Organic horticulture is often referred to as the biological approach to growing. This is because organic growers dedicate as much or more attention to microbial ecology in soil as they do to any other aspect of plant growth and vitality. One of the most effective ways to boost microbial populations is the periodic application of compost teas.
Actively aerated compost teas are an economical and effective means to improve all aspects of plant growth, including health, pest and disease resistance, water and nutrient uptake, produce quality and total yield. They are also equally useful in hydroponic systems and can be customized to meet specific goals.
Much like yogurt fosters helpful bacterial colonies in the human digestive tract to help digest food and support the immune system, compost tea aids soil dynamics. In soil, bacteria and mycorrhizal fungi (beneficial fungi) work to suppress pathogens in several ways and both can form a biological barrier at the root and foliage zones, preventing disease-causing pathogens from getting a foothold. They also produce antibiotics and enzymes that kill pathogens.
Bacteria and mycorrhizal fungi work symbiotically with plants, providing numerous essential functions in exchange for carbon sugars (exudates) secreted by plant roots. Since this plant-microbial relationship is so essential, scientists estimate that 40% or more of plant energy produced through photosynthesis is converted into root exudates used to feed soil micro-organisms.
In a highly complex ballet choreographed by seemingly near-sentient plants, individual species of bacteria and mycorrhizal fungi are chosen for the service they can provide plants, simply by altering the exudate compounds offered through roots. Mycorrhizal fungi form networks that produce acids to digest insoluble minerals, and scavenge nutrients and water to be shipped to connections at plant root tips, increasing the absorptive capacity and surface area of roots.
Bacteria also colonize plant roots and other areas of soil, producing slimes that break down organic matter into soluble nutrients. Dead bacteria and bacterial fecal matter act as soluble fertilization packets for plants. Bacteria become food for protozoa and nematodes, which excrete even more fertilizer- and nitrogen-fixing bacteria, such as azotobacter, that convert atmospheric nitrogen into plant usable forms and amino acids. Plants can even select certain microbes to alter pH at the rhizosphere as a self-defense mechanism against invading pathogens or to make nutrients more or less available.
These fascinating interactions are the reason why supplementing forests and natural ecosystems is entirely unnecessary. Mother Nature has developed these highly complex, closed-loop ecological systems, and integrating and cultivating these natural systems is the heart of the biological growing technique.
Compost tea feeds the soil with countless microbes that work symbiotically with plants, providing accessible nutrients from otherwise insoluble minerals. The benefits of using compost tea include reduced water and fertilizer requirements, increased root and foliar growth, and improved soil tilth, porosity and nutrient-holding capacity.
Compost tea exponentially multiplies the microbes found in the relatively small amount of compost used to make it. Although compost teas have been used for centuries, the modern technique of active aeration (brewing with oxygen) results in vastly superior tea. Modern compost tea is teeming with beneficial aerobic organisms including mycorrhizal fungi, bacteria, nematodes, protozoa, ciliates and yeast.
The most important factor in brewing teas is the compost that is used. Good compost can have up to 25,000 species of bacteria, 10,000 species of fungi and many species of protozoa and nematodes. Immature or improperly composted material can be a vector for pestilence, containing high numbers of pathogenic bacteria and fungus. It is critical that you first evaluate your own compost for use or locate a source with verifiable quality.
I recommend vegetarian composts, as manures can be high in salts and also pose a greater likelihood of carrying pathogens. Depending on the requirements of the plants you’re growing, a fungal-dominant tea is made by using compost high in materials such as wood chips or dried leaves. Bacteria-rich tea can be made by using compost with less woody material and by adding more sugar, such as organic black strap molasses.
Exceptional compost tea can be brewed with a 5-gal. bucket, an aquarium air pump and an air stone. As you become more proficient in brewing, more complex apparatuses can tested. Making compost tea is a fun project for both new and well-seasoned gardeners. Fill a 5-gal. bucket with water but allow the chlorine to dissipate by leaving the water out overnight.
Add one-quarter cup of un-sulphured blackstrap molasses or one cup of cane sugar. You may also add one-quarter cup each of liquid humic acid, seaweed extract and rock dust for a powerful vitamin and mineral-rich solution. Fill a cheesecloth, old tube sock or panty hose with one dry gallon of compost and submerge in the bucket. Use a small aquarium air pump and air stone to aerate the tea for 18 to 24 hours, then remove the compost bag and your tea is ready.
During the brewing process, microbes found in the compost will use the oxygen, humic acid, sugar and minerals as a food source to rapidly reproduce. The tea will then contain more microbes than the compost originally supported. Good compost tea may have up to 100 trillion bacteria in tiny drop of solution.
When the tea is finished brewing, it should have a pleasantly rich, sweet smell, and there may be significant foaming. Should you notice any foul odors, do not use the tea, as noxious odor is symptomatic of anaerobic conditions and it may harbor pathogenic bacteria that can be harmful to plants. Thoroughly clean the equipment with a light bleach solution and begin another batch with greater attention to higher quality, properly finished compost.
Aerobic compost tea should be used immediately after brewing as the useful shelf life is limited to 24 to 36 hours. For both hydroponic and soil gardens, the tea can be used at full strength or diluted with non-chlorinated water at a rate of one-tenth parts water to one part tea. Typically, compost tea is diluted to increase the coverage rate in greenhouses or large agricultural applications.
Undiluted or diluted compost tea can be poured directly on plants as a drench, or applied through a sprayer. Run the tea through a filter bag if you intend to use a sprayer to avoid clogging the nozzle with compost particles.
For hydroponic gardens, compost tea should be added to the reservoir at a rate of about one to two quarts per 10 gal. of reservoir water, although application rates are highly elastic. If adding tea to hydroponic reservoirs, it is important that your system maintain some form of constant aeration.
Stagnant reservoirs can result in an abundance of anaerobic bacteria when available oxygen is depleted. Periodic applications of sugars, such as un-sulphured blackstrap molasses, palm or raw cane sugar, will provide the food to maintain bacterial populations, while supplements such as rock dust, humates and humic acid, sea salt minerals or seaweed extract will provide mineral nutrition to both bacteria and mycorrhizal fungi, and in turn, to plants.
One important consideration when using biology for hydroponic growing is including a structure that serves as microbe habitat much like a coral reef does in the ocean. Depending on the individual hydroponic system, the grow substrate already used may be perfectly adequate, as soil microbes can colonize a wide variety of materials. Some of my favorite materials include silica stone, coco coir, biochar or growstone, based upon the structure, porosity and mineral content.
If your system uses only rockwool or is without media at all, structure can be added simply by filling a net bag of your chosen material and placing it in the reservoir. This gives the microbes something to colonize and proliferate. So far I’ve found that biochar is the most conducive to microbe habitat.
While mycorrhizae are dependent on plant roots for survival, it has been demonstrated that given proper nutrition and oxygen, mycorrhizae will colonize and proliferate in and on biochar despite the absence of plant roots. Powdered biochar added to the reservoir or chunky biochar held in a net bag is an excellent material for microbial housing structure in hydroponic systems.
So that's what you need to know to brew up your own batch of compost tea—the biological approach to growing.