COMPOST TEA: A Natural Probiotic for Soil and Plants

July 2025 · Lucile

As Dr. Elaine Ingham said, “there is no soil on this planet lacking nutrients to grow plants”, in other words there is intrinsically food (organic matter) in the soil, that we actually just enrich with mulch and compost in farming, according to the needs of our crops. However, sometimes there is not enough microbes to eat that food and release nutrients for the plants. Excess of food can turn into excessive oxidation and there is no conversion back to nutrients. Then compost becomes interesting because it contains a diverse amount of beneficial microbial population which contributes to the decomposition of the organic matter. In that sense spreading solid mature compost can be a solution, but it can be hard to do on a large cultivated area (heavy physical efforts, time consuming process). Turn it into a liquid form seems to be an easier way to do it on an agricultural scale. Dr. Elaine Ingham is the one who helped create the modern version of it : a compost tea using aerobic reaction.

Compost tea is an organic liquid fertilizer made by brewing mature compost in water with the aim to extract and multiply the beneficial microorganisms present in this compost, through aerobic processes, and introduce it to either directly to the roots or plants to rebalance and enrich the microbiome life.

Using compost tea is a permaculture practice that respects the life in the soil, understands how important it is, helps it grow without harming it while still allowing farming to continue. The main intention is important : to make all systems more efficient, in other words an healthy system which works on its own and not with the aim of exploiting the soil.

Compost tea can be poured to the roots of the plant to enrich microbial life of the soil as we just explained or it can be applied as a foliar spray by sprinkling it on the leaves to strengthen and protect them. More precisely, compost tea protects the pores on the leaves (called stomata). Because they are open areas, bacteria can deposit there. In general, good bacteria in the compost tea can live on the stomata protecting them from bad bacteria.

To make compost tea we first need to start with good cultured mature compost (6 month to 1 year in age). Mature compost means material that only grows beneficial organisms. It is important because we must be sure that nasty elements like pathogens and parasites in the compost have been killed by the composting process, which requires high enough temperature for a long enough time. As a reminder, compost should be made of green and wood materials that help bacteria and fungi grow, and their growth causes the temperature to rise in the compost pile.

Of course compost must be made based on what our soil needs. If we need more bacteria, we add more green materials. If we need more fungi, we add wood materials. It depends on what the soil is missing and what biology we want to bring back.

Then, to permit our starting culture to grow we need a specific favorable environment who provides space, medium, food and a process to make our compost tea : it’s our machine.

In this machine we have :

Contents :

  • The tea tank : who should be round shape, with no sharp corners to helps water and air move freely, avoid collect and bacteria from growing and be easy to clean.
  • A tea bag : used to hold the compost during brewing, who will be able to block solid particles and allows microorganisms to pass through. Perfect mesh size is 400-800 microns. The bag should be placed where water and air can flow through it easily and be partially filled to do not block water flow.

The medium :

  • Water to extract the beneficial organisms off the solid surface of compost.
  • Water have to be free of chlorine and chloramine which might effect the organisms in the compost extract. For that purpose we use humic acid which is adequate to kill any toxic chemicals that are present in the water. Humic acid also causes the dark brown color in the compost. By chlorine, chloramine, sulfates it decolonizes the humic acid so it is therefore its easier to notice their presence and adapt the concentration of humic acid.
  • Water we brew have to be at room temperature so that the microbial growth is flourishing.

The food :

The type of food we add to compost tea will influence which kinds of microorganisms grow. It all depends on what we want to achieve. If we want to develop bacteria, we use foods that promote fast bacterial growth. If we want to grow fungi, we need to feed the fungi directly with more complex foods. Here are some good options :

  • Jaggery (unrefined sugar) who helps develop bacteria, promoting rapid microbial growth during brewing.
  • Oat powder or rice bran powder who feeds more complex organisms like fungi.
  • Seaweed extract to adds micronutrients such as proteins and minerals, which stimulate beneficial fungi.

The Aerobic process :

who is important to remove good microbes from the compost.
To do this, water must be moved with some pressure. We apply this force using an air pump. A pipe is placed at the bottom of the tank, and air is pumped in to create bubbles. These bubbles move the water and this movement is strong enough to extract good microbes from the compost.  As a result, good bacteria and other organisms are released into the liquid. Also, the aerobic process prevents harmful pathogens from growing, because oxygen is toxic to many of them. The tank must always have strong air flow, and the water must keep moving to give oxygen and spread the good microbes everywhere.

RECIPE FOR COMPOST TEA :

  1. Fill about 80 litres of clean regular chlorine-free water in the compost tea tank (Dr. Elaine Ingham recommends letting tap water sit for 24 hours if it contains chlorine, and adding humic acid).
  2. Start with 4/5 kg of mature compost (6 months to 1 year) in the tea bag.
  3. Add 50 ml of seaweed extract.
  4. Pre-mix 100 grams of humic acid in water before adding.
  5. Add 100 ml of oat milk or a few tablespoons of rice bran powder.
  6. Pre-mix 50 grams of jaggery with water and add it to the tank.
  7. Cover the top to prevent evaporation and spillage.
  8. Brew it for at least 3 to 4 days (to be confirmed).

It’s necessary to clean the tank and pipes immediately after each batch to avoid biofilms, which can release toxic chemicals into the brew. This must be done every time, because we can’t see all parts of the system, like the inside of the pipes.