How to make compost tea and when to use it

March 13, 2025

How to make compost tea and when to use it

Compost tea sounds a lot more technical than it is, because all you're really doing is soaking finished compost in a bucket of water so the microbes and the soluble nutrients come out into the water, and then pouring that water onto your garden. It costs almost nothing, you can make it with things you already have lying about, and it's a reasonable way to stretch a bag of compost further than it would go if you just dug the lot in.

What it does, and what it won't do

What you're getting is a mild liquid feed with a load of soil microbes in it, and the honest way to describe it is that it helps a soil that's already got some organic matter in it get going a bit quicker. It is not a fertiliser in the sense that a bag of blood and bone is, so if your soil is genuinely hungry then compost tea on its own won't fix that, and it is definitely not a spray for pests or disease however often you see it written up that way. Feed the soil with it, keep your expectations sensible, and it earns its place.

What you need

Finished compost, meaning dark, crumbly and no longer recognisable as scraps
A bucket, and about 20 litres is the easy size to handle
Water, either rainwater or tap water left standing overnight so the chlorine gasses off
An old pillowcase, a mesh bag or a stocking to hold the compost
A splash of molasses if you've got some, which gives the microbes something to eat while they multiply

How to brew a batch

Fill the bag with compost and tie the top, so you're steeping it rather than tipping loose compost in.
Drop the bag into the bucket of water and give it a good squeeze and a stir.
Add the molasses if you're using it.
Leave it somewhere warm and out of direct sun for 24 to 48 hours, and stir it whenever you walk past, because the stirring puts air back in and air is what keeps the good microbes ahead of the bad ones.
Lift the bag out, tip the spent compost onto a garden bed, and use the liquid the same day.

Where to pour it

Use it as a soil drench, straight onto the ground around the root zone rather than over the foliage, and water it in the way you'd water anything else. For seedlings, water it down to about half strength first, because they don't need it at full whack and there's no benefit in pushing them. You'll see compost tea recommended as a foliar spray all over the internet, and I'd leave that alone on anything you're going to eat raw, because compost is made from raw organic matter and spraying it onto salad leaves is a food safety problem you don't need to have.

The mistakes that waste a batch

Using compost that isn't finished. If it still smells sour or you can tell what it used to be, it isn't ready and neither is the tea.
Letting it go anaerobic. If the bucket smells rotten rather than earthy, the oxygen has run out and the microbes you wanted are gone, so tip it on the compost heap and start again.
Storing it. It starts dying the moment you stop stirring it, so it's a same-day job and there's no point making more than you can use.
Going straight from the tap. Chlorinated water knocks off a fair share of what you've just spent two days growing, and letting it stand overnight costs you nothing.

Shop related products

Brunnings Ultimate Dig It In Compost 30L. A finished compost, so it's ready to brew with or dig straight in.
Naturally Good Mushroom Compost 25L. Gentler and good on beds that have had a hard season.
Go Go Juice 1 Litre. A bottled soil probiotic for when you haven't got two days to wait.
Brunnings Blood and Bone Based Fertiliser 5kg. The actual feed, for a soil that needs more than a tonic.

All of it sits in our garden centre, and if you're not sure whether your soil wants feeding or just waking up, bring a handful in and we'll talk it through.

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How to make compost tea and when to use it