Compost & Teas

Understanding the debate

Why not just add compost?

Adding compost directly to soil has a clear benefit: you keep the solid organic matter. It contributes to soil structure and helps the soil hold water and nutrients. A strained tea leaves much of that material behind in the bag. They aren’t interchangeable forms of the same soil amendment. Oregon State’s compost guide explains those soil benefits.

Tea brings a different mixture into the picture: water-soluble compounds, fine particles, and organisms carried out of the compost. What’s in that liquid depends on the starting material and how it’s prepared. With added ingredients, you’re also evaluating what those ingredients contribute, rather than compost alone.

Can the liquid actually feed plants?

Yes. In University of Hawaiʻi experiments with pak choi, vermicompost tea improved growth and nitrogen uptake compared with water controls. Soil biological activity also increased. The researchers linked much of the difference in plant response to nitrogen availability. That gives the feeding side of compost tea a real basis in research. Read the pak choi study.

Some nutrients can already be dissolved in forms roots can take up, while nutrients still bound in organic material need further breakdown. So a tea can contribute available nutrients without delivering everything in the original compost. Its nutrient content matters more than how dark it looks.

Where the disagreement starts

A plant responding to tea doesn’t tell us which part of the tea caused the response. Was it the nutrients? Other dissolved compounds? Microbes? A combination? Those are separate questions, and a comparison with plain water doesn’t fully answer them.

For example, a soybean field study compared living tea with the same tea after heat sterilization. The living tea produced no significant advantage in growth, yield, or the soil bacterial community they measured. That challenges the idea that adding live tea microbes automatically improves soil. It doesn’t tell us whether either tea would have outperformed plain water, because that wasn’t the comparison. Read the soybean study.

What about getting a top-dressing cycling?

This is closer to the reason I use tea with a top-dressing. Washington State researchers reported that certain compost teas enhanced residue breakdown in controlled incubation experiments. That’s encouraging evidence for an effect on decomposition. The next question is how well it carries over to different soils and amendments outside those experiments. Read WSU’s project summary.

For me, compost and top-dressing remain part of the soil’s foundation, and tea is another part of the routine I’ve developed over years of growing. My results are why I use it. The research helps explain what may be contributing to those results without pretending every bucket is identical.

These are the recipes I like to use and have had the best results with: microbial tea, fungal tea, veg tea, and flowering tea. They share the same basic setup, with a few ingredient changes for each one.

There’s plenty of room to experiment here. I encourage you to try different products and recipes and find what works best in your own garden. I’ve tried all sorts of combinations over the years, and these are the ones I keep coming back to. You don’t have to use the exact same brands I do. Change one thing at a time and keep a few notes so you can follow what’s working for you.

I use reverse osmosis (RO) water, a 5-gallon bucket, a TeaLAB brew bag, two Active Aqua air pumps, and two air stones. I run the pumps throughout the roughly 24-hour brew. The full recipes below use about 4–4½ gallons of water in that 5-gallon bucket. When I make less, I reduce the ingredients in proportion to the water. The fungal recipe has an extra preparation step before it goes into the bucket.

When I say “halfway,” I’m talking about filling the brew bag with compost or worm castings. The quantities below are for each recipe made in that setup.

What the air is doing

Why aerate a compost tea?

AACT stands for actively aerated compost tea. The air pumps have a job: helping replace the dissolved oxygen that microbes use while they feed and grow. Oxygen moves from the air into the water, where those organisms can use it.

The compost brings living organisms into the mixture. As oxygen-using microbes break down food, they consume oxygen. Aeration helps replenish that supply. That’s the reason air is part of this brewing method.

Aerobic and anaerobic: what’s the difference?

These words describe how organisms live in relation to oxygen. Bacteria are one group of microbes; fungi and other microscopic organisms are part of the picture too.

Some microbes can do either. These are called facultative anaerobes: they can use oxygen when it’s available and switch to another way of getting energy when it isn’t.

An organism’s oxygen needs alone don’t tell you whether it helps plants or causes disease. The point of aerating an AACT is to maintain oxygen for the organisms that use it. Bubbles show that air is moving; they don’t measure how much oxygen is dissolved in the tea.

Temperature and the brewing spot

Give the brew steady conditions

From my own experience, warmer conditions seem to get things moving faster, including when temperatures are in the 80s. There’s a reason that can happen: warming toward a microbe’s preferred temperature can speed up its activity and growth. Different organisms have different preferences, though, so the whole bucket doesn’t respond exactly the same way.

