What Is Compost Tea? (And How To Make Compost Tea)

Last updated on October 23rd, 2023 at 08:53 pm

Oxygenated compost tea (O.C.T.) is a mixture that is not yet widely used in France, but is more widely used in the United States, where it was developed by Elaine Ingham, a specialist in soil microbiology, following the example of French scientists Lydia and Claude Bourguignon. It is not a tea to be enjoyed with friends over macaroons in the afternoon, well installed in the garden, no, its name comes from the fact that it is a kind of maceration of compost in water. Let’s see how to prepare this compost tea and what are its properties.

utilisation du thé de compost au jardin

Properties of compost tea: what is it used for?

Compost tea, resulting from an aerobic maceration, offers a biologically active product, containing the “good” bacteria of compost. Therefore, its properties are anti-parasitic on the one hand, and fertilizing on the other hand, but not as systematically as an ad hoc product: compost tea boosts the biological activity of the soil and this is why it has the indicated effects.

What Is Compost Tea? (And How To Make Compost Tea)

The beneficial micro-organisms (bacteria, fungi…) contained in the compost tea that will be sprayed on the plant or on the soil will favor the microbial life, allowing to better prevent plant pathologies by creating a kind of protective guard of the aerial parts and/or the roots. The pathogens combated can be mildew, grey rot, rust, leaf spots, as well as beetle larvae, cutworms, beetles, etc.

In addition, these “beneficial” micro-organisms will produce amino acids and enzymes that promote the release of nutrients that would not otherwise be present: nitrogen, phosphorus, potassium, calcium, but also trace elements (magnesium, iron, sodium, copper, manganese, cobalt, zinc, selenium, boron, molybdenum).

Making compost tea

Making compost tea is done when it is warm so that the materials are at +/- 20°C, and you need a good compost, not a common “garbage heap” or “manure pile”. The compost must be of good quality so that the tea, which will be its reflection, will be as well. Whether it is a compost of vegetable origin (green waste) or of animal origin (manure), it must be mature, that is to say at least 8 months old and 4 to 6 months old respectively, but not more than that, otherwise it will be less interesting from a nutritional point of view. The concentration of the compost tea will depend on the maturity of the compost and the purpose of the application of the tea. To get started, you need to prepare :

  • a large black plastic garbage can or large bin, waterproof and clean
  • filter cloth or fine canvas bag
  • 1.5 to 5 kg of mature compost
  • 50 liters of rainwater (no chlorinated water or wait 24 hours for the chlorine to evaporate)
  • aquarium bubbler

Put the compost in the cloth bundle or canvas bag and immerse it in the water in the tank. Install the bubbler and run it for 24 hours. If there is no bubbler, you will have to stir very often because if the oxygenation is insufficient, it will increase the duration of the process which may favor the development of pathogenic elements. If you did not use a bag to brew the compost, you must filter the maceration.

Your compost tea will be ready to use in 24 hours, with a brown color like tea and no smell.

Compost tea can be used as is, but it can also be enriched with additives to improve its properties by feeding the micro-organisms it contains: algae, rock powders or organic molasses, among others.

Use of compost tea

Compost tea is not preserved, it must be used as soon as it is ready and within a maximum of 6 hours because, afterwards, the micro-organisms will die, as they will not have enough food to feed themselves, and therefore the tea will lose its properties.

The dilution of the compost tea is generally done at 20%, that is to say 1 liter of tea for 5 liters of water. There are 2 ways to use the diluted compost tea in the garden:

  • as a foliar spray (top and back of leaves + stems) ;
  • by watering the soil.
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Compost tea should be applied in the shade, either when the sky is overcast or at dawn, on moist soil or, at the very least, on plants that do not show a lack of watering. The periods of use cover the beginning of autumn, before the fall of the leaves, as well as spring, according to the needs, with a frequency which can be of 3 weeks or more tight (1 week) according to the situations.

After a few experiments, you will master a little better and you will know how to adjust according to the compost used, its maturity, possible additives, etc. There is no need to concentrate the mixture too much, as you risk seeing your plants grow too quickly and become more susceptible to disease. All of this remains very empirical. It’s up to you!

What Is Compost Tea? (And How To Make Compost Tea)

Organic, highly effective and easy to make yourself: compost tea for the garden.

Plants absorb their nutrients dissolved in water by transporting the water through the roots via the stem to the leaves. What could be more natural than to provide the power mix for healthy and rapid growth in liquid form. The manufacturers of liquid fertilizers have long seen the sense of this. Compost tea is easy to make yourself, all natural, and cost-efficient to boot.

Junges Gemüse

Compost tea is, roughly summarized, compost dissolved in water and enriched with microorganisms. These are not added, but develop on their own. The mixture is aerobic, so it must be constantly supplied with oxygen. As a result, fewer putrefactive gases develop, and the organisms that help the plants really grow.

