“The problem we found with our manure bin,” my colleague, Matt, said, “is a dark brown material liquid out of the holes and staining the floor.”
We were discussing using homemade compost bins to heat polytunnels in winter.
My colleague had successfully heated his polytunnel over the winter with manure and some garbage bin composters.
But the drawback was a liquid spilling all over the floor!
That liquid is called leachate.
What is leachate?
Leachate is simply the liquid that escapes from your compost.
(At least sometimes. In the photo below, I placed a bowl to catch leachate under a bin, only for none to appear!)
Leachate can be dark, light or yellow in appearance. It can smell pungent – or not smell at all.
The liquid is full of nutrients and can be a valuable plant fertilizer.
But there’s also a chance that it might contain harmful substances, so it’s worth handling with care. We’ll cover that further down this article.
What about worm leachate?
As you might guess, worm leachate is the liquid that leaks from a worm bin.
Worm bins are usually designed to deal with leachate.
Some come with a tap which allows you to easily capture it, while my homemade worm bin is simply set above a container to catch escaping leachate.
How does compost make leachate?
As we saw in the science of composting, compost is made when a combination of bacteria, fungi, and other organisms breaks down organic material.
In the process, water is released.
This water trickles down through the compost. As it does so, it picks up various substances – including nutrients but also potentially pathogens and heavy metals in the compost.
How do I reduce the amount of leachate produced?
As leachate contains nutrients that can be lost from compost, you may want to minimize the amount produced.
To do so:
- ensure that your compost is at the correct moisture level
- balance very wet compost materials with drier ones
- protect from rainfall
- avoid adding too much water
All of this is good practice in any case, as too much water slows down the composting process.
If you follow the five rules of hot composting, you will also find that you get less leachate, as more of it will evaporate instead of leaching out of the bottom.
Learn more about moisture levels in composting.
Is Compost Leachate Good or Bad for Home Gardeners?
Nutritional value
Research has shown that leachate contains valuable nutrients, including carbon, nitrogen, phosphorus, potassium, and trace elements.
These nutrients can vary depending on the type of leachate.
Green waste (i.e. compost from plants) is likely to be the highest in potassium while manure compost tends to be the highest in phosphorous.
Leachate impact on plants and soil
One study found that leachate increased plant growth and improved the amount of available nitrogen in the soil.
However, it’s important to note that this looked at treated landfill leachate, which may be different from the leachate from your compost.
A separate study found that worm leachate had no impact on the growth of tomato plants unless applied as a foliar spray.
I’d like to see more research before drawing any firm conclusions!
Potential pathogen risk – but not always borne out by research
Many composters are concerned about the risk of pathogens from compost leachate.
The composting process is usually quite efficient at reducing the number of pathogens found in composting. Still, it’s true that some pathogens can survive, and leachate is also produced through the composting process.
However, the research is quite reassuring. For example, a study by Carlos et al found no pathogens in the leachate they tested.
Still, this research has focussed on worm leachate.
Worms do help reduce pathogens levels in compost, but there is one example of a worm farm in Wales that had salmonella and e-coli detected in leachate.
Heavy metals in leachate
Some studies have found that leachate can contain heavy metals.
However, most of these studies have focused on leachate from landfill, which is likely to be very different from the leachate from your compost.
Reassuringly, worms can reduce the harm of heavy metals – so if you are vermicomposting, or have plenty of worms in your compost, there may be even less need to worry.
Leachate’s impact on the environment
One concern about leachate is its impact on groundwater and rivers.
High nutrient levels can cause problems in water, such as algae blooms in rivers.
However, most of the research that demonstrates harm has focussed on landfills, which produce far more leachate than a home composting system and are more likely to contain toxic substances or heavy metals.
However, it’s still probably a good idea to protect compost heaps from rain, or use bins where you can collect leachate, in order to minimize any possible harm.
How to collect and use compost leachate at home
It’s not always possible to collect leachate – it depends on the design of your bin!
If you have a pallet bin, the size of the bin usually means it is not feasible to collect and use it.
However, many commercially available bins are designed to allow you to collect leachate. That includes my favorite – the HotBin.
If you make your own bin, you can drill holes in the bottom of the bin, place it on bricks and place a container underneath to catch the leachate. Here’s how!
