Updated: Aug 9, 2018
What Is Compost Tea?
Compost tea is a living solution. It is a process that involves growing soil micro-organisms, or microbes, found in healthy soil and compost by aerating water in the presence of organic microbe foods.
All of these components play key roles in creating optimal conditions for aerobic microorganisms to grow and replicate. This is the goal of what is called Actively Aerated Compost Tea (AACT).
Microbes perform the vital function of creating soil, this does not just happen. Tending a compost pile concentrates this natural process, and compost tea concentrates the process even further. Performing the process of brewing compost tea is simple and a very effective way to increase the diversity and biomass of beneficial aerobic microbes in the soil and on the leaf surface of crops.
Think of soil microbes like construction workers. Your job as the contractor is to consistently bring them to the job site so that they may build the neighborhood. Once the neighborhood is built it takes on a life of its own and the soil will be working for you, mitigating pests and disease, and reducing the need to irrigate and fertilize to support growth.
What is the Soil Food Web?
Soil microbes are comprised of bacteria, fungi, protozoa, and nematodes. Together with the macro-organisms like earthworms, rolly polly’s, spiders, etc. they make up what is called the “soil food web”.
The soil food web, like any food web, works through what are called “trophic levels”, or life levels. In other words, the big fish eats the small fish.
Another good analogy to understand how important soil microbes is to consider the soil food web like you would all of the fish and organisms in the ocean. Think of bacteria as the plankton, and the larger nematodes as the sharks. Apex predators like sharks only appear in mature ecosystems, and the average landscape or garden does not have mature soil, and therefore does not have the higher organisms present. This is the source of common landscape and garden pest and disease issues - there is simply nothing there to eat what is eating your plant.
What would happen if you took all of the plankton out? Life as we know it would end. This lack of life is the state of the average conventional landscape, garden, and farm, and compost tea is a simple way of bringing back life so it can seek its own balance in the soil.
Much of the reason we take microbes for granted is we cannot see them. Some stats:
Microbes are small. Up to 500,000 bacteria can fit in the period of the exclamation point at the end of this sentence! There’s another universe down under your feet!
Microbes are magical. Humans cannot accomplish the vital processes undertaken to create health soil. The soil food web not only creates perfect plant food, but they help plants eat it and protect them from stress, pests, and disease. Plus they work as Nature’s recycler to mitigate contaminants and environmental toxins.
Microbes are abundant. A teaspoon of native grassland soil contains 600-800 million bacteria comprising ~10,000 species, plus approximately 5,000 species of fungi, the mycelia of which could be stretched out for several miles. In the same teaspoon, there may be 10,000 individual protozoa of over 1,000 species, plus 20-30 different nematodes from as many as 100 different species.
Microbes are extraordinarily prolific. According to the book Secrets of the Soil, a single microbe reaching maturity and dividing within less than half an hour, can, in the course of a single day, grow into 300 million more, and in another day to more than the number of human beings than have ever lived. In four days of unlimited growth, bacteria can outnumber all of the protons and even the quarks estimated by physicists to exist within the entire universe.
Why Brew Compost Tea?
Increased Nutrient Cycling: Microbes make perfect plant food, that is how soil works. Think of microbes like tiny fertilizer factories! Nutrient cycling is what helps make the nutrients and minerals in the soil into a form that is available for plants to uptake. When you apply organic fertilizer, you’re not directly feeding the plant but rather the microbes in the soil that will work to convert the nutrients into an ionic form available to plants.
Healthy Soil Structure: Typically, humans move the Earth to create good soil structure, but microbes will do this for you over time. Fungal hyphae helps in creating soil aggregates, and bacteria and archaea assist in breaking down organic matter and aerating the soil.
Use Less Water: Growing with microbes increases the soils ability to retain water through correcting soil structure, increasing organic matter and exchange capacity, and through the presence of the living organisms themselves. Compost tea can reduce water usage 20-40% and in many cases eliminates the need for irrigation in landscapes entirely.
Use Less Compost: Rather than haul organic matter over large (or small) areas, now you can spray it! Plus, because you have grown the population of microbes through the brewing process, it is much more cost effective and efficient.
Use Less Fertilizer: Fertilizer is a crutch, it is a compensation for the soils inability to provide fertility for plants. Think about it, you don’t have to fertilize a forest and it grows trees! Growing the microbial soil food web can help you ween yourself of the requirement for fertilizer to grow a garden over time.
Higher Yields: Improving the fertility and maturity of the soil automatically enhances the garden or farms ability to grow larger and more abundant crops.
Treat Disease: Disease organisms are merely microbes that are eating your plant who have nothing to eat them. Many commercial biological biocides are derived of microbes found in compost tea and in healthy soil. Rather than trying to kill the disease, you can eliminate it with microbial balance.
Mitigate Pests: Many soil microbes seek protein, and the exoskeleton of pests is protein, so consistent applications have shown compost tea to have pesticidal properties. But the most effective pest control is healthy biologically diverse soil and a healthy plant.
Reduce Weeds: Weeds are indicators of mineral and biological imbalance in the soil. For example, clover grows to regenerate nitrogen in the soil. Almost 80% of the air we breathe is nitrogen, so you get it for FREE from nitrogen-fixing bacteria that thrive in healthy soil. It should be noted that this is best experienced in lawns as any soil left uncultivated or without perennial plants like turf will experience weeds.