Protection for the leaves and catalyst in the soil

Compost tea is not simply a fertilizer that allows plants to increase their nutrient uptake. The microorganisms dissolved in compost tea protect plants. Sprayed on the leaves, compost tea can strengthen the plant and destroy pests. If the plant is watered with it, unwanted pathogens in the soil also cannot spread, and the roots are protected from harmful fungi. In addition, the tea acts as a catalyst, it allows the plant to better absorb nutrients from the soil.

Different methods of making compost tea

There are different methods of making compost tea. One of them is to simply stir chopped vegetable scraps and plant waste into chlorine-free water and dissolve it in it. This mixture must be left for up to four days and stirred repeatedly so that the desired organisms can multiply with oxygenation. When mushroom threads are drawn through the liquid, the compost tea is ready for use. However, this simplest method of production really requires frequent stirring to avoid anaerobic fermentation.

Another form of this miracle product is created in the bokashie bucket. There, too, a liquid emerges from the plant residues sprayed with microorganisms and slowly composted, which (strongly diluted) is an effective fertilizer. Undiluted, however, this liquid is so strong that it is only suitable as a drain cleaner. And that is to be taken literally. Diluted, it can protect the leaves of plants, can prepare the soil around the roots and help the plant grow.

Standard method of making compost tea is to mix compost and molasses in chlorine-free water in a plastic barrel. A tablespoon of cooking oil prevents foaming during fermentation. A commercial pond aerator pump provides the necessary oxygen. The brew should bubble away for about 18 to 24 hours, then it’s ready to use.

Alternatives: beet syrup instead of molasses, shredded kitchen waste instead of compost
Compost tea can also be made with beet syrup instead of molasses (which, after all, you can’t buy in every supermarket). refined raw sugar is also suitable as a substitute.

Whether only garden waste is used for the compost part of the mixture, whether lawn clippings or fruit peels are used, is ultimately the same. All unpolluted, not further processed plant material is suitable for it. However, heated food waste is unsuitable, especially if it contains spices. Animal waste products should also be avoided.

What is compost tea used for?

There are about 1.6 trillion living organisms in 0.3 cubic meters of healthy soil. That’s 266 grams distributed among bacteria, ray fungi, other fungi, protozoa, algae and rotifers, nematodes, mites and springtails, bristle worms, earthworms, snails, isopods and more. Many of these organisms are there to recycle dead animal and plant matter; they feed on it. The larger inhabitants of the soil, earthworms and beetle larvae, for example, burrow through the upper soil layers and use their tunnels to ensure that oxygen and water can reach the soil. If these organisms are even partially absent, plants become sickly. This is because the waste products of the microorganisms are what provide the nutrients for most plants in the first place, and only water can absorb them. Earthworm-rich soil is therefore always soil that is conducive to plant growth.

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Artificial fertilizers are tailored to the needs of plants and feed only the plants. The microorganisms needed in the soil are not there, as a result, bugs, millipedes and worms cannot grow either. This leaves the oxygen-poor, nutrient-rich soil susceptible to all sorts of fungi and other organisms that also harm the plant. Solid, air-poor soils are not conducive to plant growth.

Compost tea favors healthy, species-rich soil that is oxygen-rich and water-permeable. Only such soil will be able to sustainably nourish healthy plants. Therefore, compost tea is always the better choice over artificial fertilizers.

Sprayed on the leaves, compost tea acts as a protective agent: the microorganisms it contains do not harm the plant, but leave no room for harmful organisms. They simply occupy the space and thus protect the plants. This is gentler on plants and the environment than conventional pesticides, which usually simply kill both desirable and undesirable organisms, creating a vacuum in which all sorts of (undesirable) things can subsequently colonize.

How is compost tea made?

Basic recipe for 100l CompostTeaBrew according to Dr. Ingrid Körner

  • 100 l chlorine-free water[1] (rain or well water)
  • 500 g compost
  • 500 g molasses (sugar beet syrup)

Dr. Ingrid Hörner recommends as additives:

  • 250 g rock flour (recommended)
  • 50 g BioAktiv (recommended)
  • Necessary equipment
  • 120 l compost tea kettle
  • 1x immersion heater 100-150 W [2]
  • 1x pond aerator pump [3]


  • Fill in chlorine-free water [1],
  • add molasses ( dissolve in lukewarm water)
  • add compost (in a tea bag [4])
  • set temperature to 25° C
  • switch on aerator pump
  • Aerate for 12-18 hours

Compost Tea Application

  • #Drain compost tea mixture and filter through a sieve
  • Dilute solution with chlorine-free water
  • Apply within 4 hours
  • Clean compost tea kettle thoroughly!
[1] chlorinated tap water should be filled 12 hours in advance and aerated with the compost tea kettle approximately 4 hours before starting compost tea production.

[2] If the outside or room temperatures are appropriate, immersion heating may not be necessary. We are still working on alternatives.

[3] the air capacity/minute is approximately equal to the water volume (for 100l compost tea about 6000l/h)

[4] tea bags (linen bags, laundry net, ladies stockings,…)

[5] If the compost tea is to be sprayed, it is advisable to pass the solution through a fine filter again when draining it (for smaller plants, a tree woollen cloth or a nylon pointed sieve from honey production is sufficient)

[6] Watering for soil treatment 1:5, spraying for foliar treatment 1:10, starter for compost preparation 1:1, for the required quantities you need rain barrels or your own well.

