Compost tea and mycorrhiza: what our trial shows
K0A− water only (control) · K0A+ water with mycorrhizal spores
K1A− compost tea · K1A+ compost tea with mycorrhizal spores
K2A− compost tea with additional Streptomyces · K2A+ compost tea with Streptomyces and mycorrhizal spores
KsA− autoclaved (sterilised) compost tea · KsA+ autoclaved compost tea with mycorrhizal spores
PSM fungicide (plant protection product) as control
A− = without, A+ = with added mycorrhizal spores. The dashed red line is the mean of all variants.
Compost tea contains no mycorrhizal fungi. They mostly do not survive the composting process. Even so, the roots in our trial were colonised by mycorrhiza much more strongly with compost tea than without. And added mycorrhizal spores brought no additional increase.
What mycorrhiza is
Mycorrhizal fungi live in and on the roots. Their fungal threads reach much further into the soil than any root hair. They fetch water and nutrients, above all phosphorus, and in return get sugar from the plant. Most crop plants have arbuscular mycorrhizal fungi, AMF for short.
Anyone who wants AMF often buys spores in a bag or a bottle. We wanted to know: does it work without? And what happens if you combine both?
The trial
The trial was part of a master’s thesis at ZHAW Wädenswil (2021), using our brewing system and our compost. The plant was creeping bentgrass (Agrostis stolonifera), as grown on golf greens. The substrate came from a real green and was not sterilised. Eight weeks in the greenhouse.
We brewed the compost tea in the EdaLife V60: 1.8 litres of EdaBiom compost and 153 g of nutrients in 60 litres of water, aerated for 24 hours. Per dish, 30 ml went onto the leaf, four times at intervals of about 15 days.
We compared nine variants, each with and without added mycorrhizal spores (Glomus species, at least 1666 spores per dish): water only, compost tea, compost tea with additional Streptomyces, autoclaved compost tea (that is, sterilised, only the nutrients remain) and, as a control, a fungicide. After 55 days we stained the roots and counted under the microscope how much of them carried mycorrhiza.
Three observations
First: compost tea promotes colonisation. With water alone, around 45 % of the roots were colonised. With compost tea it was 60 to 68 %. So the substrate already carried its own fungi. The compost tea promoted them.
Second: added spores brought nothing extra. With compost tea, colonisation with spores was practically the same as without (61 versus 60 %, 68 versus 64 %). With water and spores it also stayed at around 48 %. That surprised us.
Third: the life in the tea counts. With autoclaved compost tea, colonisation fell to 31 to 40 %, lower than with water. The nutrients are still in there, the organisms are not. In my opinion a fertilising effect then acts rather than an inoculation effect: the nutrients are no longer bound in living cells. And the fungicide pushed colonisation down to around 15 %.
Why this might be
We do not know for sure. But there are plausible explanations. The fungi need hard-to-degrade carbon such as humic substances, and compost tea supplies these. And some bacteria promote mycorrhiza directly. Compost tea contains many of them.
In my opinion the trial shows above all one thing: it pays to feed the soil life instead of buying in single organisms. Whether a fungus from a bag gets a foothold in the soil at all depends on the soil. A compost tea, by contrast, brings the whole environment in which fungi thrive.
What you take from this
Protect what is already there. Fungicides and high nitrogen and phosphorus rates slow mycorrhizal fungi. And give the soil food and living microorganisms, best with fresh compost tea. How to make and test the tea is explained in the post How to check the quality of your compost tea. How to inoculate seed and young plants with it is in the post Inoculating seed and young plants with compost tea. You will find our brewing systems in the shop, and if you have questions we are happy to help in the advisory service.
You can read the full thesis at ZHAW.
Frequently asked questions about compost tea and mycorrhiza
Does compost tea contain mycorrhizal fungi?
No. Mycorrhizal fungi mostly do not survive the composting process. Even so, the roots in our trial were colonised by mycorrhiza much more strongly with compost tea than without.
How strongly does compost tea promote mycorrhizal colonisation?
With water alone, around 45 % of the roots were colonised, with compost tea 60 to 68 %. With autoclaved, that is sterilised, compost tea colonisation fell to 31 to 40 %, with fungicide to around 15 %.
How did the trial work?
It was part of a master’s thesis at ZHAW Wädenswil (2021), using our brewing system and our compost. The plant was creeping bentgrass on substrate from a real green, eight weeks in the greenhouse. The compost tea came from the EdaLife V60 with 1.8 litres of EdaBiom compost and 153 g of nutrients in 60 litres of water, aerated for 24 hours.
Is it worth buying mycorrhizal spores?
In the trial, added spores brought no additional increase. In my opinion it is better to feed the soil life instead of buying in single organisms. Whether a fungus from a bag gets a foothold in the soil depends on the soil.
What slows mycorrhizal fungi?
Fungicides and high nitrogen and phosphorus rates slow mycorrhizal fungi. Protect what is already there, and give the soil food and living microorganisms, best with fresh compost tea.
Last updated: November 2026. Author: EDAPRO GmbH, Ruswil, Switzerland. Sources: master’s thesis Rieger 2021 (ZHAW Wädenswil), EDAPRO trials, leaf sap analyses and field experience.
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