Compost tea is reported to be great for growing plants and some companies even call it a compost tea fertilizer. What is the NPK value of this magic potion?
I started this post about a year ago and at that time there were many products on the market but I could not find one that provided the Compost Tea NPK value. I checked again today and was surprised that the number of compost tea products available is down significantly. Maybe it was too expensive to ship all that water around the country? Maybe it did not work and people stopped buying the product?
Interestingly, the number of commercial DIY kits for making your own tea is up significantly. When you click on Google images for ‘bottles of tea’ you are taken to pages selling kits. Even with kits the same question needs to be asked. What is the NPK value of the tea these kits produce. Very few of the manufacturers I looked at provided such information, but many did say their compost ingredients were the best. Easy to say if you don’t give any data.
Even if you are not interested in compost tea – the following discussion will show you how companies are misleading consumers.
Contact the Manufacturers
I decided to contact several manufacturers of compost tea (the liquid form, not the kits) and asked them for information about their NPK values. Only one company thought this question was worth an answer, and that was Southern Organics. Compliments to them for caring about their customers.
Here is the email I sent:
“I am interested in your compost tea. What is the NPK value for this product?”At the time of contact they had only one compost tea on their web site called Ultimate Compost Tea. I thought the above question was a fair one since their advertisement for the product says “Ultimate Organic Compost Tea a highly concentrated liquid ……. that injects beneficial microbial life into the soil and the foliage of plants, improving soil structure and adding nutrients, making your plants healthier, stronger, and more robust.”
If it is highly concentrated with nutrients – they must know the NPK value!
Their email reply is as follows:
“Unfortunately we are not allowed to state the NPK level because we do not register that particular product as a fertilizer. The NPK is negligible though. It is roughly 1 – 1.5 % Nitrogen. Its power to chelate and deliver macro-nutrients is what makes the product so valuable. It makes a fertilizer much more effective, longer lasting and feeds the soil and microflora.”It is illegal to tell me the NPK value because it is not considered to be a fertilizer! I don’t believe that it is illegal. I do agree it is not a fertilizer. The reply suggests that the NPK value is 1.5-0-0. However, in subsequent emails they sent me the chemical analysis data for their starting material which has 1.5% nitrogen, based on the dry weight. So that number is not the value once it is diluted in water to make tea. Based on the amounts of Humic acid in the starting material and the finished product, the Nitrogen in the Tea is closer to 0.1%. The starting material has no P and K. The NPK value is therefore 0.1-0-0, at most.
A fertilizer with an NPK value of 0.1-0-0 has virtually no nutritional value for your plants. That is one reason they don’t call it a fertilizer.
The Power of Chelation
The second part of the email response points out that the “power to chelate … makes the product valuable”. Let’s look at this.
I asked some clarification questions and got the following response:
“It helps fertilizer last longer by a process called Chelation. See – http://southland.wistia.com/medias/3006ujfpyv
“The colloids in the humic, fulvic and ulmic acids also help keep nutrients from slipping away much like the chelation properties. While NPK is important and considered the 3 macro nutrients, there are 72 trace elements and nutrients that are just as important as NPK. Depending on the crop or species a deficiency in a single micro nutrient can be as detrimental as a deficiency in a macro nutrient (NPK). The product is also extremely high in vegetative carbon. Carbon source is more important than NPK. It plays a vital part in photosynthesis. Soil carbon is the most important factor of soil health. It increases the Cation Exchange Capacity. “They have now introduced some scientific terms like chelation and colloids that most people don’t understand – which is good for them since it allows them to present more false claims – a common tool used by marketing departments.
When fertilizer is added to soil, P, K and some other minor nutrients tend to bind tightly to soil. What this means is that even though your soil has enough nutrients, plant roots can’t easily get to them. A chelating compound will act like the soil and also hold nutrients, but it binds loosely to them so that roots can use them.
Chelation is not needed for nitrogen since it does not stick to soil, and their product has no P and K. so is chelation really an important feature of their product?
