Compost tea is a liquid made by steeping finished compost in water, so that the nutrients and soil organisms in the compost are carried into a form you can pour or spray onto plants and soil. It is not a fertiliser in the ordinary sense — it is weak in nutrients compared with a bag of feed — but gardeners value it as a gentle tonic: a way to add biology and a small amount of soluble nutrition to the soil, to give seedlings a mild boost, and to get more value out of a limited compost supply. Used well, it is a useful extra. Used as a miracle cure, it disappoints. This guide explains what compost tea is, how to brew the aerated kind safely, how to use it, and where its limits are.
There are two main kinds. Aerated compost tea is brewed with an air pump bubbling through the mixture for 24–36 hours, which keeps oxygen levels high and favours the aerobic organisms that help plants. Non-aerated tea, sometimes called compost extract, is simply compost steeped in water for a few hours or overnight without a pump; it is simpler, weaker in biology, and lower-risk. This guide covers both, with the aerated method as the main recipe.
What compost tea does — and does not do
Compost tea adds a small amount of soluble nutrients — nitrogen, potassium and trace elements leached from the compost — and a large number of bacteria, fungi and other soil organisms. When poured on soil, those organisms join the soil food web and help break down organic matter, which slowly releases nutrients. When sprayed on leaves, the organisms are traditionally said to occupy leaf surfaces and compete with disease organisms, though the evidence for disease suppression is mixed and modest; it is best treated as a possible bonus, not a reliable treatment. Compost tea does not replace compost itself, and it does not replace fertiliser for heavy feeders such as tomatoes and cabbage. Think of it as a supplement to a good soil routine, not a substitute for one. Our composting basics and fertilizing vegetables guides cover the foundations it sits on.
What you need to brew aerated compost tea
You need a clean bucket (10–20 litres), an aquarium air pump with an air stone, finished mature compost, water, and a food source for the organisms — usually unsulfured molasses at about 15 ml (one tablespoon) per 10 litres of water. The compost must be mature, dark, crumbly and earthy-smelling; immature or smelly compost makes poor tea and can carry pathogens. Do not use compost that contains fresh manure, meat, dairy or diseased plants. If your tap water is chlorinated, let it stand uncovered for 24 hours before brewing so the chlorine dissipates, or use rainwater. Chlorine kills the very organisms you are trying to grow.
A paint strainer bag, old pillowcase or fine mesh bag holds the compost so the finished tea does not clog your watering can or sprayer. You will also want a thermometer or a sense of the weather: brewing works best at 15–25°C. Below 10°C the organisms grow slowly, and above 30°C the brew can turn anaerobic quickly.
Step-by-step: brewing aerated compost tea
1. Prepare the water. Fill the bucket with dechlorinated water, leaving a few centimetres at the top. Add the molasses and stir until dissolved.
2. Add the compost. Put 2–3 litres of mature compost (roughly 1–2 kg) into the mesh bag, tie it, and lower it into the bucket. The ratio is roughly one part compost to five parts water by volume.
3. Start the air pump. Drop the air stone to the bottom of the bucket and switch the pump on. You want vigorous bubbling throughout the water, not a gentle trickle. Keep the pump running for the whole brew — if the air stops, oxygen drops fast and unwanted anaerobic organisms take over.
4. Brew for 24–36 hours. In warm weather (20–25°C), 24 hours is usually enough; in cooler weather, allow up to 36 hours. The tea should smell earthy and slightly sweet, like good compost. If it smells sour, rotten or like ammonia, throw it away — it has gone anaerobic and should not be used on plants. Do not brew longer than about 48 hours; the food runs out, organisms die, and the brew turns.
5. Strain and use immediately. Lift out the bag, let it drain, and use the tea within a few hours. Compost tea loses its biology quickly once the air pump stops, and it does not store well — do not keep it overnight. The leftover compost in the bag can go back on the compost heap or be spread on the garden.