Untold Benefits: We know very little about the diversity of microbes in the soil food web. Brewing compost tea will help establish good populations of nitrogen-fixing bacteria, nitrogen and phosphorus-solubilizing bacteria, and all sorts of other beneficial microorganisms, many of which have yet to be identified and fully understood in their role in soil health.
Other Types of “Tea”
The compost tea conversation can become complicated due to the different methods available for leveraging microbes to benefit the garden or farm. Here is some discussion around the compost tea landscape.
Actively Aerated Compost Tea (AACT): This is the primary focus of our attention here, and has been described above in some detail. The result of brewing is very high populations of aerobic soil microbes found in healthy soil.
Compost Extract: Compost extract is where the microorganisms are stripped from the soil aggregates using water and extracted into a liquid form. This process will contain good biology for soil drenches, and can be made very quickly, as it does not require a brewing process. It does however require a large amount of compost relative to the final liquid product, and is primarily used in large commercial productions. Extracts are often advertised as “instant compost tea”, so appear advantageous relative to AACT, but typically contain a fraction of the activated microbes found in AACT.
Compost Leachate: These teas are what you will find seeping out of a worm farm or compost pile during the composting process. Leachates will consist primarily of soluble nutrients, but will also contain some small amount of biology. By all means use this liquid in your garden application, but it is not sufficient to rely on as a means of establishing a healthy soil food web.
Plant Teas: This is where plants such as nettle or comfrey are soaked directly in water for an extended period of time. This is an anaerobic process, so does not really aid in growing the soil food web, however, anaerobic microbes are very good at extraction nutrition from the plant material and it can be a good tool in the garden.
Manure Tea: Manure tea may be the most popularized method of making “teas” simply because of how many old timers have performed the process. It amounts to no more than liquefying the manure for ease of application. The microbes that make manure in the gut are anaerobic, so this process will not aid in the development of a healthy soil food web.
How To Brew AACT
There are many ways to brew compost tea, and many commercial brewing systems on the market, but the process can be accomplished in just about any container.
Think about the process like you would creating an aquarium for fish. Rather than aerating water for fish to breathe, we are aerating the water for aerobic microbes to breathe.
Compost Tea Brewer: The brewer is the hardware of the compost tea brewing application. It is important because you could be using the best microbial compost and food sources available, but if your brewer doesn’t maintain adequate dissolved oxygen (DO) levels or thoroughly mix the liquid, then the quality of your tea will suffer.
There are many different brewer designs on the market. For hobby gardening it is best to DIY and brew in a 5 gallon bucket (or larger suitable container), as the cost you will pay for a molded or small commercial unit will not be worth the price.
DO is accomplished by infusing air into the solution from an air pump. An air pump does what it says, it pumps air. As opposed to a water pump that pumps water. This is typically accomplished through air tubing, which should match the diameter of the air pump and airstone you are using.
As a general rule, you want .05-.08 cfm per gallon of water when selecting an air pump. It is better to err on the side of more air than less, aeration cannot be overdone.
Using airstones are commonplace on smaller brewing units. An airstone simply breaks up the large bubbles coming out of the air tubing from the air pump creating more surface area for the air and dissolving more oxygen.
Assuming you have enough aeration occurring, methods involving airstones do a decent job mixing the solution adequately in the container. But using the air lift method when brewing compost ta is a superior way to achieve increased mixation, DO, and oxygen retention in solution.
Air lifts involve using air to displace water up to the top of the container so that a drain can replace it and allow for circulation of the compost tea solution. The best brewers that accomplish this create a vortex spiral that constantly exposes the skin of solution to the air and sucks it in, like a roiling river. This method does not require airstones, as the air is simply used to move the solution around.
Compost (humus): Even the best compost tea brewer won’t make a good tea if you don’t start with good compost. This is where your beneficial microorganisms come from, so it’s a vital part of the process.
Without a microscope and some experience, it is not easy to determine the quality of compost visibly. In general, avoid municipal or commercial compost because they are typically not made for quality, often have uncomposted animal manures, and tend to be made to recycle industrial waste.
Because worms perform much of the composting process in their gut, worm castings (or worm manure) are much safer sources of humus for brewing compost tea. However, worm castings do not have a complete soil food web in their gut, so it is a good idea to seek out as many healthy sources as possible when getting started brewing. You can add these sources to your own compost situation and make your own brewing inoculant over time.
If you find yourself needing to purchase commercial compost to get started brewing. Ask the source if they have done biological testing on their compost. Ask them about their ingredients, and how long the compost was allowed to process before being sold. If the compost is still hot to the touch at the point of sale this is an indication that it is not finished the composting process. And if the material is dried out this can harm the microbes, you want the moisture level to be generally what it would be for plant growth, like a wrung out sponge.
One good way to test the biology in a compost source is to add a food source and give it some time to see what happens. You should see a bit of white fuzz develop on the top of the compost or if added in higher amounts it may actually cause the compost to heat up. This is caused by the bacteria and other microorganisms reproducing rapidly to use up that food source, which produces heat.
Microbe Catalysts: Many in the compost tea brewing realm do not focus on catalysts in the brewing process, but they are very important. These are materials such as sea minerals, rock dusts, or other diversified mineral sources that do not represent food for growing microbes, but that are used by them to create the enzymes that allow them to operate. Microbes do not chew on the banana peel in the compost pile, they manufacture enzymes out of specific mineral elements that work to chemically break the organic matter down. The enzyme potential for microbes is so large that we will probably never discover all of them. Not allowing a diversity of elements for enzyme production is like hiring micro