One of the most hotly debated questions in vermicomposting is this, is vermicompost leachate good or bad for plants? Some say it is great for plants and use it all of the time. Others warn that it should not be used because it contains phytotoxins (plant poisons) and pathogens.
What I find most interesting about this debate is that both sides claim to be correct and yet neither side takes the time to do a search for some scientific evidence to support their position. I guess I approach things differently and the science in this case is easy to find – one Google Scholar search will do it.
Let’s look at the facts surrounding vermicompost leachate.
What is Vermicompost Leachate?
There is even confusion about this, but usually from beginners in the hobby. Vermicompost leachate is the liquid that drains out the bottom of the worm farm. This is different than the tea made from finished vermicompost, which is correctly called vermicompost tea, or just compost tea. I have discussed the value of this in Compost Tea – Does it Work?.
To better understand vermicompost and its nutrients, have a look at Vermicompost – Is it Really That Great?
Should a Worm Bin Produce Leachate?
Before we tackle the meat in this topic let’s examine another debated question. Should a worm bin produce leachate?
I see a lot of comments from people who say, if you have leachate you are doing something wrong with your worm farm. That is just another myth – it’s simply not true.
A worm farm can be running just fine and produce vermicompost leachate. In fact many farm operations and even commercial operations run their worm farm to produce leachate. It’s one of their main goals.
A drier bin that gets drier food will not have leachate. A moist bin that gets wetter food will have leachate. Both are equally good and both grow healthy worms. Being drier may be a good thing if you don’t want to bother emptying the leachate.
The Arguments Against Leachate
There are two main arguments against leachate.
- Vermicompost leachate contains phytotoxins that harm plants.
- Vermicompost leachate is anaerobic and therefore contains pathogens.
Let’s look at each claim.
Vermicompost Leachate Contains Phytotoxins
Phytotoxins are chemicals that are harmful to plants. Where would these come from? These chemicals are produced by plants, microorganisms, or by naturally occurring chemical reactions. The term is also used to describe compounds produced by plants to ward off pests.
We can certainly expect vermicompost to contain phytotoxins since they have a lot of bacteria and added plant material.
One key point that discussions seem to ignore is that any phytotoxins in leachate came from the material in the worm bin. If leachate contains them, the vermicompost also contains them, and yet, vermicompost is never described as being toxin to plants. You might think that most of the toxins get washed into the leachate, but what about vermicompost that is made without leachate? It must be full of phytotoxins?
Vermicompost does contain phytotoxins, but the amounts are clearly not enough to cause problems to plants. There is no reason to think leachate is any worse.
Vermicompost leachate improves the growth of corn when diluted by 50% and the phytotoxicity was tested and found not to be a factor. This review summarizes numerous studies that all found improved plant growth from vermicompost leachate.
Phytotoxicity can be tested by exposing germinating seeds to the liquid in question. When leachate is applied to seedlings at full strength it can reduce germination and slow root growth. These effects go away once it is diluted. It is not clear if the toxicity is due to phytotoxins or to the higher nutrients/salt content. What is clear is that vermicompost leachate does not harm plants if it is diluted before use.
Vermicompost Leachate Contains Pathogens
A pathogen is an organism that causes disease. In most cases a human disease organism will not cause disease in plants and vice versa. So, is the concern here about human pathogens or plant pathogens? Comments about pathogens are sometimes followed with examples and E.coli and salmonella are commonly mentioned.
E. coli and salmonella are human pathogens. E. coli does not cause harm to plants and most strains cannot enter a plant. Scientists may have found one strain that can get into the roots of lettuce, but it does not travel to the leaves. Salmonella can infect plants through their leaves, but probably not through the roots. It is unlikely that human pathogens will harm plants.
Some may be concerned about surface contamination of food crops. If that is a concern, then pour the leachate on the soil, not on the plants.
These pathogens originated in the vermicompost, which is considered safe to use. The concern with the leachate is that it becomes anaerobic and may be a better place for the pathogens to grow. Both E. coli and salmonella are facultative organisms, they grow in both aerobic and anaerobic conditions.
What does the science say? This study found no E. coli, slamonella or shigella in leachate. They suggested that the healthy microbes in the bin killed off the pathogens. Another study using leachate on sorghum found no pathogens.
What about the claim that, “some leachate can contain harmful pathogens”. It is true, but it is also true that every plant surface and all soil also contains harmful pathogens. They are even on your hands. That does not mean there is enough of them to cause harm.
Pathogens don’t seem to be an issue.
Uncomposted Bad – Composted Good
This is another weird claim I see. Water running out from the bin, early in the process, is washing “things” off uncomposted material and these things, which are never defined, are harmful. They go on to claim that this is very different from making tea from finished vermicompost because now it is composted and the things have magically disappeared.
If you can’t explain the “thing” – don’t make such illogical comments.
But ….. Some People Used it And Killed Their Plants
Every time this discussion comes up on social media, someone will tell the story of how they used leachate on their plant and killed it. That is proof enough that it is harmful.
There are also lots of cases of people burning their lawn with too much fertilizer – it does not make fertilizer harmful. Vermicompost leachate washes the water soluble nutrients out of the compost. You can expect the concentration to be much higher in it, than in vermicompost. It will also contain higher levels of sodium which is very soluble. Putting too much fertilizer on plants will harm them.
Unless someone tells the story, and also describes the dilution they used, it is not a very useful story for understanding the effects of leachate on plants.
Does Vermicompost Leachate Harm Plants?
Vermicompost leachate does contain phytotoxins and microorganisms but so does the vermicompost. If one is good for soil and plants there is no logical reason to suspect that the other is harmful. Several scientific studies have looked at this issue and clearly show that leachate is good for plants.
It is a good idea to dilute vermicompost leachate by at least 50%, and a 1:10 mixture is even safer. Dilution ensures that the nutrient/sodium level is low enough so it does not harm plants.
Every leachate is different and if you want to be sure that yours won’t harm plants, try the simple test in this video, using leachate instead of water. It is similar to the test scientists use.