How to Handle Leachate Safely at Home
While the research doesn’t suggest any major concerns, it’s worth bearing in mind that the compost they tested may be different from yours.
If you are composting animal or human waste, it’s worth taking extra care, as there could be an increased risk of pathogens.
It’s also true that if you have pests such as rats, you may have an increased risk of pathogens in the leachate.
If you have any concerns, wear some waterproof gloves when handling leachate, and wash your hands after you have finished handling it.
How do you use compost leachate?
Simply dilute it with water and use it as a fertilizer.
Most sources suggest diluting it by 10-20 times.
One study by Carlos et al found that, at least for Maize, a dilution of 50% worked best.
However, unless I was actually fertilizing Maize, I would stay safe and dilute it by 10 times for hungry plants like tomatoes and by 20 times for more sensitive plants.
This will avoid burning the plants with too many nutrients and also dilute any potentially harmful substances (phytotoxins).
If you have plenty of plants, you could also experiment with stronger leachate on one or two and see what the results are.
Is it actually worth collecting leachate?
Leachate produces a valuable fertilizer.
However, there are many ways to make fertilizer.
If you don’t have reservations about using urine, you can simply mix your pee and ash to make fertilizer.
I find this is one of the easiest ways to make fertilizer, and it’s also very plentiful (especially after a tip to the pub!) It also works like a dream, and its use is backed up by solid research.
Still, if your composting systems are already set up to collect leachate, it doesn’t take much extra work to turn it into a fertilizer!
The Difference Between Leachate and Compost Tea for Home Composters
Compost leachate is often confused with compost tea.
As we’ve seen, leachate is simply the liquid that leaches from compost heaps and bins.
In contrast, compost tea is produced by brewing microbes with air and water.
It’s time-consuming, controversial, and probably not worth the bother unless you have a lot of time on your hands!
Wrapping up
In summary, leachate is a valuable soil amendment that is full of nutrients.
Research suggests we don’t need to be too concerned about pathogens, although it’s still worth taking some basic precautions.
There are other ways to make fertilizer, but if you use leachate, and dilute it, it should help your plants and your soil.
Just make sure you don’t make the same mistake as my colleague Matt, and let it leak all over the floor!
FAQs
What is the composition of compost leachate?
The composition of compost leachate varies depending on the materials used in the compost pile. Generally, it contains a high level of organic matter and nutrients including nitrogen, phosphorus, and potassium. It can also contain trace minerals that are essential for plant growth.
What can you do with compost leachate?
Compost leachate can be used as a liquid feed for plants. Dilute with water by 10-20 times and water around the plants you want to fertilize.
What is the difference between compost leachate and compost tea?
Compost leachate and compost tea are both liquids, but they’re produced in different ways. Compost leachate is the liquid that naturally drains from the bottom of your compost pile or bin as organic material breaks down.
Compost tea can refer to two things; compost steeped in water to extract its nutrients or a liquid that is ‘brewed’ with the help of an aerator. See our guide to compost tea for more detail.
If I can’t use all my compost leachate, how should I dispose of it properly?
If you have more compost leachate than you can use, it’s important to dispose of it properly since it may contain high levels of nutrients that could be harmful to waterways. One option is to pour it back onto your compost pile, which can help keep the compost pile moist and speed up the composting process. However, ensure that this does make the compost too moist and force oxygen out of the pile.
Another option is to dilute it with water and pour it on non-edible plants such as your lawn or flower bed. Do not pour undiluted leachate directly into a storm drain or waterway, as it can contribute to water pollution.
Read next
How worms work their magic in compost
Should you add worms to your compost bin?
External resources
- Antonio et al, (1991) Formulation of a liquid fertilizer for sorghum (Sorghum bicolor (L.) Moench) using vermicompost leachate (Science Direct)
- Compost Leachate: Will it harm plants? (Garden Myths) An evidence-based look at the impact of leachate on plants. The comments are also quite interesting.
- Nutrient in Leachate of Biowaste Compost and its Availability for Plants
- Effects of Landfill Leachate application on Crops growth and Properties of a Mediterranean Sandy Soil
- Vermicomposting Leachate (Worm Tea) as Liquid Fertilizer for Maize ( Zea mays L.) Forage Production