Why Compost Tea Earns Its Place on the Plot

Finished compost is the finest thing you can give your soil, but there is a limit to how much you can fork in around a single tomato plant in a grow bag or a courgette in a pot on the patio. Compost tea solves that particular problem. Steep compost in water and you draw out the soluble nutrients and a good deal of the microbial life, giving you a liquid you can pour directly onto the roots of plants that need a lift. It is not a replacement for a balanced fertiliser, and anyone who tells you it will double your runner beans is overselling it. But as a gentle, regular tonic through the growing season it is hard to beat, and it costs you nothing beyond a bucket and a little patience.

What You Will Need

There is no need for specialist kit. An old pillowcase, a hessian sack or a pair of tights will do perfectly well for holding the compost, and a clean bucket or water butt is all the vessel you need.

• A bucket or trug, roughly 10 to 20 litres
• A sack, pillowcase or length of hessian, plus string
• Two or three good handfuls of finished compost or worm castings
• Rainwater from a butt, ideally, or tap water left to stand overnight
• A stick for stirring, and a second bucket for straining

Use compost that is genuinely finished — dark, crumbly and smelling sweetly of woodland floor. Anything sour, slimy or made from diseased plant material stays out of the brew. If your water company uses chloramine rather than chlorine, leaving tap water to stand will not shift it, so rainwater is by far the safer bet.

Brewing a Simple Batch

Half fill your sack with compost, tie the top and drop it into the bucket. Cover with water — rainwater if you have it — and leave it somewhere sheltered and reasonably warm, such as the greenhouse or beside the shed. Give it a stir each day with your stick, which keeps the brew moving and stops it turning stagnant.

In high summer, twenty-four to forty-eight hours is plenty. In cooler weather, give it two or three days. What you are after is a liquid the colour of weak tea, with a pleasant earthy smell. If it develops a sour, sewage-like whiff, or a film across the surface, tip the lot onto the compost heap and start again — that is the brew having gone anaerobic, and it will do more harm than good.

When it is ready, lift out the sack and let it drain, then strain the liquid through a second sack into a clean bucket. The spent compost goes on the border.

Diluting and Applying

Never use it neat on growing plants. Dilute roughly one part tea to five parts water for established hungry crops, and one part to ten for seedlings, young plants and anything in a small pot. Think of it as the colour of weak tea again once mixed.

Water the soil rather than the foliage, ideally in the evening or early in the morning when the ground is already moist. Compost tea is at its best fresh, so use it within a few hours of straining — it does not keep, and a bottle left in the shed for a week is a bottle of something else entirely. Feed fortnightly through the season, and weekly for tomatoes, peppers and aubergines in containers or the greenhouse border.

One word of caution on hygiene: if your compost includes manure, avoid drenching salads, herbs or soft fruit you intend to eat raw within a fortnight of picking. Water the ground around them, not over the leaves, and wash your harvest as usual.

Is an Aerated Brew Worth the Fuss?

You will see a great deal written about bubbling air through the brew with a pump to multiply the beneficial microbes. It is a genuine technique, but it is not necessary for the average allotmenteer. A simple steeped brew, used fresh and diluted, gives you most of the benefit for none of the faff. If you already own an aquarium pump and fancy experimenting, by all means try it — just do not let the lack of one put you off making tea at all. The plants cannot tell the difference nearly as clearly as the internet suggests.

Which Crops to Feed, and When

Compost tea suits anything that is working hard. Tomatoes, courgettes, squashes, cucumbers, runner beans and brassicas all respond well, as does anything growing in a container where the roots cannot go looking for extra. Leafy crops such as chard and kale enjoy a mid-season boost too.

Start feeding once plants are growing strongly, usually from early June, and carry on into August. Hold off on newly planted seedlings until they have settled in, and stop feeding tomatoes a few weeks before the season ends so the last fruits ripen rather than the plant putting on more leaf. Keep a bucket brewing on rotation through the summer and you will always have something to hand when a plant looks pale, tired or simply in need of a drink with a bit of goodness in it.