Compost Tea as a Foliar Stimulant: What Really Matters
Applying compost tea in the evening—foliar application belongs in the cool hours.
Hardly anything in compost tea is discussed as much as the fungal-to-bacterial ratio. In my opinion, it is a secondary criterion for use as a foliar stimulant. The decisive factor is the quality of the compost—and that is exactly where we place our focus.
In short: Compost tea is produced by brewing mature, high-quality compost—at least 2% to 10% of the water volume—in well-aerated water for 24 to 48 hours, without added sugar such as molasses. A good compost tea machine provides enough air capacity so that the dissolved oxygen remains above 6.5 mg/l throughout the entire brewing process (rule of thumb: tank volume times 0.6 = required air volume in liters)—this is achieved, for example, by our EdaLife Chrome brewing systems with an air-to-water ratio of almost 1:1.
Any compost fed with wood contains fungi. In our Petri dish tests, Trichoderma species, known as promoters of plant health, regularly grow. Nevertheless, their hyphae are not always visible under the microscope. A microbiometer also detects fungi that are not microscopically recognizable. So, if you only count what you see in the microscope, you are not measuring the effect—but the visibility.
The five criteria we pay attention to
Why we avoid molasses
Molasses is standard in many recipes. We do not use it—and there are two solid reasons for that.
The first is oxygen. Sugar is immediately available to bacteria. Between the eighth and twelfth hour, this leads to such rapid multiplication that the dissolved oxygen crashes. At temperatures above 20°C, this is further exacerbated.
The second reason carries more weight. If enterobacteria are present in the compost, they are co-multiplied by the molasses. We had this verified with a microbial analysis: In the variants with molasses, genera appeared that you do not want on the leaf.
The same result was reached by USDA investigations as well as a study in Compost Science & Utilization.
A practical side effect: The viscous consistency makes dosing tedious. That alone would not be a reason—but combined with the rest, it is one more.
More compost means more effect
Worldwide, usually 3% to 10% compost relative to the water volume is used. In German-speaking countries, the proportion is often below 1%. The reason is rarely technical in nature, but rather mechanical: Many machines lack the air capacity to keep the oxygen stable at higher concentrations.
Those who have the air capacity win in several ways: more microbial diversity, more humic and amino acids, more plant hormones like auxins and cytokinins. For foliar application, the higher concentration of secondary metabolites is what counts most—they constitute the biostimulating effect.
What this means for the application
Compost tea is not a fertilizer. The nutrients it contains are negligible in the overall balance. What works are the microorganisms and their metabolic products—around 70% of the effect comes from this fraction.
🎬 The whole thing in a video
Production and application step by step—recorded on our farm.
Compost tea production and application—watch on YouTube. You can find more films in our FAQ under “Videos”.
