Compost Tea: How to Make Liquid Fertiliser from Compost

If you have a compost heap working away at the bottom of your garden, you are already doing something genuinely useful for the environment and your soil. But there is a way to take that hard-won compost and multiply its benefits many times over – by turning it into compost tea. This liquid fertiliser is simple to make, costs almost nothing, and delivers a concentrated hit of nutrients and beneficial microorganisms directly to your plants’ roots and leaves. It is one of the most practical things a home gardener can do with finished compost, and once you have tried it, you will wonder why you ever bought bottled plant feed from the garden centre.

This guide covers everything you need to know: what compost tea actually is, why it works, the two main methods of making it, how to apply it safely, and a few common mistakes to avoid. Whether you are growing vegetables on an allotment in Yorkshire, tending raised beds in a Cardiff back garden, or managing a few pots on a London balcony, compost tea can make a real difference.

What Is Compost Tea?

Compost tea is exactly what it sounds like: water that has been steeped with finished compost, drawing out its soluble nutrients and – depending on how you make it – its microbial life. Think of it as a liquid extract of everything good in your compost heap, in a form that plants can absorb almost immediately.

There are two distinct types of compost tea, and understanding the difference matters.

Passive compost tea (sometimes called compost leachate or cold-brewed compost tea) is made simply by soaking compost in water for a day or two. It is low-tech, requires no equipment beyond a bucket, and is perfectly useful as a general nutrient feed. However, because the water is not aerated, the brew is dominated by anaerobic bacteria, which can include some less desirable microorganisms if your compost was not fully finished.

Actively aerated compost tea (AACT) involves pumping air through the mixture continuously during the brewing process. This encourages aerobic bacteria, fungi, protozoa, and nematodes to multiply rapidly, creating a genuinely microbe-rich liquid. Aerated compost tea is considered the more effective of the two by most serious gardeners and soil scientists, though it does require a small amount of equipment.

Both methods have their place, and this guide will walk you through each one in full.

Why Bother? The Benefits of Compost Tea

Solid compost is excellent, but it acts slowly. When you dig it into beds or use it as a mulch, nutrients are released gradually as the material breaks down further. Compost tea bypasses that waiting period. Because the nutrients are already dissolved in water, they are available to plants almost immediately – useful when you spot a deficiency mid-season and cannot wait weeks for solid compost to do its work.

Beyond nutrients, the real value of aerated compost tea lies in its microbial content. Healthy soil is teeming with billions of microorganisms: bacteria that fix nitrogen and suppress disease, fungi that help roots access water and phosphorus, protozoa that release nutrients by feeding on bacteria, and nematodes that control pest populations. Intensive gardening, heavy rainfall, and repeated digging all deplete these communities. Compost tea helps restore them, effectively re-inoculating your soil with the biology it needs to function properly.

There is also good evidence that applying compost tea as a foliar spray – directly onto leaves – can help suppress fungal diseases such as powdery mildew and botrytis. The beneficial microbes colonise leaf surfaces and outcompete pathogenic fungi for space and resources. It is not a guaranteed cure, but many gardeners find it a useful part of their disease management routine.

Finally, there is the cost argument. A quality organic liquid fertiliser from a UK garden retailer can cost £8 to £15 per litre. Compost tea costs you almost nothing if you already have compost, a bucket, and access to tap water.

What You Will Need

The shopping list is short and largely made up of things you may already own.

For passive compost tea:

• A clean bucket (10-20 litres is ideal)
• Finished, mature compost – roughly one part compost to five parts water
• A piece of old hessian sacking, muslin, or a purpose-made compost sock to act as a straining bag
• Rainwater if possible, or tap water left to stand for 24 hours to allow chlorine to dissipate
• A watering can or garden sprayer

For actively aerated compost tea:

• Everything listed above
• An aquarium air pump (a basic model costs around £8-£12 from pet shops or online; Interpet and Blagdon are two widely available UK brands)
• Airline tubing and an air stone (these are usually sold together as a kit for fish tanks)
• A source of food for the microbes – unsulphured blackstrap molasses is the most commonly used option (roughly one tablespoon per 10 litres of water); some gardeners use liquid seaweed extract or fish hydrolysate instead

You do not need to buy specialist compost tea brewing kits, though they do exist. Several UK companies including Wiggly Wigglers sell purpose-built brewers, but a standard aquarium pump and a clean bucket work just as well for home use.

Choosing the Right Compost

The quality of your compost tea depends entirely on the quality of your compost. This is not a method for rescuing poor or unfinished material – it is a way to amplify the benefits of compost that is already good.

Finished compost should be dark brown, crumbly, and smell earthy – similar to woodland soil after rain. It should not smell sour, ammonia-like, or like rotting food. If your compost smells unpleasant, it has not finished decomposing and likely contains anaerobic bacteria or partially broken-down material that could introduce pathogens into your brew.

If you are making aerated compost tea specifically for its microbial content, home-made compost from a well-managed heap is preferable to bagged compost from a garden centre. Bagged compost is typically heat-treated and may have a reduced microbial community. That said, even bagged compost will produce a useful nutrient tea – it simply may not have the same biological richness as a good home brew.

