Compost Tea & Brew a Living Spray
Multiplies microbes
Enter your brew volume to get the compost and molasses recipe, the diluted spray volume and the area it covers for soil and foliar feeding.
Plan your brew
Next: brew 10 kg compost in 100 L with 500 ml molasses and an air pump for 24–36 h, dilute ~4×, and spray within a few hours.
Use mature aerobic compost and non-chlorinated water; unaerated/old tea can grow pathogens — use promptly.
Compost tea — key facts
- What it is
- Aerated brew of beneficial microbes
- Compost
- brew volume × ratio (~1:10)
- Molasses
- volume × ml/L (feeds microbes)
- Brew time
- 24–36 h with an air pump
- Use it
- Diluted and fresh
- Area covered
- diluted volume ÷ rate
- Quality
- Mature aerobic compost, no chlorine
- Privacy
- Runs in your browser; nothing uploaded
A bucket of microbes for your soil
Compost tea takes the living part of good compost — the bacteria and fungi that cycle nutrients and protect roots — and multiplies it in aerated water so you can spray it across far more ground than the compost alone would cover. The recipe is simple: compost at roughly one part to ten parts water, a splash of molasses to feed the microbes, and an air pump to keep the brew aerobic for a day or so. Get the ratio right and you turn a kilo of compost into a barrel of biological inoculant.
This tool sizes the brew for you — how much compost and molasses to add, the diluted spray volume, and the area one brew covers at your application rate. Use mature aerobic compost and non-chlorinated water, brew 24–36 hours, then dilute and apply fresh; old or unaerated tea can grow pathogens. Pair it with the Compost & Manure, Vermicompost and Micronutrient Spray tools to run a complete organic feeding programme.
Scale the recipe
Exact compost and molasses for any brew volume.
Cover more ground
Turn a little compost into litres of spray.
Brew it right
Aerated 24–36 h, diluted, and used fresh.
Feed soil & leaves
Plan a soil drench or a foliar microbial spray.
Frequently Asked Questions
What is compost tea?+
Compost tea is an aerated brew that multiplies the beneficial microbes from mature compost in water, so you can spray a living dose of bacteria and fungi onto your soil or leaves. It isn't a fertiliser in the NPK sense — it's a microbial inoculant that improves soil biology, nutrient cycling and plant health.
How much compost do I need for a brew?+
Compost = brew volume × ratio. A common rate is about 1:10 compost to water, so roughly 1 kg of compost (or its volume) per 10 litres of brew. This tool takes your brew volume and ratio and gives the exact compost quantity, so you can scale from a bucket to a barrel.
Why add molasses to compost tea?+
Unsulphured molasses is a simple sugar that feeds the microbes during the brew, helping bacteria multiply quickly. A typical dose is a small amount of molasses per litre of brew water. Too much can favour the wrong organisms and turn the tea anaerobic, so the calculator keeps the dose to a sensible ml-per-litre figure.
How long should I brew compost tea?+
Brew for about 24 to 36 hours with an air pump running continuously to keep the tea aerobic. Less time and the microbes haven't multiplied; much longer and the food runs out and populations crash. Use the tea fresh — ideally within a few hours of stopping the pump — because the microbes start to die once aeration stops.
How do I dilute compost tea?+
Dilute the finished concentrate with non-chlorinated water before applying, commonly several parts water to one part tea for a foliar spray, less for a soil drench. The tool reports a diluted volume so you know the total spray you'll have to work with and can match it to your area.
How much area does a brew cover?+
Area covered = diluted volume ÷ your application rate. Foliar sprays use a light rate per unit area; soil drenches use more. Enter your rate and the tool shows the area one brew will treat, so you can plan how many brews a field or garden needs.
What compost and water should I use?+
Use mature, sweet-smelling aerobic compost or worm castings — never raw manure or anaerobic, sour-smelling material. Use non-chlorinated water, because chlorine kills the microbes you're trying to grow; if you only have tap water, let it sit or aerate it first to off-gas the chlorine.
Is compost tea ever risky?+
Yes — a brew that goes anaerobic or sits too long after the pump stops can grow human pathogens, and adding molasses to a poor brew makes this worse. Keep the tea well aerated, use clean mature compost, and apply it fresh. Avoid foliar-spraying edible parts close to harvest with tea of unknown quality.
How often should I apply it?+
Many growers apply compost tea every two to four weeks through the growing season, as a soil drench to build biology and as a foliar spray to coat leaves. It complements, rather than replaces, your compost and fertiliser programme — think of it as topping up the soil's microbial life.