly very illuminating to read about the different porous materials, their uses, and the combinations best to keep different plants alive. I love a good table and/or chart; it adds a feeling of authority to your information, even when you are going by rule of thumb. I personally prefer to let my succulents battle against their un-ideal soil conditions, like an evolutionary gauntlet on my windowsill. However, if I ever get a plant from an auntie whose fate will remain the perennial subject of old lady gossip, I will be sure to heed your advice and keep that sucker alive with proper aeration.
I love how clear and concise all of your information is organized. It is brief but just as enlightening due to your use of an easy-to-understand list of typical percentage makeups of soil for different types of plants, while previously explaining what these types of materials accomplish for plants. I briefly worked on a farm over the summer, so I know some of what you’re talking about, but there is definitely a large difference between indoor and outdoor growing, which is quite fascinating.
I am thoroughly impressed with your immense knowledge of greenery and plants! I myself am the furthest thing from a green thumb, so this post was incredibly eye-opening and helpful in my own pursuits to becoming more green. You blog actually reminded of a project I did in high school where I and a group of students worked to incorporate more greenery and plants throughout my school, especially in areas with poor air quality. Our research mostly focused on ZZ and Spider plants, and we did a lot of research into the type of soil needed and how much water/sunlight is ideal. I would love to hear your opinion about these two plants and hear if you think they would be good plants for a beginner like myself!
Ava, Spider plants (Chlorophytum comosum) and ZZ plants (Zamioculcas zamiifolia) are both excellent houseplants for beginners. Their care needs are slightly different, but both are fairly forgiving. Something interesting about the ZZ plant is that it actually has tubers (almost like potatoes) that are just under the surface of the soil at the base of the stems which store water for the plant. This means that if you forget to water your ZZ for a couple weeks (sometimes even months) the plant will be totally fine because it will stay alive by drawing water from these tubers. If you are serious about trying to get started with some indoor plants, make sure to check back to my blog in a couple weeks as I plan to make a post about the best houseplants for beginners!
Wow! I never knew that developing the right potting soil was such an in-depth procedure. Whenever I would do any such planting with my family, we would always use the traditional potting soil (which explains why our plants eventually died haha!) Also, through your post, I can tell that you know a lot of technical information and are passionate about the topic. I look forward to reading your next blog as I am not well versed in this topic and will be able to learn a lot!