Worm castings (vermicompost) are an excellent addition or alternative. They are extremely rich in beneficial microbes and nutrients. If you run a wormery, adding a handful of castings to your compost tea bucket is well worth doing.

How to Make Passive Compost Tea: Step-by-Step

1. Fill your bucket with water. Use rainwater from a water butt if you have one – it is free of chlorine and at a more neutral pH than most UK mains tap water. If using tap water, fill the bucket and leave it uncovered for 24 hours before adding compost. This allows the chlorine to off-gas naturally.
2. Fill your straining bag with compost. Use roughly two large handfuls of finished compost per 10 litres of water. Tie the bag securely and submerge it in the bucket, weighing it down with a clean stone if necessary.
3. Leave it to steep. Cover the bucket loosely with a piece of cloth or an old lid – not sealed tight, as some gas exchange is useful. Leave it in a cool, shaded spot for 24 to 48 hours. Avoid leaving it in direct sun, which can overheat the brew and damage microbial life.
4. Stir once or twice. Give the bucket a stir with a clean stick or spoon each time you pass it. This helps draw nutrients from the compost into the water more evenly.
5. Remove the bag and use the liquid. After steeping, lift out the bag, allow it to drain back into the bucket, and then squeeze gently. The remaining liquid should be a warm amber to light brown colour. Add the spent compost in the bag back to your heap or dig it directly into a bed.
6. Dilute before use. Passive compost tea should be diluted to a ratio of roughly 1:10 (one part tea to ten parts water) before applying it to plants. If the liquid is very dark, dilute it more. It should look like weak black tea.
7. Apply immediately. Use within a few hours of removing the compost bag. As the brew sits without aeration, the microbial balance can shift towards anaerobic conditions.

How to Make Actively Aerated Compost Tea: Step-by-Step

1. Prepare your water. As above – rainwater is best, or tap water de-chlorinated by standing for 24 hours. Fill your bucket to about three-quarters full.
2. Set up your aeration equipment. Place the air stone at the bottom of the bucket and connect it to the airline tubing and then to your aquarium pump. Plug the pump in and check that bubbles are rising vigorously through the water. The water should be actively churning, not just gently bubbling.
3. Add your compost. Either place the compost in a straining bag and lower it into the bucket, or add it loose. Adding it loose extracts slightly more microbial life, but straining the finished tea is messier. A compost sock – a tube of fine mesh – makes the process much neater and is reusable.
4. Add a microbial food source. Dissolve one tablespoon of unsulphured blackstrap molasses in a small amount of warm water and add it to the bucket. The molasses feeds bacteria and encourages rapid multiplication. Do not use sulphured molasses, as sulphur is toxic to many beneficial microorganisms. Liquid seaweed, available from UK suppliers like Maxicrop, works well as an alternative and also provides trace minerals.
5. Brew for 24 to 36 hours. Keep the pump running continuously throughout. The ideal temperature for brewing is between 15°C and 25°C – summer conditions in the UK are well suited to this. In cooler months, brewing in an indoor utility room or greenhouse will give better results. You may notice foam forming on the surface, which is a sign of healthy microbial activity.
6. Check the smell. A healthy aerated brew smells sweet, earthy, or faintly mushroomy. If it smells bad – sulphurous, sour, or like sewage – something has gone wrong. Discard that batch, clean your equipment thoroughly, and start again with better-quality compost.
7. Use within four hours of finishing the brew. Once the
   aeration stops, the microbial populations begin to decline rapidly. Apply the compost tea directly to soil or foliage without delay to get the maximum benefit from the living organisms it contains.

How to Apply Compost Tea

For soil drenching, pour the compost tea around the base of plants, watering it in so it reaches the root zone. Use roughly one to two litres per square metre, diluting with water if your brew is particularly concentrated or dark. For foliar application, strain the liquid through a fine cloth or old tights to remove any particles that might block a spray nozzle, then apply it to both the upper and lower surfaces of leaves in the early morning or late evening, avoiding the heat of the day. This reduces evaporation and gives the microbes time to colonise leaf surfaces before the sun dries them off.

Compost tea works best as part of a regular feeding programme rather than a one-off treatment. Applying it every two to four weeks during the growing season — from late spring through to early autumn in most parts of the UK — gives plants a steady supply of nutrients and helps maintain healthy soil biology over time. It is particularly useful when establishing new beds, transplanting seedlings, or giving struggling plants a boost. Avoid applying it immediately before heavy rain is forecast, as a downpour will simply wash it away before the microbes have had a chance to establish.

Conclusion

Compost tea is a practical, low-cost way to make more from the compost you already produce. Whether you opt for a simple cold-water steep or a fully aerated brew, you are putting the biological activity of your compost to work in a form that plants can absorb quickly and directly. It requires little specialist equipment, fits naturally into an existing composting routine, and reduces the need for bought-in fertilisers. With a modest amount of attention to temperature, aeration, and timing, most gardeners across the UK will find it a straightforward and rewarding addition to their growing practice.