How Potting Soil is Made: Commercial & homemade

Potting soils can be the most mysterious element of gardening for beginners. If you are serious about keeping your plants happy, you can gain a lot from learning how potting mixtures are made, what’s in them, and how to make your own formulas at home. 

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Commercial potting manufacture

Potting mixture manufacture is big business. Typical mineral ingredients for potting soils include perlite (pieces of light, white volcanic rock), sand, and bentonite clay. Common organic (meaning made from biological sources) ingredients are peat mosses, peat, composted organic matter, bark chips, and coconut coir. Manufacturers also include some fertilizer into the mix.

At the manufacturing facility, these ingredients are blended in different ratios for different plant types. The potting mix is sterilized before it is bagged up to make sure there are no weeds or plant-borne diseases lurking inside.

These potting mix factories often have their own composting facilities and bark recycling facilities. Potting soil can be made year-round with customer demand highest from customers during spring and summer.

Peat and peat moss are extremely unsustainable, so responsible potting soil manufacturers are starting to phase these ingredients out.

Peat is the soil unique to habitats called peatlands and peat moss is the vegetation that grows on it. Peat stores a lot of carbon and removing it from the environment releases the gas into the atmosphere.

In fact, peat in potting mixes is one of the highest contributors to carbon emissions in container gardening, alongside fertilizer (which tends to be manufactured using petrochemicals) and plastic use. Peat habitats are also becoming endangered due to over-extraction. Gardeners should opt for potting mixes that contain substitutes like coconut coir instead.

Making potting mixtures at home

If you’re starting out with indoor gardening, all soils seem alike. However,  different potting materials in different ratios offer quite different chemical and physical properties. Some plants are very particular about what kind of material they grow in and you'll see a noticeable difference in their health when planted in different potting mixes.

Depending on what materials they contain and the ratios in which they are combined, different soils have different drainage rates (how quickly water passes through the soil), textures (how loose it is, usually determined by the concentration of tiny air pockets in the soil), and nutrient profiles (the types and ratios of the minerals that plant feed on).

Here is a list of all the possible materials you can use in your potting mixture. There are two types of soil elements: organic materials (made from plant-based materials) and mineral materials (made from rocks).

Mineral potting mixture ingredients

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1. Perlite
   
   Horticultural perlite are small pieces of a light, porous volcanic mineral. It is useful for adding air pockets to your mix. Air pockets are essential for root transpiration: the exchange of gases between the soil and the roots.
   
   
2. Vermiculite
   
   Another crushed mineral ingredient, good for achieving high drainage
   
   
3. Horticultural sand
   
   You can get horticultural sand in a range of grains, from very coarse to very fine. This is useful for succulents and cacti which prefer dry, low-nutrient potting mixtures.
   
   
4. Oyster shell
   
   An underrated potting mixture that increases drainage and improves the chemical quality of your soil. Read more on why you should use oyster shell here.
   
   
5. Horticultural charcoal
   
   This is a great soil conditioner that absorbs impurities and excess moisture

Organic potting mixture ingredients

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1. Bark pieces
   
   These are great for adding a air pockets and a light crumbly texture to potting mixes. Orchids love these in their mixes.
   
   
2. Compost
   
   Homemade or store bought, compost is made up of decomposed pieces of biological matter: kitchen scraps, garden trimmings, forestry and agricultural waste, and manure can all go into it.
   
   
3. Worm castings
   
   Worm poo is great for tropical houseplants that needs lots of nutrition. You can use it in any houseplant mixture to give your plants a nutrient boost. Read more about this great potting mix ingredient.
   
   
4. Coconut coir
   
   This is a great substitute for peat, the main component in most houseplant mixes but one that is extremely harmful for the environment. Coconut coir adds air pockets and drainage to the mixture.

Most of the time, you don’t use any one of these on their own. You should be mixing together two or more to produce a potting medium with just the right balance of properties for your plant. See some suggested recipes below.

Making your own potting mixtures

Making the right custom mix for fussy roots can be very satisfying. When you get it right, these precise formulas can work a lot better than the general-purpose houseplant mixes you buy at the store.

Experienced gardeners know through trial and error which materials in what ratios will keep different plants happy. To start out, you may need some guidance.  Here are three basic mixes to get you started. Each one is geared towards differing moisture and nutrient needs.

1. The heavy-feeding tropical houseplant (e.g. Amazonian Elephant Ears and other Alocasia, Ficus elastics, Ficus Benjamina)

   Tropical houseplants that hail from rainforests enjoy soils rich in decomposed organic matter, just like in their native environment.
   
   The best recipe for these plants is a mix of equal parts composted bark, ordinary compost from scraps or manure, and sand. If your plant also appreciates high drainage (like the ficus), add in one part perlite.
   
   
2. The desert-dry cactus
   
   One part compost, two parts coarse sand, and two parts perlite
   
   
3. The damp shade-lovers (e.g. fittonia, ferns)
   
   Mix equal parts potting compost, coco coir or perlite, and charcoal
   
   Supplement with a bit of worm casting, about a teaspoon or two depending on the size of the container.

If you have many plants belonging to the same species, you can experiment with these mixes by adding or removing ingredients and seeing what works best.