Making your own biostimulants: what really works on the farm

If you make compost tea, compost extract or ferments yourself, you have the microbiology fresh in the tank instead of sitting in a canister for months. That is the whole point. Anything that has to keep is either slowed down in its metabolism, or mainly the organisms that manage without oxygen survive in it. Three methods, one brewing system – and for each there is a moment when it is the right one.

Fresh or bottled: that is the difference

Aerobic microorganisms need oxygen. In a sealed canister, it is gone after a short time. What is left are the organisms that can manage without it – or the product is slowed down in its metabolism from the start, so that anything arrives at all.

How clear-cut this can be is shown by Lorenz Rieger’s 2020 germination trial at ZHAW Wädenswil. In the cucumber seedlings, a commercial single-strain product used as the positive control came to 0.73 g fresh root weight after fourteen days. The water control was at 0.75 g. The best compost tea variant reached 0.98 g.

That is one trial, with one crop, in seed trays. It says nothing about yields in the field. The full evaluation is in the post on the germination trial.

Compost tea: multiplying the microorganisms

With compost tea, you multiply. Compost, water, food and 24 hours of aeration – this produces not only a higher cell count, but also what the microorganisms excrete in the process: humic acids, amino acids, secondary metabolites. It is exactly these metabolic products that make up the effect on the leaf.

For this to work, dissolved oxygen has to stay above 6.5 mg/l for the whole brewing time. If it drops below, the batch tips into anaerobic conditions, and you multiply the wrong organisms.

The amount of compost is the second lever. Worldwide, people work with 3 to 10% of the water volume; in German-speaking countries the share is often below 1%. We use at least 2%, in practice around 3%.

We do not use molasses. Sugar is immediately available, and the bacteria multiply so fast that oxygen collapses between hour 8 and 12 – especially above 20 °C. On top of that: if there are enterobacteria in the compost, you multiply them too. Exactly that was visible in our 16S analysis. Instead, we work with complex carbohydrates, proteins and herbs, as found in the EdaBiom+ microbial food.

These four hours are the decisive limitation. You need the tea where you brew it, and you have to know when you are going out.

Compost extract: washing out the microorganisms

With compost extract, you multiply nothing. Water flows through the compost, the organisms come loose – that is all that happens. No heating, no food.

Everything else follows from that. The full diversity of the compost is preserved, because nothing is selectively promoted. Any pathogens are not multiplied. On the other hand, the metabolites that form during brewing are missing.

The extract keeps for about a week, and it is ready in half an hour. At sowing, that is often more important than any percentage of effect: when the weather decides at short notice whether you go out, you do not have 24 hours’ lead time.

It is made with much more compost than tea – a ratio of 1:10, i.e. 10%. The larger amount simply inoculates the soil more effectively. How I make it in detail is in the post on compost extract.

When to use what

Compost tea when you go onto the leaf and are looking for the biostimulant effect: growing season, warm soils, active plant. And in propagation, where the tank is right next door anyway.

Compost extract when you want to inoculate the soil, when it is cold, when you need fungi rather than bacteria – or when you cannot apply within four hours. It can also go straight onto the seed or into the seed slot.

In early November, the soil temperature on our farm is below 10 °C. So for the last pass of the year I no longer brew tea, but make an extract.

Ferments and hay tea for slurry

The third method works the other way round: anaerobically. Lactic acid bacteria lower the pH, and the batch keeps for weeks instead of hours. It is used in slurry storage and for surface composting, not on the leaf. What happens in the store is described in the post on slurry additive.

Four things have to be in balance: an energy source, nutrients, water and a microbial inoculant. As the energy source here, we use molasses.

In compost tea, molasses would be a mistake – there it eats up the oxygen. In the ferment, that is exactly what you want: the batch is meant to go anaerobic so that the lactic acid bacteria get the upper hand. Same ingredient, two opposite goals.

On our farm, this runs in winter, when there is time anyway. The herbs for it come from our own flower strip.

What the brewing system has to be able to do

The bottleneck is never the tank, but the air output.

The fact that people in German-speaking countries brew with less than 1% compost rarely has technical reasons. Many machines do not bring in enough air to keep the oxygen stable at higher compost amounts. So people reduce the compost instead of increasing the air output – and lose exactly what makes the tea. If you are looking at a brewing system, ask about the air output, not the tank volume.

The second point is utilisation. The same brewing system makes compost tea, compost extract, ferments and hay tea. A brewing system that only runs six weeks a year is hard to justify financially.

Where the limits are

• The compost decides everything. Multiplying also means multiplying mistakes. And the extract cannot release anything that is not in there.
• Mycorrhiza is not in it. It does not survive the composting process. In our trials, mycorrhizal colonisation in grasses still increased – added spores, on the other hand, brought no additional increase.
• It does not replace fertilisation. The nutrient amounts are far too small for that. This is about biology, not kilos of nitrogen.
• None of the methods can be stored indefinitely. Four hours for tea, one week for extract. If you have no time for planning, you are better off with the extract.

Then there is monitoring. You cannot see from outside what is happening in the tank – how to check it is explained in the post on quality testing.

The next step

Start with one batch. One area treated, one next to it untreated, and after fourteen days a spade test. Alongside that, a cress test on the windowsill – it costs nothing and tells you more about your compost than any brochure.

If you want to know whether what you apply actually reaches the plant, measure it two to three weeks later with a leaf sap analysis. And if you want to work out what makes sense on your farm before buying a brewing system: get in touch for advice before you order.