Inoculating seed and seedlings with compost tea: what the trial shows

When you water your seed with compost tea, the seedling gets a whole microbial community to take along – exactly at the moment the rhizosphere forms. In cucumber seedlings, this gave around 31% more root mass after fourteen days in a trial at ZHAW Wädenswil. How big the effect is depends heavily on the crop, though, and in maize it was not measurable. Both belong in the same post.

How the trial worked

In 2020, Lorenz Rieger tested at ZHAW Wädenswil what compost tea does during emergence – and whether its effect can be steered via the added nutrient. The tea was brewed with an EdaLife Hobby V15, 24 hours, following our instructions.

Two composts were used, plus three nutrient variants: EdaBiom+, molasses or nothing at all. The crops were maize and cucumber, i.e. one monocot and one dicot. Some of the trays were also inoculated with Pythium ultimum, the pathogen that causes root rot in seedlings.

More root in fourteen days

In cucumber, all six compost tea variants were above both controls. The strongest was the KEX compost with EdaBiom+: 0.98 g fresh root weight versus 0.75 g in the water control, i.e. 30.7% more. The difference from the water control is statistically significant (p = 0.008). Across all eight variants, however, the ANOVA sits exactly on the threshold at p = 0.050 – the effect is real, but not huge.

I find the last bar remarkable. The bought-in product of the positive control came in at 0.73 g – below the water control. So a ready-made product off the shelf did not even beat tap water, while every variant from our own brewing system was above it.

What compost tea can do against Pythium

Pythium ultimum hit hard in the trial. Without the pathogen, 99 to 100% of the seedlings emerged. With the pathogen, only 5% of the cucumbers were left in the end – and the collapse happened between day 6 and day 8, i.e. after the plants had already emerged.

Maize coped with the pathogen much better: 50% emerged. In this crop, compost tea was able to make a difference. The negative control lost 70% of its plants, while the best compost tea variant reached a 90% emergence rate.

The nutrient steers what grows in the tank

For me, the second part of the work is the more interesting one technically. 42 samples were analysed by 16S rRNA sequencing, five replicates per variant.

Two findings from it:

• Production is reproducible. All replicates lie close together in the principal coordinates analysis. That is the basis for being able to work with the method at all.
• The nutrient shifts the microbiome – within 24 hours. Without an additive, the tea resembles the compost used. With an additive, the microbiomes of the two completely different composts converged. Gammaproteobacteria were strongly promoted, at genus level above all Acinetobacter.

The price for this is interesting: diversity decreased. Both nutrients had a C/N ratio of 22 and a similar composition, so they worked in the same direction.

In practice, this means: your choice of nutrient helps steer which groups multiply. If you are uncompromising about diversity, the tea without an additive is the more honest option – the result of the worm compost without nutrient fits with that. For anything that needs an effect within a tight time window, we work with EdaBiom and EdaBiom+. How to check what ends up in the tank is explained in the post on quality testing.

How we do it on the farm

For sowing, we rely on two methods, and the choice has mainly a practical reason.

Compost tea keeps for about four hours. That works if you water in the greenhouse or the propagation area and the tank is on the farm. It is easy to apply via the irrigation.

Compost extract keeps for about a week, because nothing is multiplied. For field sowing, that is the big advantage: you can apply it to the seed or put it straight into the seed slot without running out of time. How to make it is explained in the post on compost extract.

Then there is planning. At sowing, the weather often decides at short notice whether you go out – and then the quantity has to be ready immediately. Compost tea needs 24 hours’ lead time; an extract is ready in 30 minutes. The microbiology is only washed out of the compost, not multiplied. That costs metabolites and saves a whole day.

With the EdaLife V900 you make 900 litres of compost extract in a minimum of 30 minutes. To do this, you disperse 90 litres of compost in 900 litres of water in an aerated bath. The extraction can also run longer.

The next step

If you set up one area with and one without treatment in spring, you will have your own result after fourteen days – with a spade test and a kitchen scale. Root weight is the value that showed the difference in the trial.

And if you want to know whether the young plants are actually better supplied afterwards, measure it two to three weeks later with a leaf sap analysis. And what happens in the soil itself is shown by the Haney soil vitality analysis – it measures microbial activity rather than just nutrient reserves. You can guess without us. If you would rather look at the figures together, come to us for advice.