What's Actually Living in Your Potting Mix?

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Mix

A pot isn't sterile. But there's no soil outside it moving back in — what lives there is mostly what went in with it.

A bag isn't soil

Dig a handful of garden soil and you're holding more living things than there are people on earth. Bacteria, fungi, protozoa, nematodes, mites, springtails, worms — a crowd, arguing over every scrap of dead leaf, and between them turning it into something a root can drink.

A bag of potting mix is not that. It isn't sterile either — composted bark and coir carry plenty of microbes — but processing, composting and storage leave a community that's usually much simpler and less settled than the soil outside. More to the point, a bagged mix is engineered for its physical properties: structure, drainage, consistency, no weed seeds. Its biology is whatever happened to survive the process. Nobody designed it.

Outdoors that doesn't matter, because the soil around a plant is crowded and life moves in on its own. A pot on a windowsill is more like an island. New biology still arrives — on roots, in water, on your hands — but there's no surrounding soil community constantly moving back in. What lives in the pot is mostly what went in with it.

Two kinds of life in a pot

It helps to split the biology into two groups, because they do different jobs and they get into a pot in different ways.

The first is the community. Bacteria, the decomposing fungi, the general crowd. Their job is to break organic matter down and cycle the nutrients in it into forms roots can take up. They don't need the plant; they'll live on anything dead. Give them organic matter to eat and somewhere to live and they multiply on their own.

The second is the specialists — the mycorrhizal fungi that partner directly with the root, growing into it and out from it, trading minerals and water for sugars. They're not decomposers. They can't build a working network by feeding on dead material; to grow and reproduce they need a compatible living root. That difference is the whole story of this post.

You can feed biology. You can house biology. You can't always add it.

So the practical question is what you can actually put in a pot to stop leaving that biological layer to chance. For the community, the answer is two things — one is food and residents, the other is housing — and they do different jobs.

Frass

Frass is insect manure — in our case, from black soldier fly larvae farmed on food waste. It's a by-product, not a product: nothing is grown, mined or made to produce it. What it brings to a pot is three things.

Nutrients, released gradually

Frass contains nitrogen, phosphorus, potassium and trace elements, much of it bound up in organic material. Microbes break that material down over time, so its contribution is slower and less immediately available than a liquid mineral feed. That's a different job, not a lesser one: a liquid feed delivers minerals now; frass keeps the cycling going in the background.

Biology, as well as food for biology

Frass naturally carries the microorganisms associated with the larvae and the material they were raised on. How much stays active depends on how it's processed and stored — which is why it matters that the frass in our Booster has a specified microbial count, rather than a hopeful assumption. Unlike an inert mineral amendment, it arrives with residents.

Chitin

The larvae shed their skins as they grow, and those skins — chitin — end up in the frass. Chitin is what frass is known for beyond its nutrients, and it's worth being straight about what the research does and doesn't show. Chitin feeds a specific set of microbes that can digest it, and those microbes are useful ones to have. There's also a body of work linking chitin in soil to a primed defence response in the plant — the plant's own system, switched on early. Most of that research used chitosan, a processed form, applied directly; how much carries over to raw frass in a pot is still open. So: it's interesting, it's real, and it's not a pesticide or a cure. If you've got a pest problem, frass is not the fix.

Biochar

Biochar is habitat, not food. It's charcoal made by heating plant material without oxygen, which leaves a structure riddled with pores — a large internal surface in a very small volume. Microbes colonise those pores. The same surface holds water and dissolved nutrients, so less of what the community releases washes straight out through the drainage holes. In a pot, that means the residents have somewhere to live that doesn't collapse when the mix is dry, and more of what they release stays in the root zone long enough to be used.

Which is why the two belong together. Frass brings food and residents; biochar gives them somewhere to settle. Either alone does something. Together they're a system.

What our Booster actually does

Our Soil & Microbe Booster is frass and biochar blended, in one jar. Its name says "booster," and the honest version of what that means is this: it doesn't make whatever's already in your mix stronger. It adds organic matter for the community to eat, biochar for it to live in, and frass carrying its own microbial community — so the biological layer of the pot is something you've chosen, rather than something you've left to whatever survived the bag.

Stir it through fresh mix at repotting, or work it into the top of a pot that's been in the same mix a while. It's an amendment, not a feed — keep feeding as normal — and because it's organic matter, it's one more reason to let the mix dry back properly between waterings: a wet pot full of organic matter is what fungus gnats are looking for.

The partnership a pot can't grow on its own

Now the specialists. Mycorrhizal fungi are one of the most important partnerships happening around roots outdoors. They grow into the root and out into the soil, extending the root's reach many times over, pulling in phosphorus and water the root couldn't reach alone, and taking sugar in return. Most land plants form some kind of mycorrhizal partnership. Many of the plants on your windowsill would, given the chance.

Indoors they rarely get the same opportunity. These fungi need a compatible living root to establish and reproduce. They don't build a functioning network by feeding on the dead organic material in a potting mix, so a healthy decomposer community doesn't automatically give you a mycorrhizal one. Outdoors, a root grows into soil that already has them; the partnership is inherited from whatever grew there before. In a pot on a windowsill, there is usually nothing to inherit from. A plant can live for years alongside a thriving decomposer community and never meet the fungus it evolved with.

That's the gap. You can feed the community and house it, and our Booster does both. The specialists are a different problem, and it's not one a frass-and-biochar blend solves. What would is a question we'll come back to.

Repotting is the easiest time to intervene

One more thing worth knowing. Repotting doesn't wipe the slate clean — whatever's living on the roots and in the old mix comes with the plant. But it does put a large volume of fresh mix around that root ball, and fresh mix is the part that's been left to chance. You're already mixing, the roots are in your hands, and whatever goes in now sits exactly where the next season of root growth will happen. Repotting in spring is the moment to decide what goes in with the plant.

Mix gives roots somewhere to live. What moves in is up to you.