There’s another piece to this: warm water holds less dissolved oxygen than cool water. As microbes become more active, they can use oxygen faster too. That’s why temperature and aeration belong in the same conversation.

So is there one perfect temperature? The studies give different answers depending on the compost and what researchers measured. One temperature comparison found the strongest antifungal activity at 68°F and 86°F. That supports taking temperature seriously, but it doesn’t establish the 80s as the best range for every tea.

Feeding the microbes

Why I use molasses

I use Grandma’s Molasses because it’s inexpensive, it’s worked well for me, and I can pick it up at the grocery store. There’s no need to make a special trip to the grow shop for it. Keeping these recipes simple is a big part of why I’ve stuck with them.

The sugars give many of the microbes in the tea an accessible food source. They break those sugars down for energy and use some of that carbon to build new cells. With food and the right conditions, they can grow and multiply, increasing their population in the brew. That’s the reason for including molasses.

Think of it as putting food on the table for organisms already in the compost. Sugar is one food source; the compost supplies other organic material and nutrients too. As oxygen-using microbes become more active, they can consume oxygen faster. That connects directly to why I keep the tea aerated.

What about other sugar sources?

Molasses and brown sugar both appear in compost-tea research, and plain sugar has been studied too. Coconut water comes up in growing discussions because it naturally contains sugars. These ingredients differ in concentration and in what else they contain, so they aren’t equal spoon-for-spoon replacements.

Unsulfured, non-GMO, and certified organic describe different things. If you specifically want an organic product, look for that certification on the package.

Sugar can also feed unwanted organisms if they’re present. Aeration doesn’t sterilize a brew, which is why the starting materials and clean equipment matter.

I use microbial tea for seedlings, transplants, and watering in top-dressings. When I top-dress, this or my fungal tea is what I reach for.

I fill the brew bag about halfway with compost, worm castings, or a combination of the two. This recipe uses bokashi, liquid seaweed, and molasses alongside that base, with a brew time of about 24 hours.

Why I sometimes add an old root ball

Sometimes, if I have an old root ball on hand, I’ll add that to my microbial tea too. The roots and the soil clinging to them are home to microbes. The root material also contains carbon and nutrients that decomposers use as they break it down.

Adding it can bring some of that root-zone community into the brew, along with organic material for microbes to feed on. That’s its potential value here. The roots won’t disappear or release all their nutrients during a 24-hour brew. Keep diseased roots out, especially anything from a plant with suspected root rot, so you aren’t carrying that problem into the bucket.

This is the same tea recipe I used in the Undeniable Heat masterclass. It’s one I like to turn to when my soil needs some attention, and the tea I prefer to use more often. It takes more planning, though: I prepare the compost and castings a few days before brewing.

Prepare the base

1. Put compost, worm castings, or a combination of the two into a container with a lid.
2. Add a handful of bokashi or ground oatmeal and mix it throughout.
3. Sprinkle another small handful of the same ingredient over the surface.
4. Moisten it lightly, then close the lid.
5. Let it sit for about 3–4 days. I look for visible fungal growth across the surface before using it as my tea base.

Then brew the tea

I use that prepared material as the base, add the liquid seaweed and molasses, and brew for about 24 hours.

Watch the Undeniable Heat masterclass

This is the recipe I use for veg. I use the same compost or castings base, with alfalfa meal and kelp meal added to the mix. The brew time stays at about 24 hours.

For this recipe, I use Down To Earth Rose & Flower Mix in place of the alfalfa meal. The kelp meal, bokashi, liquid seaweed, and molasses stay in the recipe, and I brew it for about 24 hours.

Down To Earth product information

Make the amount you need

Making a smaller batch

My full batch uses about 4–4½ gallons of water. A half batch uses half that water and half the ingredients. The same goes for a quarter or three-quarter batch.

Pick your recipe above, then use the matching column below. Only include the ingredients listed in that recipe.

For the fungal base, reduce the compost or castings and the handfuls of bokashi or ground oatmeal by the same fraction. The smaller amounts refer to the ingredients, rather than a new fill line on the bag.

For another water amount, divide it by the water amount in your full batch, then multiply each ingredient by that fraction. Use the same full-batch water amount throughout the calculation, whether that’s 4 gallons or 4½.