What compost tea is and why you'd make it

Compost tea is water that has had finished compost steeped in it, the way you'd steep tea leaves. The result is a liquid you spray or pour on soil and plants. The idea is that it carries nutrients and beneficial microorganisms from the compost into the soil faster than working solid compost in by hand.

You make it because it's a way to spread the benefits of compost across a larger area than you could with solid compost alone. A bucket of finished compost can make several gallons of tea. It also works faster—plants can take up nutrients from liquid form more quickly than they break down solid matter in the soil.

There are two main methods: passive tea, which is straightforward and takes longer, and aerated tea, which uses an air pump and takes less time. Both work. Passive tea is what most home gardeners make because it requires no equipment beyond a bucket.

Key Takeaways

  • Passive compost tea needs only finished compost, water, and a bucket; you let it sit for 24 to 48 hours without stirring.
  • Aerated tea uses an aquarium air pump to bubble air through the mixture, which speeds up the process to 12 to 24 hours and produces more active microbes.
  • Use finished compost that is dark, crumbly, and smells earthy—not fresh or hot compost.
  • Strain the tea through cloth or a fine mesh before using it so solids don't clog a sprayer.
  • Use compost tea within a few hours of making it, or within a day if you keep it cool; the microbes die off quickly once exposed to air.

Making passive compost tea in a bucket

Fill a 5-gallon bucket about one-quarter full with finished compost. Pour in water until the bucket is nearly full, leaving a few inches at the top. The ratio is roughly 1 part compost to 4 parts water, but it does not have to be exact. Stir it once to mix, then leave it alone.

Let it sit for 24 to 48 hours at room temperature. You do not need to stir it again. After the time is up, the water will be brown and will smell earthy and rich. Strain the mixture through cheesecloth, an old t-shirt, or a fine mesh strainer into a clean bucket or watering can. Compost solids stay behind; the liquid is your tea.

Use the tea right away by pouring it around the base of plants or spraying it on leaves. If you need to store it, keep it in a cool place and use it within a day. Once the tea is exposed to air, the microbes in it begin to die. Aeration (air exposure) kills them faster than you might expect, so fresh is always better.

Making aerated compost tea with an air pump

Aerated tea follows the same basic recipe but adds an aquarium air pump to bubble air through the mixture. Fill a 5-gallon bucket one-quarter full with finished compost, add water to nearly full, and place an aquarium air stone (the porous piece that bubbles come from) at the bottom of the bucket. Attach the air stone to the air pump with tubing, turn the pump on, and let it run for 12 to 24 hours.

The bubbling aerates the water and feeds oxygen to the microbes in the compost, which makes them multiply faster. After 12 to 24 hours, turn off the pump and strain the tea the same way you would passive tea. The result will be darker and will smell stronger because there are more active microbes in it.

An aquarium air pump costs $15 to $40 new, and the tubing and air stone are inexpensive. If you make tea regularly, the equipment pays for itself quickly. If you only make it once or twice a year, passive tea is simpler and costs nothing extra.

Choosing and preparing your compost

Use finished compost only—the dark, crumbly material at the bottom of a pile or bin that smells like rich soil. Do not use fresh or hot compost (the kind that is still heating up and breaking down). Fresh compost has not finished its work and will not make good tea. It may also contain pathogens that have not yet been killed by the heat of decomposition.

If your compost is lumpy or has large pieces in it, break it up by hand or run it through a screen before adding it to the bucket. This increases the surface area and lets more nutrients and microbes dissolve into the water. You do not need to pulverize it—just break up the biggest clumps.

Some gardeners add other ingredients to compost tea: a tablespoon of unsulfured molasses (which feeds microbes), a handful of worm castings, or a few leaves from a healthy plant. These are optional and not necessary for basic tea. If you are new to making tea, start with compost and water alone.

Straining and storing your finished tea

Strain the tea through cloth or a fine mesh to remove solids. If you plan to spray it with a pump sprayer, strain it twice—once through cloth and once through a finer mesh or coffee filter. Particles left in the liquid will clog the sprayer nozzle.

Use the tea the same day you make it if possible. If you must store it, keep it in a sealed container in a cool, dark place and use it within 24 hours. Once the tea is exposed to air, oxygen begins to kill off the beneficial microbes. A sealed container slows this process but does not stop it. After a day, the microbes will have died and the tea will be mostly just water with some nutrients left in it.

Do not refrigerate compost tea. Cold slows down microbial activity and can make the tea go bad. Room temperature or slightly cool is fine.

How to use compost tea on plants and soil

You can pour compost tea directly on the soil around the base of plants, or you can spray it on the leaves. Both methods work. Soil process feeds the microbes and nutrients into the root zone. Leaf spray (called foliar feeding) lets plants absorb nutrients through their leaves, which works faster but covers less area.

For soil process, pour the tea slowly around the base of each plant, the way you would water. Use it on vegetables, flowers, shrubs, and trees. For foliar spray, use a pump sprayer or spray bottle and coat the leaves lightly until they drip. Spray in the early morning or late evening, not in full sun, because wet leaves in direct heat can burn.

There is no risk of overdoing it with compost tea the way there is with chemical fertilizer. The worst that happens is that you waste the tea. Most gardeners explore it every two to four weeks during the growing season. Some explore it monthly. There is no hard rule—use it as often as you have time to make it.

When compost tea does not work and what to do instead

Compost tea works best on soil that already has some organic matter in it. If your soil is very poor, compacted, or depleted, tea alone will not fix it. In that case, work solid compost into the top few inches of soil first, then use tea as a maintenance tool afterward.

If you do not have finished compost yet, you have two options: buy bagged finished compost from a garden center (which is more expensive but ready to use), or wait until your pile is done. If you are just starting out, buying a bag or two to make your first batch of tea is reasonable. After that, your own compost will be ready.

If you live in a very hot climate, compost tea may break down too quickly in the heat. In that case, make smaller batches and use them when ready, or make aerated tea, which is more stable because it has more active microbes.

Frequently Asked Questions

Can I make compost tea with kitchen scraps instead of finished compost?

No. Kitchen scraps are not finished compost—they are still breaking down. Tea made from them will smell bad, may contain harmful bacteria, and will not have the stable microbes you want. Always use finished, dark, crumbly compost that smells like soil.

Do I need to add anything else to make the tea work better?

No. Water and finished compost are enough. Molasses or worm castings can boost microbial activity, but they are not necessary. Start straightforward and add extras only if you want to experiment after you have made basic tea a few times.

What if my tea smells bad or looks slimy?

Bad smell usually means anaerobic (airless) conditions developed, often because the compost was too wet or the bucket was sealed too tightly. Slime is usually a sign of mold or bacterial overgrowth. Throw it out and start over. Make sure your compost is finished and not too wet, and do not seal the bucket during passive brewing.

Can I use compost tea on seedlings or newly planted transplants?

Yes, but dilute it first. Mix one part tea with one part water and use that on young plants. Full-strength tea can be too strong for delicate seedlings. Once plants are established, you can use full-strength tea.

How long does compost tea stay good if I refrigerate it?

Refrigeration does not help much. The microbes will still die within 24 to 48 hours, and cold actually slows their activity. Make tea fresh and use it the same day. If you need to store it, keep it at room temperature in a sealed container and use it within 24 hours.