Both gardening and composting are full of controversy.
Compost tea is no different.
You’ll find experts and scientists that believe compost tea makes no difference to plants – and could even be harmful. You’ll also find other experts that are convinced it can prevent disease and introduce beneficial microbes to your soil.
Let’s take a look at what it is, how to make and use it – and what research and experts says.
- What is it?
- How does it work?
- Making compost tea
- Applying compost tea
- What the research says
What is compost tea?
To add to the confusion over compost tea, there’s also disagreement over what it is. Some composting books describe suspending compost in a pail of water to allow the nutrients to leach into the water. In this case, compost tea is simply a leachate. (Manure tea is made in a similar way, but uses manure instead of compost.)
Other sources are very specific in defining compost tea as something that is brewed – and is not a leachate. These refer to a mixture uses microbes grown aerobically (with air). This is often called Aerated Compost Tea or ACT.
How does compost tea work?
Leachate compost tea
Compost contains insoluble nutrients. These nutrients are highly valuable in improving the soil, but the benefit is a long process rather than an immediate boost to plants. Compost or manure tea releases some of those nutrients in soluble form which is immediately available to plants.
Brewed compost tea
Compost tea brewers are more concerned with microbes than nutrients. As we’ve seen before, microbes are tremendously important for soil structure, maintaining balance in the soil and in releasing nutrients to plants.
Lowenfels and Lewis, in the book Teaming with Microbes, argue that:
- You need to apply a lot of compost to get results.
- The microbes in compost tea reach plant roots a lot faster than the microbes in compost.
- Unlike compost, compost tea can be applied to plant leaves. Here they compete for space with pathogens, and in some cases protect the leaves from attack.
- Using compost tea helps increase the diversity of microbes in your soil.
How do you make compost tea?
To make compost tea in the traditional way, simply take a porous bag, such as a burlap sack. Put a generous amount of compost in the bag, and suspend it in container of water. Allow it to soak, then use the water to fertilise your plants.
Brewing Compost Tea
Brewing compost tea takes a lot more work.
First you need to choose your site with temperature in mind. If your compost tea gets too cold, the microbial activity will slow. If it’s too hot, many will die. Aim for room temperature. You should also site your compost tea away from the sun, as ultra-violet rays can kill some microbes.
Mature, high quality compost is placed in a large container of chlorine free water at a ratio of around 5 pounds per 25 gallons. Some brewers choose to suspend the compost in a porous material such as a stocking. You can also use worm castings or a concentrate.
Some brewers choose to activate the compost before adding it to the water. You can do this by mixing the compost with simple proteins such as oatmeal.
A pump is then used to aerate the compost tea for 24 to 36 hours. You can choose to use a commercial pump or compost tea brewing system. Alternatively, make your own with a container and an aquarium pump. You can find instructions for a simple composter brewer on Garden Therapy.
While the compost tea is brewing, some brewers choose to feed the compost tea. Sugar will help feed the microbes, or you can choose to vary the food to encourage fungal or bacterial growth.
Lowenfels and Lewis recommend adding complex sugars and fish emulsion for bacterial growth, and kelp, humic acids and phosphate rock dust for fungal growth.
It is worth being careful, as some amendments may encourage pathogens such as Salmonella (see research below).
Signs that compost tea is becoming ready include a coffee-brown colour and a fresh earthy smell. At this stage, some brewers also choose to add mycorrhizal fungi. If tea gives off an unpleasant smell, it is probably a sign it is not ready.
How do you apply compost tea?
A successful brewing process will create huge numbers of microbes, which will soon start competing for nutrients. So it’s important to use your compost tea as quickly as possible. Try to apply outside the hottest parts of the day to avoid UV damage.
The tea can then be applied to the soil in a plastic watering can. You can also use a spray, but you will need to strain the tea first. You can also spray leaves, aiming to cover at least 70% of the leaf.
Does it work?
There is huge controversy over aerated compost tea, and research findings have not always been consistent.
Some studies have found that compost tea can improve growth and deter diseases. One study by Pant et al found that different compost teas could improve the growth and nutrient content of Pak Choi. Another study by Kim et al found that different compost teas had different effects, with some encouraging more root growth while others encouraged foliage growth. Martin and Braithwait state that a number of studies have found that compost tea can suppress soil-borne disease.
On the other hand, Dr Linda Chalker Scott chronicled an experiment on cherry trees to see if compost tea could prevent cherry blossom rot. Half the trees were treated with compost tea and half with water. The trees treated with compost tea either performed the same or worse than those not treated with compost tea.
In 2007 Dr Scott chronicled some of the problems with compost tea research.
- Compost tea is highly variable, which makes it difficult to study its effects.
- Studies with inconclusive results are not published, even though they are just as important as those with conclusive results.
- Laboratory results can be positive, but results have proved more variable in the field.
- Some studies have not been peer-reviewed.
Although that review was written in 2007, Dr Scott’s opinion remains the same, telling me in in an email in 2021 that:
“I’ve not seen anything in the literature to change my views, and in fact the peer-reviewed research that has come out only supports my skepticism.”
Some research has also found that aerated compost tea amended with molasses can cause the growth of human diseases such as Salmonella and E-Coli. Lowenfels and Lewis are sceptical about this, but suggest leaving out extra ingredients if you are worried about E-Coli.
Some gardeners and experts swear by compost tea, while others are dismissive. But while the benefits of using compost are clear, the evidence for using compost tea is inconclusive.
Making compost tea is also a time-consuming process. If you do choose to experiment with it, it may be easier to start by buying a pre-prepared mixture rather than brewing it yourself.
Featured image by Lubiana Wines.