Non-aerated method. If you have no pump, steep 2–3 litres of mature compost in 10 litres of water for 12–24 hours, stirring several times a day. Strain and use the same day. This gives you a nutrient extract with fewer live organisms, but it is simple, safe and still useful as a mild liquid feed.
How to use compost tea
As a soil drench. Dilute aerated tea roughly 1:1 with dechlorinated water and pour it around the base of plants, using about 1–2 litres per square metre of bed. Apply in the morning or evening, not in hot sun, and ideally after rain or watering so the soil is already moist. Use it every two to four weeks through the growing season on vegetables, flowers and fruit. Seedlings and transplants benefit from a drench at planting time, which is traditionally said to help them settle in.
As a foliar spray. Dilute the tea 1:1, strain it finely through cloth so it does not clog the sprayer, and spray leaves until lightly wet, top and underside, in the early morning or evening. Spraying in hot sun can scorch leaves and kills the organisms quickly. Do not spray on edible leaves within a few days of harvest, and wash all produce before eating.
On houseplants. Compost tea can be used on houseplants such as monstera deliciosa and golden pothos, diluted 1:1 and applied monthly at most, in place of a regular watering. Use it sparingly indoors: overuse can bring fungus gnats and smells, and houseplants need far less feeding than outdoor vegetables. See our fertilizing houseplants guide for the feeding routine it fits into.
On compost and mulch. Undiluted tea can be poured over a new compost heap to add organisms, or over mulch to help it break down. It is also useful for charging biochar before it goes into the soil — soak the biochar in the tea for a day or two. See our biochar for garden guide.
Common mistakes
The most common mistake is brewing too long or without enough air, which produces a smelly anaerobic brew that can harm plants — if it smells bad, discard it. The second is using immature compost or fresh manure in the brew, which risks pathogens; only mature compost belongs in compost tea. The third is storing brewed tea: it must be used within hours, not days. The fourth is expecting it to replace fertiliser — heavy feeders will still go hungry on tea alone. The fifth is spraying in hot sun or on leaves close to harvest. And the sixth is using chlorinated water straight from the tap, which kills the organisms before the brew starts — always let tap water stand for 24 hours or use rainwater.
Safety notes
Compost tea is a living liquid, so hygiene matters. Use only mature, finished compost — never fresh manure, pet waste, meat or dairy. Wear gloves when handling compost and tea, wash your hands afterwards, and wash all fruit and vegetables before eating, especially leafy crops. Do not use compost tea on crops that will be eaten raw within a few days of application. If the tea smells sour, rotten or like ammonia, do not use it — pour it on the compost heap instead. If you are unsure about the safety of your compost, check your local extension service for advice on home composting and safe use.
Frequently asked questions
How often should I use compost tea?
Every two to four weeks through the growing season is a reasonable routine for outdoor beds, and monthly at most for houseplants. It is a supplement, not a staple — your plants still need compost in the soil and proper feeding for heavy crops.
Does compost tea replace fertiliser?
No. Compost tea is weak in nutrients compared with fertiliser or even a compost top-dressing. It adds biology and a small amount of soluble nutrition, but heavy feeders such as tomatoes, cabbage and cucumbers still need proper feeding. See our fertilising guides for the main routine.
Can I make compost tea without an air pump?
Yes. Steep mature compost in dechlorinated water for 12–24 hours, stirring several times a day, then strain and use the same day. The result is a compost extract: useful as a mild liquid feed, but with fewer live organisms than aerated tea.
Why does my compost tea smell bad?
A bad smell — sour, rotten or ammonia-like — means the brew went anaerobic, usually because the air pump was too weak, stopped, or the brew went on too long. Discard it on the compost heap and brew a fresh batch with stronger aeration and a shorter time.
Can I spray compost tea on leaves?
Yes, diluted 1:1 and finely strained, applied in the early morning or evening. It is traditionally said to help leaf-surface biology, but the evidence is modest — treat it as a possible bonus, not a disease treatment. Do not spray edible leaves close to harvest, and always wash produce before eating.