Chelation will NOT make fertilizer last longer. It can however make it more accessible to plants and it might reduce the speed of leeching. But once an ion is absorbed by the plant it is gone – it does not last longer.
The Lies Keep Coming
Trace elements are important but are there really 72? Their lab only tests for 9 micro-nutrients and that is for the raw material, not the tea. Based on their testing they don’t know if they are in the tea. But you might as well make claims for all 72! In my post, Trace Mineral Fertilizers – How Many Nutrients Do Plants Need, I reviewed the ridiculous claims that plants need a large number of different nutrients. Plants only use about 24 nutrients.
“The product is extremely high in vegetative carbon”. Now that is interesting and is not mentioned in the product description. What is vegetative carbon? It is carbon that comes from plants. Oil would be an example of this, as would diamonds. This may seem like an odd statement, but their so called compost is actually mined from the ground and is more like coal than what most people think of as compost – but that is a topic for another post. They also say that “Carbon source is more important than NPK”. The main source of carbon for plants is the CO2 in the air. They don’t get much of their carbon from chemicals in the soil. These statements about carbon are nothing more than science mumbo-jumbo.
It says “extremely high”, but what is high? The analysis is a bit confusing but it looks like the value is about 6% carbon. Remember that this is a liquid product so most of the jug is water. Compared to compost which is 25% carbon, a value of 6% is not extremely high.
“It (vegetative carbon) plays a vital part in photosynthesis” – no it doesn’t. CO2 from the air does. But they might argue that without carbon chemicals in the plant you can’t have photosynthesis?
“Soil carbon is the most important factor of soil health” – not even close to being true. A pile of coal has lots of soil carbon, but it is not considered healthy soil as far as plants are concerned. They are probably talking about organic carbon, but if that is the case, what the heck is vegetative carbon?
Summary
A few statements from the manufacturer are true or are partially true. Chelation in soil is important. Micro-nutrients are important to plants. Humic acids are beneficial to plants – more on that in a future post.
These facts are then submersed in a bunch of mumbo-jumbo making the whole thing sound factual and scientific. A bit of science lingo lets people say all kinds of stuff and make it sound valid. That is why the internet is full of so many myths and products that add no value to the garden.
The reality is:
- compost tea contains few nutrients – see below
- spreading real compost or manure onto the soil provides all of the benefits of tea, at a fraction of the price.
Compost Tea NPK Value
So what is the NPK of compost tea? Here are some numbers I found.
Ultimate Tea from Southern Organics: 0.1-0-0
Cutting Edge Solutions – HumTea Original – Compost Tea: 0-0-1
Super Compost Tea: 0.02-0.2-0
Compost Tea – Does It Work: 0.2-0-0
The actual values will depend on the compost used to make the tea, but you can see from the numbers above that none of them contain significant levels of nutrients. Keep in mind that these values will be further diluted before the tea is added to the garden. Sounds a lot like homeopathic medicine!
Some proponents of compost tea say that the nutrients are not the important ingredient. It is the microbes that are the key value in compost tea. In my post, Compost Tea – Does It Work ,I look at the effect of these microbes on the growth of trees.
To date, most of the research does not support the claims made by proponents of compost tea. There is little evidence that it is any better than just using compost.
Related Posts
Compost Tea – what is it and what are the benefits?
References
- Photo source; Suzie’s Farm
I absolutely love your work. A lot of this stuff is coming from the highly popular permaculture movement, which is giving out some very bad advice to very large numbers of novice gardeners. My personal beef is referred to above in the actual promotion of planting of invasive plant species; an issue I had to fight OSUs Massive Online course the Introduction to Permaculture for the right to include a piece on really knowing your plant before sticking it in the ground. Honestly? I feel like we are headed towards a dark age.
