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Uses for Bat Guano: What It Is Good For, and the Real Risks

Bat guano is a fast, high-nitrogen or high-phosphorus fertiliser depending on the bat's diet. How to use it, how to dilute it, and why the dust is dangerous.

By Zakaria 4 min read Save to Pinterest — opens Pinterest in a new tab
Hardened bat guano deposits crusted on a cave wall, lit by torchlight
Photo: shankar s., CC BY 2.0, via Flickr
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Bat guano is a fast, concentrated organic fertiliser — and one of the few where the nutrient profile depends on what the animal ate. It is also the one where the safety note is not boilerplate.

Which guano you are buying

Guano types and what they are for
TypeTypical NPKBest for
Insectivorous bat~10-3-1Leafy growth, greens, greening up a tired lawn, early vegetative stage
Fruit-eating bat~3-10-1Flowering, fruit set, root development, bulbs
Seabird guano~10-10-2A more balanced general feed
Composted manure~1-1-1Soil structure and slow background feeding

Buy for the job. The single most common mistake is feeding a high-nitrogen insectivorous guano to tomatoes or roses at flowering, which produces magnificent foliage and very little fruit.

The four ways to use it

1. Guano tea — the default.

  • ¼ to ½ cup per gallon of water
  • Stir, then steep one to two days, bubbling an aquarium air stone through it if you have one
  • Strain, then water in around the root zone
  • Every two to four weeks in the growing season

This is the right method for containers, seedlings and anything you are feeding regularly, because the dilution removes the burn risk.

2. Topdressing. A light scatter on the soil surface around established plants, scratched in and watered. Follow the bag rate — typically a tablespoon or two per plant, not a handful.

3. Mixed into potting soil. Roughly 1 to 2 tablespoons per gallon of mix, blended thoroughly before planting. Never leave a concentrated pocket where a root will run into it.

4. Into the compost heap. A modest addition speeds a slow, carbon-heavy heap along by supplying nitrogen and microbes. Do not build a heap out of it.

It burns because it works

The reason guano acts within days is that its nutrients are already in a plant-available form. That is the same reason it scorches roots and foliage when over-applied. If you are guessing, halve the rate — under-feeding with guano costs you a week, over-feeding can cost you the plant.

The health risk, properly stated

Dry guano dust can carry Histoplasma capsulatum, a fungus that grows in soil enriched by bat and bird droppings. Inhaled spores cause histoplasmosis: usually mild and flu-like, occasionally serious, and genuinely dangerous for anyone immunocompromised or with existing lung disease.

The risk is concentrated in disturbing an accumulation — a loft, a cave, a barn floor — not in opening a bag of heat-treated commercial product.

If you are clearing any quantity:

  • An N95 respirator at minimum, fitted properly. A dust mask is not this.
  • Gloves, goggles, and clothing you can wash separately
  • Dampen the material first with a light mist so it does not aerosolise
  • Ventilate the space, and keep everyone else out of it
  • Bag it sealed, and do not sweep dry material with a broom

Bats are protected — check before you touch a roost

In the UK, the EU and much of North America, bats and their roosts are legally protected, and blocking a roost during the maternity season seals flightless pups inside to die. If the guano in question is accumulating in your house, that is a bat exclusion job for a licensed operator, timed around the maternity season — not a fertiliser supply.

Is it worth it?

Honestly, for most gardens: as a supplement, yes. As a staple, no.

  • It is expensive per unit of nutrient compared with composted manure or a bagged organic feed.
  • It adds no organic matter worth counting, so it does nothing for soil structure, water holding or earthworms. Compost does that job and guano cannot.
  • It is excellent at what it is for: a fast, targeted correction — a nitrogen boost to a sulking leafy crop, or a phosphorus push before flowering.
  • Sourcing varies in quality and in ethics; harvested-sustainably claims are worth checking, because cave mining disturbs the colonies the supply depends on.

Use it the way you would use a liquid feed: precisely, occasionally, and on top of good soil rather than instead of it.

Common questions

What is bat guano used for?

Mostly as a concentrated organic fertiliser. Insect-eating bats produce a high-nitrogen guano, typically around 10-3-1, that drives leafy growth; fruit-eating bats produce a higher-phosphorus guano, often around 3-10-1, used to push flowering and fruiting. It is also used brewed as a liquid tea for a fast feed, and as a soil amendment because it carries microbes that help break down organic matter. Historically it was mined at scale for fertiliser and for the saltpetre in gunpowder.

Is bat guano a good fertiliser?

It is genuinely good and genuinely easy to overdo. The nutrients are highly available, so plants respond within days rather than weeks — which is the appeal, and also the risk, because the same availability means it will burn roots if applied neat or heavily. Used at label rates, or brewed into a dilute tea, it is one of the stronger organic fertilisers available. It is not a substitute for compost: it feeds plants, it does not build soil structure.

Is bat guano dangerous to handle?

The dust is the hazard, not the material itself. Dry guano can carry Histoplasma capsulatum, a fungus whose spores cause histoplasmosis — a lung infection that is usually mild but can be severe, especially for anyone immunocompromised. Anyone disturbing accumulated guano should wear an N95 or better respirator, gloves and eye protection, and dampen the material first so it does not aerosolise. Bagged commercial guano is heat-treated and far lower risk, but the same dust precautions are still sensible.

What is the NPK of bat guano?

It depends entirely on what the bats ate. Insectivorous guano runs roughly 10-3-1 — nitrogen-dominant, for leafy growth and greening up. Fruit-eating bat guano runs roughly 3-10-1 — phosphorus-dominant, marketed for bloom and fruit set. Seabird guano is different again. Always read the bag, because buying the wrong one means feeding leaves when you wanted flowers.

How do you make bat guano tea?

Roughly a quarter to half a cup of guano per gallon of water, stirred and left to steep for one to two days, ideally with an aquarium air stone bubbling in it. Strain it, then water it in around the root zone rather than over the foliage. It feeds far faster than a topdressing and is the usual way to use guano on containers and seedlings, where a dry application would be too strong.

Should I collect guano from bats roosting in my house?

No — deal with the roost first, and mind the law. Bats are protected in many countries and in much of the US, and sealing a roost during the maternity season traps flightless pups inside. An accumulation in a loft or wall cavity is also a structural and health problem rather than a resource. Get a licensed bat excluder to handle the timing and the exclusion, and treat the cleanup as hazardous dust work, not as harvesting.

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