How to Brew Compost Tea

Compost teas are a great way to give your garden some of the same benefits you’d get from adding compost without having to shovel and cart heavy compost around the yard. Like compost, compost tea (when brewed correctly) is brimming with beneficial bacteria, fungi, nematodes and nutrients. For instance, one teaspoon of compost tea may contain up to 4 billion (good) bacteria! This fantastic concoction does wonders in the landscape when applied to a plant’s leaves and the root zone or to the soil in general.

Things to Consider

As is true of compost, not all compost teas are equal. Teas may vary by bacteria and fungi populations and by nutrient content. Some of this is controlled earlier during the composting process, before brewing compost tea. For instance, to make compost tea with high nitrogen content, you should use finished compost that was created with special attention to nitrogen.

While nutrients are important to plant health, so are the microorganisms that inhabit compost. Although all plants need beneficial bacteria and fungi, some plants prefer different proportions. For instance, long-lived trees, shrubs and perennial plants prefer compost tea recipes that are rich in fungi, while annual plants such as flowers, vegetables and turfgrass prefer more bacteria than fungi.

Finished compost is at the heart of every compost tea recipe. Completely finished compost has an earthy, sweet smell, and usually contains both fungi and bacteria. Although worm castings are safe for compost tea brewing, composts that contain animal manure may harbor e-coli bacteria. The tea-making process should kill e-coli, but it’s better to be safe and avoid composts with manure than to take the risk.

Oxygen-loving (aerobic) bacteria will thrive in a compost tea brewer if provided the correct environment. Water should be dechlorinated or the bacteria won’t survive. Oxygen needs to be injected with an air pump to keep beneficial bacteria alive and growing. A small amount of sweet, sugary food, like non-sulfured molasses, maple syrup, cane syrup or even fruit juice, helps bolster bacteria populations. Although all compost contains some amount of bacteria, vermicompost is especially rich in these microbes because bacteria live in worms’ digestive systems.

Fungi, on the other hand, have a tough time competing with bacteria during brewing. In their book Teaming With Microbes, Jeff Lowenfels and Wayne Lewis give some tips to bolster your fungal populations. About one week before brewing, mix your finished compost with a fungi food source like oats (oatmeal, powdered baby-food oatmeal or oat bran), powdered malt or soybean meal. Add 3-4 tablespoons of fungus food for every cup of compost.

Fungi need moisture to grow. After adding the food source, check to make sure the mixture is damp but not drenched. If you need to add a little moisture, remember to use dechlorinated water. Store the mixture in a dark, warm area — about 80 degrees Fahrenheit would be best, although room temperature should be sufficient. At the end of the week, the mixture should be full of fine, silver strands of fungal hyphae.

During brewing, it helps if fungi have some added surface area to grow on. Add some bulky ingredients, such as fruit pulp, rock dust, kelp or fish hydrolysate (basically, ground-up fish), to harbor fungi. Fruit pulp is easily on hand for any juicing fans, and kelp can be bought from many garden centers.

How to Make Compost Tea

1. Gather the brewing supplies. Before you start brewing your compost tea, you'll need a few supplies. You can find commercial products to help you do the job, or you can make your own system. All you need is a 10-gallon bucket and an aquarium pump. The most important things, of course, are good quality compost, a water source, aeration and some compost catalyst for good measure.

2. Fill your bucket with water. If you're using tap water, let it sit for a day in the bucket. This allows the release of chlorine, which could otherwise kill beneficial micro-organisms.

3. Add the catalyst. Dump the compost catalyst into the bucket. The catalyst is a commercial mixture of nutrients that essentially wakes up the micro-organisms in the compost and encourages them to multiply.

4. Float your compost. Put the compost in a "sachet" in the bucket. A sachet? Think of the wire-mesh containers used in brewing the tea you drink. (If you don't have a compost pile at home, bagged compost from your garden center is a fine alternative.)

5. Pump it. The air pump is the final step to starting the brewing process. It circulates air up through the bottom and throughout the solution. It's essential for the beneficial fungi and bacteria in the compost to start working.

For 24 hours, let the air pump through the compost tea. The finished product is a rich, frothy brew. The foam is an indication of the nutrients, bacteria, fungi, nematodes and protozoans that are going to do their work on your plants. And there's no wasting anything from the brew; the solid compost can still be spread in the garden.

How to Use Compost Tea for Healthy Leaves

Did you know that bacteria and fungi live on plant leaves? The foliar bacteria and fungi that are most familiar to gardeners tend to be undesirable diseases and pathogens, like powdery mildew or leaf spot. Although a fungicide or other pesticide treatment can help clear up or stop the spread of the bad microorganisms, these treatments also kill the beneficial bacteria and fungi that live on the leaf surface.

One would think that sterile foliage would be a good thing, but it actually isn’t. When leaves are covered with the sort of beneficial bacteria and fungi that are found in compost tea, there’s more competition for resources and less space for undesirable microbes to take hold. Rather than treating diseases with fungicides after they’ve already taken hold, consider applying compost tea to foliage preventatively at the beginning of the growing season.

Properly brewed compost tea shouldn’t burn leaves. However, the ultraviolet (UV) rays in sunshine can kill the microorganisms; so it’s best to apply before 10 am or after 3 pm during the long days of summer. A standard garden pump or backpack sprayer can be used, although a concrete sprayer will be just as effective and better able to handle the occasional solids that make it through a strainer. It’s fine to dilute compost tea as long as the water used is free of chlorine (which would kill the beneficial organisms in the tea). Apply liberally to the top and bottom of leaves.

How to Use Compost Tea for Healthy Roots

Drenching with compost tea is a great way to quickly deliver organic fertilizer and beneficial organisms from compost to a plant’s root zone. Simply pour the tea mixture onto the soil in the area below the plant’s leaves (or under the “drip line”). The liquid tea will carry beneficial bacteria, fungi and other microorganisms down to the area around the roots.

Once there, the bacteria will help unlock nutrients that are already in the soil but were previously in a state the plants couldn’t use (for example, converting unusable ammonium nitrogen to nitrite, then to useable nitrate). The mycorrhizal fungi will create a symbiotic relationship with plant roots that will benefit both the fungus and the plant. In exchange for some of the carbon in the roots, mycorrhizae will extend further into the soil to bring back nutrients for the plant. Some estimates suggest that mycorrhizal fungi can extend a plant’s reach in the soil by 95%!