Summary
Compost leachate is the brown-yellowish liquid that drains out of a compost bin when the contents are wetter than the bin can hold. It is not the same thing as ‘Compost tea’, ie the brown-yellowish liquid made by soaking compost in water and then using the brown-yellowish water.
Neither is a consistent, predictable fertiliser. In a well‑running bin, leachate is usually something to avoid collecting in the first place.
What compost leachate actually is
Leachate is not “compost juice” in the way most people imagine. It is simply water that has moved through partly decomposed material and picked up whatever is easy to dissolve or carry with it.
That means leachate can contain a mixture of:
- dissolved salts and nutrients (sometimes useful, sometimes too strong)
- organic acids and other by‑products from oxygen‑limited patches
- very fine particles washed out of the composting mass
Because composting happens unevenly inside a bin, the liquid draining out reflects whatever zones the water passed through. Two bins fed with the same kitchen scraps can produce very different leachate on different days.
What people mean by “compost tea” (and why the term causes confusion)
The phrase “compost tea” is used loosely for several very different liquids. Lumping them together is where most confusion starts.
In practice, people are usually talking about one of three things:
- Plant-based soaks – leafy plants such as comfrey soaked in water until they break down. This produces an oxygen‑limited liquid similar to a digestate. It is often called compost tea, but it is not made from compost at all.
- Compost water extracts – compost mixed with water, then strained so the liquid can be poured onto soil. This dissolves some fine particles and soluble nutrients, while leaving larger fragments behind.
- Accidental low‑oxygen brews – compost left sitting in water for too long, especially without movement, where the liquid turns sour or smells unpleasant.
All three are liquids. All three are sometimes called compost tea. Biologically and practically, they behave very differently.
The shared name creates a false sense of consistency. It sounds like a defined product, when in reality it describes a range of liquids whose behaviour depends on what was soaked, how wet it became, and whether oxygen was present.
How compost leachate is claimed to work, versus the practical reality
The common claims
You will often see claims like:
- “It’s a free liquid fertiliser.”
- “It’s full of beneficial microbes.”
- “It feeds plants instantly and boosts growth.”
These claims feel attractive because they turn a messy by‑product into a benefit.
The practical reality in a compost bin
A compost bin is a living decomposition mass with wet and dry pockets, airy zones and oxygen‑limited micro‑zones. Liquid forms when moisture builds up faster than it can be held and evaporated.
So, leachate usually appears when conditions have drifted in the wrong direction:
- too much wet input (food scraps, fruit, coffee grounds) relative to structure
- compaction and loss of air pathways
- rainfall entering an open or poorly covered bin
In other words: leachate is often a sign of excess moisture and reduced airflow, not a sign that the system is producing a premium plant feed.
When it can be useful
Leachate can sometimes act like a rough, fast‑acting feed because it may contain soluble nitrogen and potassium. However, its strength is unknown and can swing from weak to harsh.
It tends to be most usable when it is:
- produced in small amounts
- not foul‑smelling
- coming from a bin that is not waterlogged and not sour
Even then, it behaves more like an unpredictable liquid feed than a stable fertiliser.
When it should not be used as a fertiliser
Leachate is a poor choice when it shows signs of oxygen‑limited breakdown or concentrated load:
- strong sour, rotten, or “drain” odours
- thick, sticky liquid with lots of fine solids
- large volumes after heavy rain or prolonged wet loading
In these situations, the liquid is more likely to carry organic acids and reduced compounds that plants do not enjoy, especially in pots or on young seedlings.
Why leachate shows up in some bins and not others
Leachate is mostly about water behaviour.
Bins that shed rain, hold structure, and allow air exchange tend to produce little or no leachate. Bins that trap water, compact easily, or are fed heavily with wet materials tend to produce more.
A useful mental model is this:
- Good composting is mostly aerobic biology.
- Aerobic biology needs air pathways.
- Too much water blocks those pathways.
- Leachate is what happens when the system has more water than it can carry.
So the long‑term “solution” is rarely about finding the perfect way to use leachate. It is about keeping the bin in a moisture range where leachate barely appears.
A practical way to think about using it (without overthinking it)
If you choose to use leachate on plants, treat it as a strong, unknown concentrate rather than a gentle tonic.
That leads to four simple, practical rules of thumb:
- Treat it as a concentrate and assume it is stronger than it looks.
- If you dilute it, keep it heavily diluted (many gardeners use at least ten parts water to one part leachate as a safety margin).
- Use it only on established plants, not seedlings or cuttings.
- Prefer soil beds over pots, because pots concentrate mistakes.
If it smells bad, treat it as a warning sign from the bin rather than something to apply to plants.
This keeps the focus where it belongs: on composting conditions first, liquids second.
Rather than trying to replace leachate with another liquid product, it is usually more helpful to recognise when claims about liquids are drifting beyond what compost bins reliably produce.
How to spot marketing drift in leachate and tea claims
A simple test: if a product description makes leachate sound like a guaranteed benefit, it is skipping the messy reality of bins.
Leachate is not a consistent output. It changes with:
- what went into the bin
- how wet the mix became
- where oxygen was limited
- how long material has been decomposing
So any claim that treats it like a standardised fertiliser is overconfident.
Bridge: where to go next
If you want to reduce leachate at the source, the next page is:
- What materials can be composted (and which ones push moisture too far)
If you are deciding what kind of system fits your household, go to:
- Choosing a compost bin (which designs cope best with wet inputs and rainfall)
