Last updated on October 23rd, 2023 at 08:54 pm
Due to the massive use of synthetic fertilizers and sprays, many soils today resemble a sterile clinic rather than a living landscape. With compost tea, life can be restored to the soil and the nutrient uptake of the plants can be improved. This is because the constant application of synthetic fertilizers and sprays reduces soil life year after year. Plants, however, need a diverse soil fauna and flora to be able to absorb the nutrients provided in the first place. For many farmers, this increases the amount of nutrients applied each year and, of course, drives up costs. With compost tea, this vicious cycle is reversed.
What is compost tea?
Compost tea is a liquid that consists mainly of water and is made with the addition of fresh compost and sugar syrup. So when I make compost tea, I create a liquid extract from my fresh compost and help the microorganisms multiply with the sugar syrup. The result is a fertilizer of a different kind: compost tea does not fertilize the soil directly, but supplies it with important microorganisms that improve the fertilizer uptake of the plants.
Different cultures – different microorganisms
Different crops, i.e. cereals, vegetables, etc., also harbor different microorganisms. So if I want to grow vegetables, I should create a ratio of bacteria to fungi in my soil that will support vegetable growing. In the following table you will find an overview of the ratios of bacteria to fungi in different types of cultivation:
|Fungi to bacteria ratio
|Pure raw material
|“weeds”, pioneer plants
|PB ratio 0.1:1
|PB ratio 0.3:1
|PB ratio 0.75:1
|PB ratio 1:1
|PB ratio 2:1 to 5:1
|PB ratio 1000:1
Table 1: Ideal ratio of bacteria to fungi in different crops.
These ratios are already the first sticking point: depending on the ratio I want to produce, I have to prepare the compost tea in such a way that, for example, it contains slightly more bacteria than fungi if I want to grow vegetables. For the amateur gardener, it is hardly possible to precisely control the ratio of fungi and bacteria in the compost and thus also in the compost tea. However, Ms. Theby recommends a microscope for approximate determination, costing around 200€, which can be used to determine which microorganisms are present in the compost.
The multiple effects of compost tea
Compost tea improves the soil structure and water holding capacity of the soil. It also ensures increasing plant health due to the higher mass of microorganisms in the soil. In a very detailed handout on the production of compost tea, agronomist Dr. Ingrid Hörner describes the following field observations when compost tea was administered:
- “Reduction of water consumption by up to 50% in two years.
- Greater water holding capacity due to better live soil retention.
- Greater root growth and therefore better drought stress tolerance.
- More symbioses and active communication between plant and soil life
- Faster conversion of plant residues into humus
- Reduction of compaction – improvement of soil aggregate condition
- The nutritional quality of plant products is increased.
- The cycle of nature is revived (food)” (
Due to the many positive effects, many farmers report that externally purchased chemical fertilizers and pesticides can be reduced. And, of course, this is reflected in operating costs. According to Dr. Ingrid Hörner, cost reductions of 20% in the area of artificial fertilizers and pesticides have already been reported in the first year.
Compost tea concretely in use
In the ZZ2 farmers’ association in South Africa, compost tea has been used on an area of 200,000 ha for over 10 years. For this purpose 70,000l of compost tea are produced weekly, i.e. in 70 large IBC containers with central aeration. For me, this example is a very real proof of the effectiveness of the compost tea, because on this area it would be an economic disaster if the compost tea did not work. The 200,000 hectare area produces tomatoes, mangoes, avocados, apples, pears and onions, among other crops. The farmer’s association has also given rise to a method of cultivation called “Natuurboerdery” – there is no organic certification, but it is a form of nature-based conventional cultivation.
The making of compost tea
After the preliminaries, let’s get down to the nitty gritty: To make compost tea, you’ll need:
- A pond aeration pump
- An adjustable heater, power of about 125W
- 100l rainwater
- 300g worm compost or well rotted compost
- 400ml of sugar beet syrup
To make it, pour the rainwater into your container and put the heating rod in it. Set the temperature to 25°C, a temperature variance of +/- 3°C is fine. The power of the pond aeration pump should be adjusted to the amount of water in the container, it is best to set it to 80-100l per minute. After that, put all the ingredients into the container and leave it for 2-3 days. If you want to apply the compost tea with a sprayer, it is best to put the compost into the container in a permeable net, for example in a nut milk bag. After the microorganisms have multiplied, you can turn off the pump and apply the tea within four hours.
Spreading the compost tea – what to consider
Before spreading the compost tea, you should dilute it again. How much you dilute it depends on how you want to apply it. For foliar spraying, a dilution of 1:10 is recommended, and for soil application, 1:5. When foliar spraying, keep in mind that you should not apply it to fruit that is ready to harvest, and the treatment should be completed no later than 3 weeks before harvest. For salads and leafy vegetables, you should always choose the soil treatment for hygienic reasons. For both applications, we say you should apply the compost tea primarily to moist stands and ideally just before rain. For the foliar treatment, 5 applications of 0.5l concentrate per 100m² each are recommended, and for the soil treatment, 5 applications of 2l concentrate per 100m² each are recommended. The compost tea can not be overdosed. Stronger concentrations therefore do no harm.
For me, compost tea is a very exciting “fertilizer method” that actually only improves soil and plant health and doesn’t really fertilize at all. It is also relatively easy to make!