[7] After that, the microorganisms will slowly die back if not re-fed.

[8] Immediate cleaning is recommended, as the residues can then be easily removed with water.

Let your plants take a cure. Compost tea nourishes and heals the soil and only in good soil can your plants thrive optimally.

You may have heard of compost tea, but maybe you didn’t know what the difference is from worm tea and why compost tea is supposed to be so good.

Worm tea is the leachate created when organic waste is processed in worm bins. Worm tea is full of nutrients and microorganisms and can be wonderfully used as a bio-organic fertilizer when diluted 1:10 with water.

Compost tea ingredients

Compost tea boosts this even further by naturally increasing the helpful microorganisms so they can be administered in even higher concentrations. Furthermore, some additives such as silicic acid are better dissolved in compost tea. Silica, for example, helps to “build” stronger leaf walls and thus protects against pests. Our compost tea set makes it very easy to make this fertilizer fresh.

Compost tea is used in large quantities, especially by professional gardeners, to give plants a cure or simply to strengthen them preventively.

Studies have shown that compost tea not only strengthens the growth and healthy development in crops, but also has a protective effect against pests. Especially silicic acid – which strengthens the leaves of plants against stings of aphids – can be easily dissolved in compost tea and thus applied directly to the leaves.

Compost tea combines almost all the benefits of worm humus with a quick delivery method and an even higher concentration of helpful microorganisms. This means you can quickly provide your plants with valuable nutrients, but more importantly, protective bacteria and silica.

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Compost tea is “brewed” using a starter population of bacteria (worm humus or mature compost is best), a special sugary activator (molasses is best, honey is not) and plenty of oxygen for 24 hours. To do this, the activator together with worm humus and preferably chlorine-free (rain) water is aerated with an air pump for 24 hours.

During this time, aerobic bacteria in particular multiply by millions and important additives dissolve in the oxygen-rich water. For example, it is easier to supply the plants with important silicic acid.

In the activator mixture of our compost tea set are still the following ingredients that improve the tea:

Bentonite, a clay mineral which helps form the clay-humus complexes in the soil and provides better adhesion to the leaves when the finished compost tea is sprayed.

Primary rock flour, a mineral mixture of Dia Bas rock, which contains trace elements that are especially important for food. The soluble micronutrients increase the flavor and shelf life of the crop.

Algal lime, which protects the soil from acidification and supports many metabolic processes of plants. Due to its neutralizing properties, lime makes some nutrients available to plants in the first place.

Vegetable carbon – included in the activator. This certified plant carbon is guaranteed free of toxins and provides an ideal surface for bacteria and microorganisms to multiply and establish themselves.

Compost tea application

The finished compost tea is now either poured directly onto the soil around the plants, or sifted through the fleece to be sprayed as foliar fertilizer. This is best done during the growth phase of the plants and before problems with pests occur.

Typical application rates are 200 liters of compost tea per acre to the soil and 20-40 liters per acre for foliar fertilization.

Dilute before fertilizing

The compost tea should be diluted with the same chlorine-free water to avoid disturbing the bacteria.

The mixing ratio of water to compost tea, depends on the application, whether as a soil or foliar spray, as well as the biological condition of the soil and lastly whether the plant is wet or dry. Typical here is 1 liter of compost tea to 5 liters of (aerated) water.

When applying on the ground

In soil application (little leaf and much soil), 30-60 liters of compost tea are applied per hectare. Foliar applicationFor foliar application (much foliage and little soil), 20-40 liters of compost tea are applied per hectare.

The nozzle pressure

The application should be carried out with the technically lowest possible nozzle pressure to prevent damage to the microorganisms. Ideal is 1 bar pressure and 2-3 bar is a common setting.

After brewing the tea, it is important to clean all utensils well and let them dry so that “bad” bacteria do not multiply when used more than once.

Disadvantages of compost tea

A major disadvantage of compost tea is that it cannot be stored. The bacteria use up the enriched oxygen quite quickly and then begin to keel over. When the compost tea begins to smell bad, it should be discarded. Activating it again is possible, but far more difficult and less productive than starting over.

Another disadvantage arises when you have to factor in the nutrient content of the tea. This is possible due to the dissolved salts with the help of the EC value, but which macronutrients (NKP) are contained is difficult to measure. Therefore, compost tea should only be given to medium to heavy growers. Weak growers (herbs, etc.) could otherwise be overwhelmed with the amount of nutrients.

Compost tea refill set

Give it a try: water your plants with homemade compost tea and compare the effect! You can find the compost tea instructions for our set here.


  • James Jones

    Meet James Jones, a passionate gardening writer whose words bloom with the wisdom of an experienced horticulturist. With a deep-rooted love for all things green, James has dedicated his life to sharing the art and science of gardening with the world. James's words have found their way into countless publications, and his gardening insights have inspired a new generation of green thumbs. His commitment to sustainability and environmental stewardship shines through in every article he crafts.

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