Compost tea promotes root growth and with it silicon uptake: how the plant becomes more robust
Compost tea promotes root growth: in the ZHAW trial, cucumber seedlings formed 31 % more root mass than with water. More roots mean more uptake and less stress in dry spells. In addition, the silicon content in the leaf sap rises with compost tea, as we have seen in over 100 leaf sap analyses.
The photo above: wheat with and without compost tea
The photo shows two wheat plants that we dug up. On the left the plant with compost tea, on the right without. On the left, clearly more root mass hangs at the base of the plant, and the roots are finer. That is the difference this post is about.
Fine roots make up the plant’s uptake surface. More of them means more nutrients and more water. They also loosen the soil and leave food for the soil life.
The trial with cucumber seedlings
In 2020 at ZHAW Wädenswil, Lorenz Rieger measured how compost tea affects young plants. He sowed cucumbers and watered them with 10 ml of compost tea per planting spot. The tea came from the EdaLife Hobby V15, brewed from compost with EdaBiom+ as food. The control got water. After 14 days he weighed the roots.
The cucumbers with compost tea had an average root fresh weight of 0.98 g, those with water 0.75 g. That is 31 % more root mass. All compost tea variants were above both controls. A commercial Trichoderma product that ran as a comparison brought no increase.
The food in the tea counts. With EdaBiom+ the effect was largest, with molasses it was small. In maize we saw no difference in root weight in the same trial. Cucumbers apparently react more strongly.
Cress shows it too
An apprentice ran a cress test in a tree nursery, in container substrate, with and without compost tea. The cress with compost tea formed clearly more and longer roots. You can see it in the photo in rows 3 and 4. Such a test works on any farm; you only need substrate, seeds and some compost tea.
Why roots help against drought stress
A dried-out soil can get hotter than 60 °C. For the plant that is a fever. Photosynthesis then no longer works efficiently, and the plant breaks down its own energy.
One percent more humus stores around 52,000 litres of additional water per hectare. That buffers hot days. Compost tea promotes fine root growth and with it, indirectly, humus build-up. A well-fed plant does more photosynthesis and releases more sugar through the roots. This feeds the soil bacteria, and there are more of them. The bacteria do not build the humus themselves. Part of them remains in the soil as stable humus after dying: the dead cell mass, the necromass, binds to minerals.
In my opinion this is why plants with compost tea get stressed less quickly. How to recognise drought stress early and what to fertilise then is covered in the post on leaf sap analysis: Leaf sap analysis: when to do it and what it shows you.
Silicon: the second argument for compost tea
Silicon strengthens the cell walls. The plant becomes more resistant to pests and diseases and takes up phosphorus better. But silicon in the soil only helps the plant if it can also take it up.
This is where the microbiology helps. After over 100 leaf sap analyses in 2021 we saw: with compost tea, sugar and silicon content in the leaf sap rise. In my winter cereals, the silicon content in the leaf sap was recently slightly low. From experience it rises with microbial activity and with the use of compost tea. On the golf green in Rieger’s trial, silicon in the leaf sap was 476 ppm on the compost tea area and 372 ppm on the fungicide area. That was a single sample; more in the post Compost tea against disease on golf greens.
In my opinion the two are connected. More roots and an active soil life give the plant the uptake surface and the helpers to get at the silicon.
How to use compost tea
For seed and young plants, you water the tea on directly at sowing. In the greenhouse this works through the irrigation. More in the post Inoculating seed and young plants with compost tea. In the field we count on at least 100 l/ha, ideally 150 to 200 l/ha.
The tea keeps for only around four hours, so use it fresh. How to check the quality is explained in How to check the quality of your compost tea. You will find our brewing systems in the shop, and the leaf sap analysis under leaf sap analysis. If you plan a trial on your farm, we help you set it up in the advisory service.
Frequently asked questions about compost tea, roots and silicon
Does compost tea promote root growth?
Yes, measurably in cucumber seedlings. In Lorenz Rieger’s ZHAW trial (2020), roots weighed on average 0.98 g with compost tea and 0.75 g with water 14 days after sowing. That is 31 % more root mass. In maize there was no difference.
How are compost tea and silicon connected?
With compost tea, sugar and silicon content in the leaf sap rise. This is our observation from over 100 leaf sap analyses in 2021, not a controlled trial. Silicon strengthens the cell walls and improves phosphorus uptake.
Does compost tea build humus?
Not directly. Compost tea promotes root growth and with it the release of sugar through the roots. More soil bacteria live on it. The bacteria do not build the humus themselves. Part of them remains in the soil as stable humus after dying, because the dead cell mass (necromass) binds to minerals.
How much compost tea do I need per hectare?
In the field at least 100 l/ha, ideally 150 to 200 l/ha. After brewing, the tea keeps for only around four hours and is applied fresh.
How do I check on my farm whether compost tea works?
Dig up 5 plants each from the treated and the untreated area before spring and compare root mass and fine roots. A few weeks later a leaf sap analysis shows silicon and the other nutrients in the plant sap.
Last updated: November 2026. Author: EDAPRO GmbH, Ruswil, Switzerland. Sources: project thesis Rieger 2020 (ZHAW Wädenswil), leaf sap analyses and field observations by EDAPRO.
