Last week’s article on the results of on-farm research about the value of compost tea raised some questions from readers. To answer them, I’ve brought in Robert Pavlis. Robert has a background in chemistry and biochemistry, is a Master Gardener, and author of the book, Soil Science for Gardeners, an easy-to-read, practical guide to the science behind a healthy soil ecosystem and thriving plants. The book debunks common myths, explains soil science basics, and provides the reader with the knowledge to create a personalized soil fertility improvement program for better plants.
Robert and I are similar in that we both translate science into practices our readers can use. He has done some good work gathering information about compost tea and he answers some important questions for us here:
Does Compost Tea increase nutrients?
“To clarify the question, it should be stated more clearly as; “Does compost tea add more nutrients than compost alone?”
“There is no doubt that compost tea adds nutrients. But does the process of making tea increase the level of nutrients compared to just using compost without brewing? If they both add the same amount of nutrients–why bother making tea?
“If you think about it for two seconds you will realize that this is a silly notion. Think about what you are doing in making tea. You take a handful of compost and you put it in a bucket of water. Microbes take over and start digesting the compost.
“Your original handful of compost had a certain amount of nitrogen, phosphorus and potassium. No matter what process you use, you will never increase the amount of these nutrients in a plastic bucket (except for some minor organics falling in an uncovered bucket). The microbes might breed and grow and digest things, but the total amount of nutrients remains the same. In fact it might actually be less since some of the nitrogen might be converted to ammonia which evaporates into the air.”
Does Compost Tea make the benefits of compost go farther?
“The nutrient content (NPK fertilizer numbers) of say 500 ml of compost is 2.6 – 0.9 – 2 (average value for composted cattle manure; source Alberta Agriculture Department). If I now add this to a 5 gal pale (about 20 L), I still have the same ratio of nutrients, namely 2.6 – 0.9 – 24, but it is now diluted 40 times (500 ml to 20 L). The nutrient value of the tea is now 0.07 – 0.02 – 0.05. That is an extremely dilute fertilizer. For comparison human urine has a nutrient value of 11 – 1 – 2.5, that’s 160 times as much nitrogen as compost tea.
“Sure, you can probably spread the tea over a larger area than a handful of compost, but if you do that the amount of nutrients added to the soil is negligible – so why bother? The fact is that making tea from compost does not increase the amount of nutrients. It does not make the compost ‘go further.'”
The bottom line: If you want to add nutrients, just add the compost directly.
Does Compost Tea add microbes to the soil?
“There is no doubt that it does. You have a pail full of slimy microbes and if you spread it around you are certainly adding microbes. But the soil already has lots of microbes and adding a bit of tea is not going to make much of a difference.”
Do we need to add microbes to soil?
Here, Robert starts by getting us all on the same page about what microbes are.
“A microbe, also called a microorganism, is a generic term to refer to a wide range of microscopic life which includes things like fungi, algae and bacteria. Algae play a minor role but we won’t talk about that here.
“Each gram of soil, which is about the weight of a paperclip, contains anywhere from 100,000 to 1 million living microbes. That is a lot of microbes. Not only are there a lot of them, but there are also many different kinds. The kind of microbes present at any time depends very much on soil conditions such as moisture, temperature, pH, chemicals present, and available food. Even the kinds of plants growing in the soil will have an effect on the type of microbes present. So the microbes under tomato plants will be different from the ones under a zinnia.”
To the argument that poor soil (compacted, sandy or heavy clay) doesn’t have as many microbes, Robert points out that “if you add microbes and the soil conditions are not to their liking, they simply die. If the conditions are acceptable to the microbes you add, they would already be there. Adding microbes to poor soil does not work. You need to first improve the soil, and as that happens, more microbes will grow.
How about adding Mycorrhiza?
“Mycorrhiza is a type of fungi that is very important for plant growth. Companies have started packaging them and promoting them to consumers. At first they were sold as an additive to soil, but now you can find them added to many soil and soil-less products.
“Your soil already has mycorrhiza so you don’t need to add them.
“There are hundreds and maybe even thousands of different types of mycorrhiza, some of which are very specific to certain varieties of plants. Commercial products, at best, contain 4 types. Many products contain fewer types. You don’t know that the ones in the pail are the ones your plant needs!
“Mycorrhiza are fairly sensitive to high temperatures. If the container holding them gets too warm, like sitting on a truck too long, they die. You have no way of knowing that the product you buy actually contains living mycorrhiza. It may just be a very expensive, useless white powder.”
“Probiotics for soil is the same idea as probiotics for your intestines. They are a combination of microbes that you buy and add to your soil.
“How do you know if they are living? You don’t.
“Will they live in your soil environment–remember microbes only grow in environments that suit them? You don’t know.
“Probiotics for soil is just another way to fleece you of your money.”
The bottom line: Your soil contains thousands of different microbes already. If the conditions suit them, they will grow, reproduce and prosper.
The best you can do is keep your microbes happy.
As Robert says, “Your soil already has lots of microbes. Don’t add more using commercial products or compost tea. The secret is to provide the microbes you already have with a home they love. How do you do that? Feed them. Microbes eat and digest organic matter. Keep adding compost, manure, plant cuttings, wood chip mulch etc, to your soil. Just growing plants in the soil will provide organic matter for microbes to eat. Disturb the soil as little as possible.”
You can also keep them happy by following the Five Soil Health Principles:
1. Keep soil covered
2. Minimize disturbance
3. Manage for plant diversity
4. Keep live roots in the ground
5. Integrate livestock into your management
One last note
Robert notes at the end of his article on compost tea that many of the comments he received were from people with feelings about the topic, but with no scientific evidence that their feelings are correct. Neither Robert nor I intend to hurt anyone’s feelings. We’re simply conveying what the science says. As always, if you have references to science or data that adds to the discussion please do provide them. We are always interested in advancing our understanding of how things work, and will change our minds should new information become available.
This information was drawn from two of Robert’s articles: Compost Tea and Soil Microbes – Do you need to add them to the garden? They include additional references that he drew on when writing his posts. Visit his website to find more great information and science-based soil tips. You can also join his Facebook group, Garden Fundamentals, a science based gardening group focused on providing solid information backed by research.
Compost as an Alternative
While Compost Tea may not be worth the time and effort invested in it, compost is actually a very good alternative. In an effort to improve soil health, increase forage production and sequester carbon long-term, John Wick and Peggy Rathmann and the Marin Carbon Project have been running an ongoing on-ranch research project. Over a decade of data indicate that their carbon farming practices using compost can double forage production and drastically increase carbon sequestration in the soil.
If that’s interesting to you, check out this two-part series. And if you’d like to know more, send me an email.