The Truth About Compost Tea: Making it, Using it, and What to Expect from it
The internet is loaded with directions, advice, equipment and warnings about compost teas. This article explores how to make it, its benefits, potential downsides, and current best practices.
What is compost tea?
Historically, compost “teas” were made by suspending a bag of compost, or maybe manure, in a bucket of water, letting it soak for a few days until the water darkened, then using it as high-nutrient water for plants. I remember that my grandparents collected chicken manure in an open wine cask where it would be diluted with rain water, then stirred and used to water the tomatoes. Today, this isn’t a recommended practice. The awful smelling soup, now called leachate, not tea, is anaerobic, housing undesirable anaerobic bacteria and potentially pathogenic bacteria like E coli and Salmonella.
The product that is favored today is called Actively Aerated Compost Tea (AACT). The brewing technique is to use a container, chlorine-free water and some high quality, fully decomposed hot compost, preferably in a mesh bag, and an aeration system, like a multi-outlet aquarium pump with a couple of air stones to keep the water highly aerated. Some recipes suggest additives to encourage the growth of desirable bacteria and fungus. Processing time is about 2 days, and it is recommended that the tea be used immediately after processing is completed, before the thriving microbial community exhausts the available oxygen supply.
Is it worth the effort?
There are many enthusiastic supporters who believe it is. AACT goes a lot farther than compost. Five gallons is enough to dose a typical home plot. Well-made AACT can have 4 times as many microbes as regular compost. Because the nutrients are largely dissolved, they can be quickly absorbed by plants.
Furthermore, besides being used unfiltered as a soil drench, filtered AACT can be used as a foliar spray. Treated plants can then absorb nutrients through their vegetation and the “good” microbes can theoretically help limit diseases by some combination of attacking and outcompeting pathogens. It can also be used to stimulate microbial activity and decomposition in a compost heap.
Questions and Limitations
There are detractors, however. They point out that the nutrient value of an AACT batch is some fraction of the nutrients in the small amount of compost used to generate the tea, which is spread across a much larger area. There is no argument about being able to generate very high microbe populations, but there is little control over the types of bacteria, batch to batch, and there is some risk of building pathogenic populations, depending on the organisms in the original compost. Beyond this, does it really make sense to build the microbe population if there isn’t enough organic matter in the soil to feed them once they arrive? Finally, while there are many claims of added disease resistance and control, there are also legitimate studies that find little or no benefit from foliar spray applications.
Best practices for successful application
The cooperative extensions around the country offer positive but limited information about compost tea use. But there is plenty of “how to” information about AACT from organic growers and their supporting organizations. The best advice for successful application is:
- Use high quality compost, meaning fully-decomposed material that has been hot composted to kill pathogens, especially if animal manure is a source material for the compost.
- Be sure to generously aerate the tea and use it immediately after it is ready to maintain the most diverse aerobic organism populations. Salmonella and E. coli are both anaerobic organisms that don’t thrive in aerobic environments.
- Stay away from additives that may fuel an increase in harmful organisms. Good compost alone, when brewed properly, makes a very good tea.
- As a visual check, finished tea should have a sweet earthy smell, coffee brown color and bubbles on top.
- Don’t spray the tea directly on edible plant parts.
- Thoroughly clean and disinfect brewing equipment immediately after each batch is made.
How to make AACT
There are many retail kits offered for brewing AACT. However, here is a DIY approach that is practical for most home gardeners:
- Fill a clean 5-gallon bucket with 4 gallons of chlorine-free water; rainwater is ideal, well water is good. City water should be aerated in an open container for a few hours to eliminate chlorine.
- Aerate the water with an aquarium pump, preferably double outlet, connected with plastic tubing to 2 air stones set in the water.
- Add about 1 cup of compost per gallon of water to the bucket. It is a good idea to put the compost in a pillow case or stocking to reduce or eliminate the need to filter the tea later, especially if you plan to apply it with a sprayer. If adding loose compost, put it in the bucket before adding water. If the compost is contained, squeeze the bag a couple of times after adding it to the water to help water infiltration.
- Many recipes recommend adding a nutrient source to the water to further increase microbial activity. For our bucket system, add 2 tablespoons of natural sugarcane, unsulphured molasses, maple syrup or fruit juice to encourage bacterial growth. For a fungally-dominated tea, add a similar amount of kelp, fulvic or humic acid or rock phosphate. Countering this widespread practice, however, is the National Organic Standards Board requirement that organic growers have their teas tested for pathogens if nutrients are added to the compost/water suspension. Additives of most concern are sugars used to stimulate bacterial population growth.
- Bacterially-dominated teas accelerate nitrogen fixation in the soil and can assist in insect resistance and suppression of some diseases. They are best for vegetables and annuals. Fungally-dominated teas also accelerate decomposition in the soil, especially of tougher, woodier materials and have been shown to aid the fight against powdery and downy mildew. They are favored for perrenials, shrubs and trees.
- Brew for 24-36 hours, looking for the aforementioned sweet, earthy smell and bubble-covered surface. Keep it out of the sunlight to avoid UV damage to the microbial population. Keep it as close to room temperature as possible for best results.
- Apply the tea within 4 hours of completing the process — before the thriving microbial population exhausts the oxygen in the mixture. It can be applied unfiltered directly as a soil drench at the rate of 5 gallons per acre, or, if filtered, as a foliar spray at 10 gallons per acre. It can be diluted up to 4 or 5 times the original volume while maintaining the benefits.
- If you have used chemicals on your lawn, AACT is a good way to regenerate soil organisms. It will take multiple applications to rebuild the in-soil microbial population.
- There is no limitation to application
- Best to apply it early in the morning or in the evening as UV rays kill microbes
- Disassemble the brewing kit and clean it to remove the brewing film with a 5% baking soda or 3% hydrogen peroxide solution.
A final assessment
While Actively Aerated Compost Teas aren’t necessarily a panacea for all garden ailments, they can be an effective means to strengthen the soil microbe population and assist in disease and insect resistance. Where there is already adequate soil organic matter, they can boost decomposition activity and nutrient availability for plants. They are a quick means of rebuilding microbial populations that have been harmed by use of chemical fertilizers and pesticides. They are certainly easier to apply than solid compost and are a good way to stretch a limited compost supply. While nutrient content is modest, their dissolved state makes nutrients immediately plant-available. Some users even think the brewing process is fun.
But, concerns about pathogens and variable effectiveness of foliar application are to be taken seriously. At the same time, while the literature is filled with pathogen warnings and questions about batch-to-batch effectiveness as a foliar disease preventive, there are also many effusive stories about and photos of lush gardens attributed to use of AACT. And, while there are plenty of warnings, I haven’t found instances of actual harm linked to it. It has been around a long time. I know my grandparents were believers. That said, following the NOSB guidelines makes good sense.
At the least, if you are working toward following more natural and organic soil building and gardening practices, AACTs are another potential tool in your tool chest and for many of us, deserving of a fair trial.
“Compost Tea to Suppress Plant Disease,” University of Vermont Extension, http://www.uvm.edu/vtvegandberry/factsheets/composttea.html
“National Organic Standards Board Recommendation for Guidance: Use of Compost, Vermicompost, Processed Manure and Compost Tea,” https://www.ams.usda.gov/sites/default/files/media/NOP%20Final%20Rec%20Guidance%20use%20of%20Compost.pdf