Does Nettle, Comfrey OR Weed Tea Work? People Claim They Are A Great Fertiliser But What Does Science Say? Are They Myth Or Magic? More Below

People make many claims about plant based teas. They claim they add nutrients to the soil and that when used as a foliar spray it combats pathogens. Others say they add important microbes to the soil. It’s easy to get fixed on an idea we’d like to be true and ignore the evidence one way or another.

Nettles tea can be made from nettles and used as a fertiliser. But is it really nutrient rich and reliable?

Many people swear by nettle or comfrey teas. They claim their plants are much better for it and really believe it. BUT how many gardeners do actual comparison trials to compare plants with and without compost teas plus comparisons with a range of alternative nutrition options?

The answer is very few. A lot of the people that claim compost teas are great haven’t really run verifiable trials to really test the situation. I have and I want to show you some of my findings. And those of other people that have examined the use of compost teas.

Let’s run through the claims and see what evidence there is other than hearsay.

Do Nettle & Comfrey Teas Add Nutrients To Plants

The short answer to this question its yes. Teas do add nutrients to the soil and they can be taken up by plants.

But it isn’t quite that simple. Rainwater also adds nutrients to the soil. So rainwater alone can benefit plants. Now follow the link above and look at which nutrients are found in rainwater. Keep a note of them as I want you to compare them with the nutrients compost tea in tap water contains.

The next thing to consider are the compost or plants you make tea from. They contain nutrients so clearly the tea will also contain some nutrients. Or will they!

But how many nutrients? In the following video Robert Pavilis say the NPK level is about 0.2N, 0P and 0K ie a NPK ratio of 0.2-0-0. That is so low as to be barely worth bothering with. And this means it contains fewer nutrients than found in rainwater.

And the argument that it is easier to spread tea over a large area because it can be diluted is suspect. Because if you take such a weak nutrient level tea and dilute it even further it is going to be very very weak.

We could of course use the compost tea as a foliar feed. And we know leaves can take in small amounts of major and minor plant nutrients. But they can’t take all of it from a weak compost tea, diluted even more with water to ensure we can wet all the leaves, so it’s not going to supply a lot of nutrients.

Law of Conservation Of Mass

This is a physical law. It means that whatever we start with cannot be magnified. We cannot take a pinch of nutrients from the compost and multiple them by making tea. Loaves and Fishes don’t work here!

So if we start with a kilo of nettles and soak them in water the best we could get is a mix that would probably grow about one kilo of other plant. And if you’ve ever made a compost tea you’ll know that at the end of the process you are likely to have a lot of fibre and “gunge” that doesn’t go into the tea. So the process isn’t 100%. You get less out than goes in. A lot of the nutrients are insoluble and are in the fibre and gunge that gets left.

Making compost tea is a bit like perpetual motion .. it doesn’t work 100%.

Of course we can add molasses and other products to reinforce the tea. But we are still limited to the nutrient level added to the mix. We can’t produce extra nutrients out of thin air.

So, though there are some nutrients in compost teas, there aren’t a lot and they aren’t going to make a huge difference to the plant.

It Works For Me

I hear so many people say it works for them. I cant argue that. If it works for them and they are happy so be it.

But I’ve a few questions to ask about how well it worked for them.

Firstly, it worked well against what criteria? Did they test it with some plants having tea compost and some not? Was it a blind test so they didn’t favour the plants they thought were getting the compost tea against those that didn’t?

And what do they call a good crop? Just being better than last year doesn’t mean it was a good crop. Just being better than a neighbour doesn’t mean it was good. We need to compare with well grown crops to know if it was a good crop as opposed to it “working for them”.

I’m not denigrating how good or bad a grower they are or whether they are happy with their yields. That’s for them to decide. What I’m trying to do is ascertain how they measure a good crop grown with compost tea against other crops.

