What is Leachate / Liquid Fertiliser / Compost Tea?

Thursday, 15 August 2019  |  HOTBIN Composting

Water is released during the composting process. In the HOTBIN, much of this water is evaporated and leaves as steam/water vapour 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 a liquid fertiliser (leachate) and 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 liquid with the HotBin?

Collecting the HOTBIN LeachateYes.

The amount of liquid you will get will vary depending on the types of waste that are added to the HOTBIN.

Be aware things can change dramatically when high water content food waste is added. Each 3-4kg (small 5 litre caddy) of food waste can contain up to 2 litres of water (i.e. a large mixing jug).

If the HOTBIN is not operating at hot composting temperatures 40-60°c, this excess water is not evaporated as steam/water vapour and will drain to the base of the HOTBIN and out through the mesh plate (HOTBIN Mk.1 model) or collect under the base plate ready for draining (HOTBIN Mk.2 and HOTBIN Mini models).

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

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

Yes - you can collect the liquid fetiliser (leachate) and pour onto your plants and soil.

The liquid created by the HOTBIN does not have a high NKP (Nitogen, Potassium, Phosphorus) value so can be used as a fertiliser. The brown odourless liquid will be mainly Fulvic Acid and Humic Acid - both improve the uptake of minerals and can stimulate plant growth. 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 ‘why does the leachate smell’ below.

How do I collect the liquid fertiliser (leachate)?

With the HOTBIN Mk.2 & HOTBIN Mini the liquid fertiliser (leachate) will collect in the bottom of the HOTBIN under the aeration base plate, this can then be drained off by unscrewing the cap at the bottom of the bin on the right-hand side. You can put a shallow container (a takeaway tub is ideal) under the tap and collect the liquid. You should drain the liquid regularly to avoid too much liquid collecting in the base and spilling out through the mesh plate.

Do not worry if you have a HOTBIN Mk.1 you can still collect the liquid fertiliser (leachate) it will just require more of a DIY solution. Using a shallow container put this if front of the mesh plate at the bottom of the HOTBIN in the middle. Set the tray on the ground and push tight to the mesh plate. Alternatively get a sponge on a tray and set this against the mesh plate (be careful not to fully block the mesh), this will absorb the drips.

You could also place the HOTBIN on a slab and raise this up using some bricks, this allows you to place the container slightly under the HOTBIN ensuring all the liquid is collected including drips. You could also raise the HOTBIN Mk.2 & HOTBIN Mini models up to gain easier access to the collection unit.

You can also let the leachate drain away to soil if this is where the HOTBIN is situated.

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







Should I dilute liquid fertiliser (leachate) produced by the HOTBIN, what dilution?

No. You do not need to dilute the liquid fertiliser (leachate).

As the liquid fertiliser (leachate) from the HOTBIN does not have a high value of NKP it will not burn the plants and therefore can be used neat. However, if you wish to dilute this to make if go further you can do so.

If your liquid fertilser (leachate) has an odour this an indication that it is anaerobic due to depletion of oxygen, you can leave this it to stand so the bacteria can complete the digestion of the intermediary organic chemicals.

Why does my liquid fertiliser smell? What do I do if it smells?

If the liquid produced by the HOTBIN smells fruity, acidic, or is pale yellow rather than dark brown these are good indicators you have excess water in your HOTBIN and the water is leaching intermediary organic acids out as it drains to the base. These acids are produced in all forms of composting as waste is broken down into ever smaller compounds that bacteria can utilise.  

We do not recommend you apply such liquid to plants. Many of these acids can be phytotoxic (bad for plants). If you have collected such liquid, always leave it for several weeks until the bacteria (present in the liquid) consume the acids - it will turn odourless and is then safe to use on your garden.

Liquid that smells is also an indication that the contents of your HOTBIN may be too wet and requires attention to stop the contents from turning anaerobic! You can remedy this by adding in extra shredded paper/cardboard to absorb the moisture and extra woodchip to provide additional aeration in the HOTBIN.

If you have a black liquid that has a pungent drain like odour - the contents of your HOTBIN has gone anaerobic. See how to fix an anaerobic HOTBIN here.

Will the liquid stain my patio?

Whilst the liquid may leave a brown mark on a patio, this will wash off in the rain or with a stiff brush (and washing up liquid if necessary) 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 the unit and should not be confused with liquid seeping from behind the mesh plate.

What is Compost Tea?

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. Whilst there are some aerated compost tea processes, most compost teas occur as a result of anaerobic digestion (e.g. Bokashi liquid).