What is compost leachate?

Tuesday, 4 February 2014  |  HOTBIN Composting

Water is released during the composting process. In the HOTBIN, much of this water is removed as steam through the valve in the lid. However, when there is excess water it will drain down towards the ground, as it does, it dissolves humic compounds from the compost to form a dark brown liquid. This liquid is commonly called leachate. (it is sometimes called compost tea - but this term is more accurately used to describe a digester process where all waste is converted to liquid - see compost tea section at end of post).

Do I get leachate with the HotBin?

Yes. You normally only get a small amount - a tiny dribble totalling a few mm a day (maybe a cup in a week).

Collecting the HOTBIN Leachate

Be aware things can change dramatically when high water content food waste is added. Each 5 litres (small caddy) of food waste will have 3-4 Kgs of waste which can contain 2 litres of water (i.e. a full large mixing jug).

If the HOTBIN is not hot (40-60C), this water is not evaporated as steam and it will drain to the base of the HOTBIN and out through the mesh plate (not leachate). Sometimes this can be very obvious (e.g. a pool of liquid on the patio stones) and sometimes not - drains into soil.

Although we do recommend the HOTBIN is put on a slab or hard surface).

Excess water can lead to the compost/waste becoming saturated (water logged) and this prevents effective aeration - so always investigate and check excess water using the FAQ. An excess of water is not a good thing in a HOTBIN and will need remedial action.

Can I collect the leachate and use it as a fertiliser?

Yes - you can collect it and pour it on to your soil it is good for your soil. and plants. (For the technical gardeners - leachate is is not a high value 'NKP fertiliser'. The liquid draining down will not leach large amounts of nitrogen, potassium or phosphorus from the compost. The brown odourless liquid will be mainly fulvic acid and humic acid - both key humic substances and good for the soil as they hold nutrients and release them). Ensure you have collected an odourless, dark brown liquid. take care if you have collected a fruity / acidic yellowish liquid, or a black pungent drain like thick slimy goo. (see table at end)

Should I dilute HOTBIN leachate, what dilution?

No you do not need to dilute the dark brown odourless leachate. (Compost leachate does not have a high value of NKP so does not burn plants - it is manly humic substances).

The answer is is different for ‘compost teas’ (see below) where the whole NKP (ie nitrates, potassium salts and phosphates) from the food are in the tea solution, then diluting 1:10 or 20 is well advised as concentrated fertiliser can burn plants.

If your leachate has odour – do not add it around plants. it is potentially toxic to plants – dilution is not the answer – leaving it to stand for a period of time so the bacteria can complete the digestion of the intermediary organic chemicals is the answer.

How do I collect the leachate?

Many people collect the leachate from their bin and you can set up your own simple DIY system. One way is using a small plastic tray, set this on the ground and push tight to the mesh plate. Connect the two using putty (or another waterproof flexible membrane), ensure the joint is at the bottom of the mesh plate (see photo). Alternatively get a good sponge at set this against the grill (do not fully block the grill). This will absorb the water drips. Some customers also set the HOTBIN onto a stone plinth. And a tray under the stone. You can also let the leachate drain away to soil.

Use a small tray to collect leachate Leachate collected using a sponge






Why does my compost leachate smell? What do I do if it smells?

If your leachate smells fruity, acidic, or is pale yellow these are good indicators you have excess water in your compost and the water is leaching intermediary organic acids out as it drains out the base. These acids are produced in all forms of composting as waste is broken down into ever smaller compounds that bacteria can utilise. They indicate your waste is active (ie immature). We do not recommend you collect or apply such liquid to plants. Many of these acids can be phytotoxic (bad for plants). If you collect such a liquid, always leave it for several weeks until the bacteria (still in the liquid) consume the acids - it will turn odourless. It is also a sign your heap is too wet and needs attention! Always investigate and check excess water if your compost is too wet.

If you have a black liquid that has a pungent drain like odour - your heap has gone anaerobic. Again leave any liquid like this for several weeks before using it.

Will the leachate stain my patio stones?

Yes the liquid will leave a brown stain - but it is only temporary. This will wash off in the rain or when washed with over with water and a stiff brush as it is water soluble. Please note you will often see small amounts of water and wetness around the base of the bin - especially in the morning - this is just the steam condensing and running down from the valve/lid area and should not be confused with brown liquid seeping from behind the mesh plate.

What is Compost Tea?

The terms leachate and compost tea are often mixed up. Compost tea is used to describe the liquid produced during anaerobic digestion where the goal is to convert all the waste to liquid. This liquid contains all the original plant nutrients but very little humic compounds. (We accept there are some aerated compost tea processes, but most teas occur as a result of anaerobic digestion (eg Bokashi liquid).

Can I spray compost leachate and or compost teas onto plants (ie use as foliage spray)?

There are lots of contradictory story of good results and bad results. This probably comes down to the lack of precise analysis of the chemicals being sprayed. There are 100s of intermediate products produced during the decomposition of waste before you reach compost or a solution containing liquid nutrients (nitrates, potassium salts, phosphates). Some compounds have been found to be good (eg foliar sprays of fulvic acid), and some bad (Phytotoxic organic acids). Unless you know exactly what you are spraying, it is much safer to add liquids to the soil where bacteria can act and finish anything unfinished.

Summary table of liquids and sources

Anaerobic Process Both - if not allowed to complete Aerobic Process
Compost tea,

Anaerobic Digestion solution

Immature liquids – take care

Compost leachate, Worm tea

Light to mid brown, water/thin liquid, odourless.

colourless to pale yellow liquids with odour ranging from fruity, acidic and fragrant

Dark brown, thick liquid, odourless

Lots of liquid


Small amounts of liquid in HOTBIN

Liquid is water plus soluble nutrients from the food waste


Liquid is water plus soluble humic substances leached from the compost

Are compost teas and leachate a breeding ground for pathogens?

There are good bacteria (composting) and bad (pathogens) in any waste / compost pile. There are many discussion posts about the potential growth of e-coli in compost teas. From our perspective this is the same debate with any compost system. Good & bad bacteria compete to eat food. The bad guys loose in a compost heap as they are not ‘experts’ at living in compost heap they are experts at living in the digestive tract and stomach / human body. The bad guys loose faster the higher the temperature and the longer the time period.

Can Compost teas prevent plant diseases?

Rodale Institute have done a fair bit of work on aerated compost teas (ACT). Our summary of their summary: overall, the data suggests there is a lot still to learn about the use of compost tea, whether in organic or conventional systems. The widely divergent results in the three crops studied here suggest that it is difficult, if not impossible, to generalise about the efficacy of compost tea for disease suppression across all crop species.

The folks at WoodsEnd laboratories also have a good set of reports on Compost tea.