Let's get the obvious out of the way. Frass is insect poo. Specifically, it's what's left after insect larvae have worked through organic matter — droppings, shed skins, and a bit of the material they didn't finish.

It has become a fixture in organic growing over the last decade, and like most things that get popular quickly, the explanations attached to it have run a bit ahead of what's actually established. So here's what frass is, what the chitin in it genuinely does, how to use it across pots, beds and the compost heap — and, just as usefully, what it won't do.

What frass actually is

Insect frass is a by-product of insect farming. Larvae are raised on organic matter — food waste, spent grain, plant material — and frass is what comes out the other end, collected, dried and screened.

Four things end up in the bag:

Nutrients

Frass carries nitrogen, phosphorus and potassium, along with a range of trace elements, in modest concentrations. It is not a strong fertiliser and it isn't meant to be — the numbers on a bag of frass are a fraction of what you'd see on a synthetic. What it offers instead is a gentle, organic-matter-bound supply that releases as soil life breaks it down, rather than arriving all at once.

Organic matter

Frass also carries partially digested organic material, which contributes organic matter back into the soil. In soil that's structure: something for aggregates to form around, something to hold water and air. This is the least glamorous part of frass and probably the most reliably useful.

Living microbes

Frass isn't a sterile product. It arrives carrying bacteria and fungi — some from the gut of the insect that produced it, some from the material it was reared on — and they go into your soil along with everything else.

That's the difference between feeding soil life and adding to it. Most amendments do the first. Frass does both: it brings organisms in, and it brings food they can work with.

Worth being realistic about it. Introduced microbes have to establish in soil that already has its own population and its own conditions, and how many take hold depends on what they arrive into. Moist, living soil with organic matter in it is a far better prospect than something dry, compacted or worked to death.

Chitin

The interesting one, and the reason frass gets talked about differently from other organic inputs. Insect larvae moult as they grow, and those shed exoskeletons come through into the frass. Exoskeletons are made of chitin.

So what does chitin do?

Chitin is a structural polysaccharide — the same material that makes up crab shells and fungal cell walls. It's one of the most abundant organic polymers on the planet, and it has been cycling through soils for as long as there have been insects and fungi in them.

It does two things worth knowing about. The first is straightforward. Chitin is food, and it's fussy food. Most soil organisms can't do much with it; a specific subset can, the ones carrying the enzymes to break it down. Add chitin to soil and you're putting a particular item on the menu, which favours the organisms equipped to eat it. Over time that shifts the balance of who's living in your soil.

Chitin has a second property too, and it's the reason it appears in far more research papers than its nutrient value would ever justify. We'll come to it — but the practical business of using the stuff comes first.

Why we pair frass with biochar

Frass has a limitation on its own. Everything it brings — organisms, gentle nutrition, organic matter — has to stay put long enough to be useful, and in a free-draining mix or a sandy bed a fair amount of it simply moves through faster than roots can make use of it.

That's the gap biochar fills, which is why we blend the two rather than bagging frass on its own.

Biochar is biomass burned at high temperature with very little oxygen, so instead of turning to ash it becomes a stable, porous carbon skeleton. Two things follow from that structure. It holds water, which buffers a mix through dry spells. And its very large internal surface area holds onto nutrients that would otherwise wash straight through, keeping them where roots can reach them.

It's also close to inert. Biochar doesn't break down on any timescale that matters to a gardener — which is exactly why it's of interest for carbon storage — so it isn't feeding anything directly. What it provides is structure and shelter: somewhere for water, nutrients and microbes to sit.

Which is why the pairing works. Frass brings the biology and gentle nutrition; biochar gives that biology somewhere to live, and helps the soil hold onto water and nutrients.

How to use it

Frass is mild, which makes it forgiving — it's difficult to burn a plant with it the way you can with a heavy hand on a soluble fertiliser. That said, more isn't better, and the rates below are where we'd start.

Boosting potting mix

Mix it through before you plant, at 1–10% of the volume of the mix, concentrated around the root zone. This is the easiest win: you're building the biology into the medium from the start rather than trying to introduce it later. Works with any decent mix.

Vege gardens and beds

Same rate going in — 1–10% of volume, worked into the root zone before planting. For beds already planted, top-dress at 1–1.5 litres per square metre and water in. Around new or established trees, 1–5 litres per square metre. Beds have an advantage pots don't: they're connected to a much larger living system, so anything you add has somewhere to go and something to work with.

Compost heaps

Blend it through at 1–10% of volume, in with your greens and browns. A compost heap is already a microbial engine, and frass gives the chitin-degraders in there something to get on with. It also means the frass arrives in your beds already integrated into finished compost rather than as a separate application.

Established pots

Top-dress and water in. You won't get it evenly through the root zone the way you would mixing it in at potting time, but it still gets there — water and soil organisms will move it down. Repotting is the better moment if you have one coming.

Practical note: it's a dusty product and a living one. Wear gloves, avoid breathing the dust, and wash your hands afterwards — the same as with any compost or soil amendment.

One thing worth saying across all four: frass works through soil biology, and soil biology needs the conditions to be alive. Bone-dry, compacted, or sterile soil isn't going to do much with what you've added no matter how good the input is.

Plants can tell when there's a fungus about

Fungal cell walls are made of chitin. Plants, over a very long time, have worked this out.

They carry receptors that recognise chitin fragments specifically — proteins that do essentially nothing else. When those receptors pick up chitin, the plant reads it as evidence of fungi and activates defence-related responses. This is the direct route, and it's a large part of why chitin has attracted so much research attention.

There's an indirect route as well, and it's the one closer to what's properly called induced systemic resistance. Chitin favours the soil organisms capable of breaking it down; some of those organisms interact with roots in ways that prime the plant's defences more broadly. A plant that's been primed responds faster and harder when something does turn up, and the effect is systemic rather than local.

It's genuinely good science and it's worth knowing about. It is also, we'd suggest, worth holding a little more loosely than the internet holds it.

And what it doesn't do

This is the section most frass articles leave out, and it's the one we'd rather you read.

None of the above makes frass a pest control product. The ISR research is real, and we think it's worth understanding. But research on a compound is not the same as a claim about a bag of soil amendment — frass isn't registered as a pesticide or a fungicide, isn't sold as one, and shouldn't be bought as one. If you have a pest problem, deal with the pest problem — and if something is rotting, that's water and drainage, not a shortage of amendment.

It's not a replacement for feeding. The nutrient content is modest by design. If your plants need feeding, feed them — frass improves the medium, it doesn't substitute for a fertiliser.

And it's not instant. Everything frass does, it does through soil life, on soil life's timeline. If you're looking for a visible response this week, this isn't the product.

None of that makes it less worth using. It just means using it for what it is: a soil amendment that improves the medium and feeds the biology in it, over time, reliably, without drama.

Why insect frass, specifically

There's a resource argument here worth making plainly. Insect farming turns organic waste — material that would otherwise be landfill or low-value compost — into protein, with frass as the by-product. The nutrients in a bag of frass are nutrients that were already in the system, redirected rather than manufactured.

Rather than treating that material as waste, it goes back into soil, where its nutrients and organic matter keep moving through the biological system instead of leaving it. That's the argument, and it's a good one on its own terms.

We'd rather make the point qualitatively than wave a number at you, because the figures in this space vary enormously depending on what gets counted.