Of course one reason they may get a crop that is better than before, and attribute it to compost tea, is that because they are using tea they are watering far more. Watering alone can improve crops against not watering. Especially if using rainwater!

This is a bit like those adverts we see for a certain miraculous fertiliser. They used to show a pot of flowers that had received the miraculous fertiliser against one that hadn’t been fed. Obviously the one that had some fertiliser looked far better. I’ll bet that if you and I entered a race and I had been fed for the last week and you hadn’t, I’d win!

Foliar Spray Combats Pathogens

Where’s the evidence of this? I’ve searched the international research databases and can’t find any research that makes this a credible claim.

Teas Add Important Microbes To The Soil

Compost teas often stink. We often get told that proves they are potent and full of life!

The reality is there is a lot of bacteria in the brews made by decomposing plants in a barrel of water. But does that mean they are beneficial bacteria, those that are going to help plants grow? Surely it is far more likely that they are the bacteria that promote decomposition!

The other problem is that anaerobic decomposition is likely to encourage pathogens. Teas could be dangerous to use.

Compost Teas V Plant Based Teas

Now I’m going to confuse the issue a bit. The following video is entitled Does Compost Tea Work. The problem is that the presenter, Robert Pavlis, doesn’t define what the means by compost tea.

So far I’ve been careful to discuss nettle, comfrey and other plant based teas, made by rotting plants in water. Technically compost tea is different. It uses compost. which is derived from plants, to make the tea. That isn’t the same as using the “raw” plant.

There is also an issue with the use of the term “compost tea” between American and UK English.

However, whichever he uses to make his tea, Pavlis is right about the nutrient value of the teas produced by either compost or plant based teas.

To confuse things even further some research papers state that there is a risk that the teas are a pollution risk because it contains a lot of nutrients. We need to read this with Extreme Caution. The reason being they do not provide a verifiable source that evidences the high nutrient level claim. Even researchers get led astray and make assumptions some times. One thing science teaches us is that we should question everything. And sometimes even researchers fail to do so. Peer reviewing the papers help but often we get to see a pre-publication research paper BEFORE it has been peer reviewed. Don’t fall for this!

Problems With Compost Teas

The main problem in my view is that there is little credible source of info to support the process or result.

If the teas contain nutrients in useful quantities, which I doubt, there is no way we can ell how strong it is. After all the recipe seems to be take an unknown amount of plant material and soak for an indeterminate length of time in a unknown amount of water!

If the straight of a tea is uncertain we can’t know how much it should be diluted.

If we don’t know the strength of the tea we don’t know how much and how often we should use it. Or even how much to use.

Compost teas can also contain pathogens. Do you want them on your plants? Do you want to handle them?

Does Compost Tea Work? The Video

Comfrey Update

Since writing this article I have been asked why I did not include more information about the work of Lawrence Hills with comfrey. Hills was the founder of the HRRA, know known as Garden Organic who have their HQ just outside of Coventry.

Hills did a lot of research on comfrey and his work established Bocking 14 as being the best comfrey variety. However there is no mention of the nutritional content of comfrey on the GO website, or of the analysis of any the teas from it. If it is so good why do GO now provide more information?

Looking for other comfrey research there is some from the USA that indicates limited success with comfrey as a mulch but I can find none about it use as a tea. Again, if it is so good why is there not more research or information? The lack of published information concerns me.

Tag: Compost Tea

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The Myth of Compost Tea Revisited: “Aerobically-brewed compost tea suppresses disease”

Here’s a complication where research can mislead us. This paper looks at the best compost tea to use on tomatoes. But it doesn’t compare with no compost tea or a proprietary nutrient mix.

Garden Waste Compost Tea: A Horticultural Alternative to Promote Plant Growth and Root Traits in Tomato (Solanum lycopersicum L.) Plants

Tag: Plant Teas
Image credit: Norbert Nagel, Mörfelden-Walldorf, GermanyCC BY-SA 3.0, via Wikimedia Commons

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