Free compost tea calculator. Enter batch litres to find the compost and molasses you need for a home compost tea brewing batch.
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Enter the batch size in litres. This calculator plans 100 grams of compost and 15 ml of molasses per litre, so a 20 litre batch needs 2 kilograms of compost and 300 ml of molasses. Put the compost in a mesh bag, aerate or stir for the brewing time below, then use the tea immediately, since compost tea has almost no shelf life once aeration stops.
Compost per litre in this calculator is 100 grams, and molasses per litre is 15 ml. A 20 litre batch therefore needs 2000 grams, or 2 kilograms, of compost and 300 ml of molasses, while a 5 litre batch needs 500 grams and 75 ml. Recipes vary widely: one common guide uses 1 litre of compost per 10 litres of water, and aerated guides often use far less molasses, about 1 to 2 teaspoons per gallon. Treat this page as a planning ratio, and scale down the molasses for a lightly fed aerated brew.
Aerated compost tea, often called actively aerated compost tea, uses an aquarium pump and air stone to keep oxygen in the brew for 24 to 36 hours. Steeped compost tea simply soaks compost in water for 2 to 3 hours with stirring, or 12 to 18 hours in a cloth bag. Aeration grows a larger microbe population, but it needs a pump, while steeping needs no equipment and suits a quick batch. Both methods extract nutrients, humic acids and microbes from compost into water.
Chlorine in tap water harms the microbes compost tea is meant to grow, so guides insist on dechlorinated water. Rainwater and well water suit brewing directly. Tap water left uncovered for 24 hours loses much of its chlorine by off gassing, which is the usual home method. Use water at room temperature and keep the brew out of direct sun.
Brewing time for aerated tea is usually 24 to 36 hours, with some guides extending to 48 hours in cool weather. Steeped tea is ready in 2 to 3 hours with regular stirring. A warm spot near 25 degrees Celsius suits brewing, and the finished tea should smell earthy, like good garden soil. A sour, sulphurous or rotten smell means the brew went anaerobic: discard it and start again with fresh compost and clean equipment.
Compost tea dilution depends on how you apply it. As a soil drench, guides use it undiluted or diluted 1 to 5 with rainwater, every 1 to 2 weeks. As a foliar spray on leaves, dilute 1 to 10 and strain it through cloth first so sprayer nozzles do not block. As a soil conditioner, use it undiluted or at 1 to 5 every 2 to 3 weeks. One permaculture guide gives an application rate of at least 5 gallons per acre, up to 20 gallons per acre, for field use.
Molasses feeds bacteria, so a molasses rich brew leans bacterial, which suits annual vegetables and grass. Fungal teas for trees and perennials use less molasses and more fungal foods. Optional extras in competitor recipes include kelp meal or soluble kelp, humic acid, and fish hydrolysate, usually in small amounts, since more is not better with kelp. Worm castings, or vermicompost, make a popular base because they are fine, mature and microbe rich.
Take a 20 litre batch. Compost is 20 times 100 grams, which is 2000 grams, and molasses is 20 times 15 ml, which is 300 ml. Brew it aerated for 24 to 36 hours, remove the bag, and use it within 4 hours of stopping aeration, since oxygen drops fast and the microbe balance shifts once the pump stops. Do not store brewed tea for days, and for hygiene, stop spraying compost tea on vegetables or fruit about to be harvested at least 3 weeks before harvest.
This calculator plans 100 grams per litre, so 20 litres needs 2 kilograms. Many guides use about 1 litre of compost per 10 litres of water instead.
This calculator plans 15 ml per litre. Aerated recipes often use less, about 1 to 2 teaspoons per gallon, so scale down for a lightly fed brew.
Aerated tea brews for 24 to 36 hours with continuous aeration. Steeped tea is ready in 2 to 3 hours with stirring.
Aerated tea grows more microbes but needs a pump. Steeped tea is simpler and suits a quick batch, with less microbe growth.
Dechlorinated water: rainwater, well water, or tap water left uncovered for 24 hours so chlorine can off gas.
For a foliar spray, dilute about 1 to 10 with rainwater and strain it first. For a soil drench, use it undiluted or at 1 to 5.
It has almost no shelf life. Use it within 4 hours of stopping aeration, and discard it if it smells sour or rotten.
Discard it. A sour or sulphurous smell means the brew went anaerobic, and it should not go on plants. Start again with fresh compost and clean gear.