Everything About: Soil Mixes!

When you’re potting -- or repotting -- your plants, you likely use a bag of potting soil. That’s completely fine, especially for a beginner -- but it’s kind of like “one size fits all” clothing: it might technically fit each plant, but it’s not going to fit each plant well. 

So how do you fix it?

For each plant that you pot, the absolute best way is to make your own potting mix. There are a number of ingredients that can go into a potting mix, and an infinite number of combinations and proportions you can use -- don’t consider this a full list, by any means. If something you use and love isn’t on this list, that doesn’t mean you shouldn’t be using it!

So, all that being said, here’s a list of some common potting mix ingredients:

• Potting soil. This is usually the main/base ingredient of most potting mixes. What may surprise you is that this doesn’t actually technically have any dirt in it -- it depends on the brand, but it’s most often a mixture of peat moss, perlite, and lime. The peat moss is the main structure of the potting soil (and what we think of as the dirt); the perlite helps provide aeration; the lime neutralizes the otherwise-acidic pH of the potting soil. This is a good start -- though it’s usually a better starting point the better-quality potting soil you buy. Some brands will use styrofoam instead of perlite in order to cut costs; avoid this at all costs.
• Peat moss or coco coir. Peat moss is decayed sphagnum moss, collected from the bottom of peat bogs; coco coir is a coconut byproduct, found between the hard shell and the outer layer of coconut. There’s a lot of debate about what’s best to use between these two (peat moss is cheapest and most well-known; coco coir enthusiasts have concerns about the sustainability of peat moss harvesting) but they ultimately perform the same function in potting mixes: they retain moisture and nutrients, holding it next to the roots so that your plant can drink as needed. Add some more of whichever ingredient you use for plants like Calatheas that prefer to stay moist, but don’t add more to plants like succulents that like it dry.
• Perlite. This is volcanic glass that is superheated until it “pops” like popcorn, expanding suddenly and becoming incredibly porous. This has a number of uses (and is, in my opinion, the most important additive) -- it provides oxygen to the roots, retains some moisture, makes your soil mix lighter, and most importantly means that your potting mix will drain better. I add this to every single mix I make in some quantity but will add even more to mixes meant for plants like snake plants that need well-draining, quick-drying soil.
• Vermiculite. This is a material similar to mica, and it works similarly to perlite. It tends to retain more moisture than perlite and doesn’t make mixes drain quite as well. I personally use this in addition to perlite, especially in mixes where the plant prefers soil that stays a bit moist, like ferns.
• Orchid bark. This is exactly what it sounds like -- the bark of a tree (usually a fir tree), meant for use as a soilless potting medium for orchids, but usable as a potting mix additive. Adding in bark drastically improves the mix’s ability to drain water. It manages to help drain excess water while still helping to retain some moisture in the mix after watering. Add a bit more if your plant prefers it dry, but don’t go overboard.
• Earthworm castings. This is, to put it more simply, worm poop. It helps aerate the soil and increases water retention, but most importantly it acts as a fantastic natural fertilizer, providing a number of important nutrients to your plants.
• Sand. Adding sand to your mix will make it drain much faster -- it could even drain too quickly for some moisture-loving plants like prayer-plants.
• Horticultural charcoal. This is basically a very finely ground activated charcoal, and can help prevent mold and fungal/bacterial infections.
• Diatomaceous earth: Despite being called “earth”, this has nothing to do with soil; it’s actually ground-up fossils, believe it or not. This can be used sprinkled on top of your pots to help kill pests (it’s so finely ground that it actually gets under their skin and shreds them to death!), but you can also add it into your potting mix to make it drain faster, while not just retaining moisture and nutrients -- it also adds nutrients!
• Dolomitic lime, sometimes just plainly called lime: This will increase the pH of your soil (make it less acidic). Most houseplants prefer their substrate slightly acidic, so this is uncommon.
• LECA, or Lightweight Expanded Clay Aggregate -- small, expanded clay balls that are used for semihydroponics. LECA will wick moisture to where it's needed, while retaining porosity and promoting good air circulation -- all wonderful things to add to a soil mix!

So, now that we’ve talked about the different ingredients -- how do you actually decide on what needs to be in your mix, and in what proportions?

This can vary widely, based on the plant that you’re potting. You won’t want to use the same potting mix for a moisture-loving maranta that you would for a drought-resistant ZZ plant!

Here are a few things to keep in mind:

• How much moisture does this plant need? This is probably the biggest factor. If your plant needs more moisture and prefers that its soil doesn’t dry out quickly, add some more peat moss, coco coir, or vermiculite. If your plant wants its soil to dry out as soon as possible after watering, add some more perlite, sand, or orchid bark.
• Does this pot have sufficient drainage? I use the word “sufficient” here because while a pot may have a single small hole in the centre, and thus technically have drainage, that’s often not sufficient drainage. If it doesn’t, you’ll need to be more careful about water in general, and may want to adjust the moisture retention levels based on if you’re an over waterer (let it drain to the bottom of the pot!) or err on the side of underwatering (give it some extra moisture retention!)
• What pH does this plant prefer? This isn’t a huge factor, especially since most tropical houseplants prefer their soil with a neutral or slightly acidic pH, but remember that peat moss is acidic by nature -- if your plant can’t deal with an acidic pH, ease off on the added peat moss.
• Where does this plant grow in nature? If it’s terrestrial -- it always grows on the ground -- then try to mimic that texture. If it’s a desert-dwelling plant, adding some sand to your mix will help it feel like home. If it’s an epiphyte -- naturally growing on trees, for example -- then adding orchid bark will help give its roots something to hold on to.
• What do you have on hand/what can you afford? Don’t spread yourself too thin -- if you don’t have something on hand, if it’s sold out at the store, or if you don’t have the money for an ingredient at the moment, then leave it out -- use other ingredients to achieve nearly the same effect (an extra handful of vermiculite instead of some more peat moss, for example), or just remember what ingredient you didn’t have, and adjust your watering accordingly. I currently only use the first five ingredients on this list, for a combination of reasons.

A good basic proportion that I’ve found will work for your average tropical houseplant -- and, in particular, most aroids like pothos, philodendrons, or monsteras -- is:

2:2:1   Soil:Bark:Perlite

This mix will promote good drainage, while still retaining moisture. If it’s an epiphytic aroid, add a bit more bark. I’ll add some more potting mix (or less bark) for my alocasias, and because I know my Alocasia Polly, in particular, gets droopy when her feet stay wet for a minute too long, I’ll add more perlite to her mix.

And so on  -- experiment with different proportions for different plants, and you’ll eventually hit the point where you can use the different ingredients without even needing to think about it.