It’s incredible how quickly actively aerated compost tea has become so popular. A lot of people are starting to use it regularly, and it’s a shame to see them wasting their time. At least with nettle or comfrey tea you didn’t have to do much, but AACT is a whole load of faff for no reason at all!
Just wonderful!
I am of the generation who where juvenile at the same time as the World Wide Web. During my teens a lot of information had to be gathered from magazines and newspapers as internet was hard to search and breaks at the school combined with quite finite dial up speed and only one computer to book time on in the library skewed it to more mailing and other things crucial to coming of age.
Fast forward a couple of years and forums, communities and the likes flourish and die in cycles but information is still Accessible to most.
Now with a couple of behemoths gobbling up users creating customized bubbles the outlands are scarce and scattered.
Paired with “the biologist is in” “Alanbishop.proboards.com” and others there still is gold to be washed.
Thanks again /Swedish fan
You know, Robert, that your widest contribution to national/world health is the way you analyse various claims. Had we in the U.S. not an educational system beholden to and captive of every local fucking crackpot religious thought extant, barring scientific analysis on many fronts, we’d have a population (and children) who could THINK. I thank you so much. Tom Brophy
What a wonderful, informative report, Robert! It’s about time that somebody is actually working to educate the public against false claims and exaggerated promises, which are being made by manufacturers and their salespersons. You kind of remind me of the Amazing James Randi, and his medical associate, Dr. Harriet Hall. Keep up the good work man!
Thanks. I have been following James Randi for a long time and love his work. He used to write for the Skeptical Enquirer – that is probably where my interest in garden myths started.
Thanks to you larryreedjr both for appreciating Robert’s wonderful analysis of these ( and so many other) claims, but also being appreciative of the marvelous James Randi. I am glad now to know of his assistant, Dr. Hall. I’d say “God bless these people ” were I not an atheist . Best to you. Tom Brophy
Thanks Robert, I so appreciate your articles. I’m wondering if you could do a bit more work on companion plants. Based on your researches, I’m slowly eliminating comfrey from my garden and I sorely wish I’d never planted tansy, because I’ll have it forever. I’m sick of seeing all these “this goes with that” articles, for which there is almost never any evidence. I know it’s a huge field, but you seem like a person who’s up for a challenge! 🙂
Companion planting is on the list to be done, but it is a complex subject with very little real scientific data. In short, I have found very little evidence for companion plants except that a large variety of plants is better at fighting off pests, than mono-culture.
I did do a couple of posts on comfrey; https://www.gardenmyths.com/dynamic-accumulators-beneficial-garden/
Fascinating. Such a shame that science, the great revealer,, should be used to occlude. Does what you say here also hold true for the nettle tea and comfrey tea the old boys on our allotments swear by? Basically as much of the plant crammed into a bucket, topped up with water and left until it smells like a recently disinterred corpse. I use it but since I have not run a control experiment I can’t speak for its efficacy.
The same argument holds for leaf tea as well. The bucket might stink but most of the organic matter has not started to decompose, so there can be few nutrients in the water.
If you homogenize the leaves and then make tea there might be more nutrients, but making tea can not increase the total amount of nutrients. You get just as many nutrients by dropping the leaves on the ground.
It might be a belief of the ‘organo-religious’ people. The facts are not important.
I have a request from you for your opinion concerning the use of biochar in the garden.
Thanks.
Biochar is on the list to do. In short there seems little short term benefit for the gardener. There may be some long term environmental benefits.
I have no experience of the properties of commercial biochar but I do make my own charcoal by dousing the burning embers of my bonfires with water. Charcoal is powerfully absorptive of nutrients so needs to be charged with plenty of fertiliser before use.
I have been adding it to my soil and using it as an ingredient in potting compost for ten years now. I love my black gold!
I write about it on my blog!
Any commercialization of a do-it-yourself product or process is a ripoff. Homemade compost tea is a harmless way of watering your plants before you spread the compost residue on the surface. The very idea of selling the stuff in jugs is a crime against sanity
Love how you debunk many gardening myths