Compost teas are a very popular yet controversial technique used in organic and natural farming. Many people use them consistently and have adopted them as a crucial part of their operating procedures. Outside of the cannabis world, many people abhor compost teas. People call them scams, gimmicks, dangerous, pointless, and lack any real science behind them. I’m going to go over some of these points and counterpoints and give my perspective on the aerobic fermentation process of organic inoculants called “compost teas.”
Online and within social media, so many voices are given a platform on scientific processes. You can see citizen scientists performing many experiments to validate or discredit a vast diversity of practices and concepts. Compost teas are not exempt from this. Many gardeners you can find on YouTube have performed side-by-side comparisons of compost teas, many with contradictory conclusions. First, let me explain the concept of compost tea and the intentions behind its use.
An actively aerated compost tea uses water, a biological inoculant, a food source, and aeration to multiply biology and introduce it to the soil. This is to introduce beneficial biology to the soil and root zone of a plant. A compost source is full of microbes that are a crucial part of the soil food web. These microbes play an essential role in the functions a plant needs to be healthy and uptake nutrients within the soil. A compost source has a given count of microbial density. Brewing a compost tea is said to multiply that microbe count in water to stretch that compost further than its limited benefit compared to when it is introduced by topdressing or mixed into the soil.
You can find several videos online of people making compost teas, adding them to the soil, then comparing growth rates to plants given straight water. Their results show no increased growth rate or harvest yields. I personally believe this can be explained by the lack of understanding of when and why compost tea is used. First of all, compost teas are not nutrient sources. They are not a turbo button, and they are not always needed by soil. Especially in a soil full of beneficial biology already. Compost teas are meant to fill gaps in the soil food web. It doesn’t surprise me when experienced hobby horticulturalists growing veggies in their gardens see no benefit from compost teas. These people typically make their compost out of a diversity of plant matter and reintroduce it into a small plot of fertile land year after year. I expect their soil food web to be very well intact, and all roles from bacteria, protozoa, microarthropods, and grazers are filled. Introducing compost teas in that situation won’t accomplish anything. Many no-till farmers use water only cycle after cycle and grow the most incredible plants you’ve ever seen. Compost teas aren’t necessary, and they definitely shouldn’t be used on a schedule.
To properly use and integrate compost teas into your garden, you must familiarize yourself with microscopy and the soil food web. This means taking a soil sample, agitating it in water, and viewing the sample under a microscope at 400x. I cannot properly educate anyone on microscopy in this article, and I’m not going to try. I will say, though, that when looking at a soil sample and an essential microbial population is missing, it can be introduced with a compost tea tailored to multiply that specific population. If that population is already present in its appropriate numbers adding more of it will only disrupt the balance of your soil biology. This is where people get their harsh words about compost teas. Why would someone bother with any of this if they don’t have to? If the soil can be topdressed with composts or mixed in for proper biology, why bother?
There are several instances that benefit from the use of compost teas. As mentioned above, compost teas can introduce specific biology. If the soil is lacking protozoa for whatever reason, adding a compost tea rich in protozoa will help balance the soil. When soil is brand new, freshly mixed, or hasn’t had plants growing in it for a long time, it can lack bacteria. If you have a lot of soil and don’t have the budget or infrastructure to topdress or introduce compost, compost teas are an easy solution. Compost tea is a good solution if your soil has too much organic matter, and adding more for biology isn’t an option. Maybe a soil is rich in bacteria and a balanced protozoa population but lacks fungi. A fungal inoculant is needed. Large quantities of fungal inoculants are hard to come by. Agitated and aerated mushroom spawn is a fantastic way to introduce those fungal bodies to the soil. Although most fungi will not multiply in tea, this is an effective way of spreading a fungal inoculant over a large area. If growing acreage and topdressing compost by hand or machine isn’t an option, compost tea is a perfect way to build biology in your soil over time.
