Living SoilHave you ever turned over a bag of potting mix and read the ingredients, then wondered what half of them are for? Perlite, vermiculite, bark fines. It reads like a chemistry set.
Here is the secret that makes it simple: every ingredient does one of only a few jobs. Some make room for air and drainage. Some hold water and nutrients. Some give the mix structure. Once you know which is which, you can read any bag, and see why one bag cannot be right for every plant.
For drainage and air
Perlite
White volcanic rock that has been crushed and heated until it expands. It adds drainage and air and holds almost no water or nutrients. Two cautions: wear a mask when handling it because of the dust, and avoid it for plants sensitive to fluoride, such as spider plants.
Pumice
Natural volcanic rock used for the same drainage and air. It is heavier than perlite, so it does not float to the top when you water.
Coarse sand
Large sand particles that improve drainage and add weight, which also keeps tall plants from tipping. The catch is huge: it must be coarse builder's sand, not play sand. Fine play sand fills the gaps between larger particles and makes drainage worse. And it takes almost 50 percent sand by volume before a mix drains quickly, so a handful does very little.
Expanded clay pellets
Fired clay pellets with excellent drainage, used as a propagation medium.
For holding water and nutrients
Vermiculite
A light, gray, puffy material made by heating mica. It holds moisture and nutrients, and supplies some potassium, magnesium and calcium. Its weakness: it compresses over time, and used alone it packs down and holds too much moisture.
Coco coir
Coconut husk fiber and dust, used for water retention. One reference flags its salt content as something to check.
Compost
Finished, stabilized organic matter. It adds organic matter, structure, some nutrients and some disease suppression. In a container mix, one guideline says to keep it between 15 and 40 percent, because many composts are above pH 7.0.
Worm castings
A fully stabilized amendment, much more microbially active than the material the worms ate. One guideline says no more than a quarter of a potting mix by volume. More in worm castings explained.
For structure
Bark
Ground and partly composted by products of the timber industry. It adds air and structure and breaks down slowly. Two cautions: it is difficult to re wet if it dries out completely, and fresh bark can tie up nitrogen.
What about garden soil?
Garden soil is not a potting mix ingredient for seedlings. It is not sterile, it is too heavy, and it does not drain well.
And biochar? The research for this guide found no authoritative source to support it as a potting ingredient, so it is not covered here.
Why the balance matters more than any one ingredient
A cactus needs a mix that drains fast and dries out completely between waterings. A fern wants something different, and a tomato in a container something else again. They can all be built from the ingredients above, in different proportions, and that proportion is the whole difference between a plant that thrives and one that rots.
That is exactly what a recipe is for. Each Living Soil mix gives the proportions from a published reference, measured for what you are filling. Look up your plant to see which mixes suit it.