In organic farming, using plant-based inputs is very popular. Things like fermented plant juices, blended up aloe vera, liquid kelp, and polysaccharide sources. Unless formulated otherwise, these all require biology to break down organic compounds to make all their compounds and nutrients readily available to plants. Introducing these inputs to a compost tea speeds up their availability by giving biology a head start in breaking them down in the tea. This diversity of food sources also provides the foundation for a larger diversity of biology in your compost teas. I have personally verified this under a microscope. Molasses is a common food source in compost tea, but I recommend replacing it with a polysaccharide. Molasses is rich in glucose, a monosaccharide, and will only feed a narrow spectrum of biology compared to polysaccharide sources.
There are dangers and risks to be aware of when making compost teas. Compost teas have the potential to multiply bad bacteria and fungi that will harm your plants and even make you very sick. When feeding biology in a compost tea, it’s important not to overfeed. Overfeeding a compost tea with food sources will lead to large bacterial and protozoan blooms. Most bacteria and protozoa are aerobic microbes and, in high numbers, will consume too much of the available oxygen in a tea. These anaerobic conditions give anaerobic bacterial and fungal pathogens an environment to multiply, which is why using a microscope is important. If a compost tea has multiplied a fungal or bacterial pathogen, it needs to be sterilized and discarded. An invaluable tool in making compost tea is a dissolved oxygen meter. It’s important to take note of your dissolved oxygen levels when making compost tea. Potential anaerobic indicators can be found under a microscope, but a dissolved oxygen meter is a sure way to determine the dissolved oxygen levels in a tea. Monitoring your water temperature is important. Coldwater will hold more oxygen than warm water. If your tea gets too warm, it can mean bad news for your tea.
There are multiple compost tea brewer designs to increase dissolved oxygen levels and prevent anaerobic conditions. I recommend a conical brewer. Do not use airstones in your compost tea brewer. While an airstone may seem like an obvious choice for aeration, we want an air source that will better agitate your compost or inoculant. You want your tea to physically agitate the bacteria off of your compost while providing oxygen to all areas of the brewer. A conical brewer with free-floating inputs is ideal for this because your inputs fall to a center point and are then agitated and oxygenated by the air source when it falls to the lowest point of the tea brewer. Don’t be alarmed if you see foaming occur when making compost tea. Foam is the result of degrading proteins. Protein is made of amino acids, and when agitated, the bonds holding them together break and oxidize, resulting in bubbles and foam. This is not a sign of microbial activity.
There is one last concept surrounding teas I want to cover—using nutrient-dense amendments in a compost tea. Many people like to do this, but it’s important to understand how nutrients are made available and the conditions required to make them available. Typically compost teas are not nutrient sources, but they can be made to be if really desired. Nutrients become available when degraded by bacteria, which are then consumed by protozoa. This is why protozoa are so important in the soil. If you want to add an amendment source to your tea, that tea needs to be very high in protozoa. Protozoa are like the predators of the microscopic world. Their name means first animals. Its believed that all animals evolved from protozoa. Tea will become protozoa dominant with time. Typically you start to see protozoa take over a tea around 48 hours of aerating. Letting protozoa overtake a tea with amendments is key to making those amendments available. However, many amendments take much more time to break down than is practical for tea brewing. Some people will let a tea go for 72 hours or more for readily available nutrients. Keep in mind that it’s possible to brew pathogens, and one must be very careful when doing this. I recommend topdressing your amendments and using compost teas strictly as an inoculant.
Compost teas are a tool. Like any tool, they can be dangerous. Tools are also needed to build anything significant. Compost teas can promote life and bring fertile conditions to your soil. They can also kill all your plants. Be smart. Learn how to identify microbes with a microscope. Without educating yourself first, making compost teas is like shooting a gun in the dark. You might get lucky; you might not. There are other alternatives to making tea. Is it completely necessary? No, it’s not. I see compost tea brewing as an art. The biology and advantages teas provide can result in massive growth rates and plant health. Just be careful and respect nature. She can bite back.
Soil biology specialist.