How do EM work? Fermentation, competition, and the limits of the dominance principle

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A vegetable peel forgotten in a sealed bag can end up one of two ways: it rots and smells awful, or it ferments and takes on a sour smell, a bit like sauerkraut. The whole idea behind effective microorganisms (EM) comes down to this difference: steering organic matter toward fermentation rather than putrefaction.

This guide explains what really happens, what we know about the various microorganisms in the mix, and why the famous "dominance principle" promoted by the manufacturer should be taken with caution.

Fermentation and putrefaction: two fates for organic matter

When oxygen is lacking, organic matter is broken down by microorganisms that don't need air. Depending on which ones gain the upper hand, the result is very different:

• Putrefaction mainly breaks down proteins. It releases ammonia, sulfur compounds (the rotten-egg smell), and amines that smell like carrion. The material turns slimy, and the smell attracts flies.
• Fermentation goes after sugars and starch first. It produces acids (lactic, acetic), a little alcohol, and carbon dioxide. The material keeps its shape, smells sour or vinegary, and keeps well.

This is nothing new. Sauerkraut, kimchi, olives, cured sausage, and farmers' silage all rely on the same principle. What EM offer is a way to apply this age-old know-how to food scraps, manure, animal bedding, or compost. For the specific case of the bucket, see how bokashi fermentation works.

The role of lactic acid bacteria: acidifying to preserve

Lactic acid bacteria are the real workhorses of EM. They consume sugars and give off lactic acid. The environment quickly becomes acidic: a successfully activated EM has a pH of 3.2 to 3.6, and the EM-1 stock solution is in the same range.

Most putrefying bacteria grow poorly in such an acidic, airless environment. This is exactly what happens in a well-sealed silo: the pH drops, the material is "locked in place," and it keeps for months. This mechanism has long been established, it is used throughout the food industry, and there is nothing unique to EM about it.

Two conditions are essential for it to work:

• Sugar to feed the lactic acid bacteria. That is the role of the molasses in EM-1 and in fermented bran.
• Little or no air. Hence the importance of packing it down, sealing it, and not opening it constantly.

Key takeaways

The most solid effect of EM is simple: acidifying organic matter away from air, so that it ferments instead of rotting. Anything that encourages acidification (sugars, no air, mild warmth) encourages a good result.

Yeasts and photosynthetic bacteria: what they are credited with

Yeasts

Yeasts turn sugars into alcohol and carbon dioxide. They are what makes a container of EM swell during activation, and they contribute to the fruity, slightly alcoholic smell of a good ferment. Lab analyses confirm their presence in EM products.

Photosynthetic bacteria

The manufacturer presents them as the key to the mix: they supposedly use light and heat to turn waste and foul-smelling gases into substances that benefit plants. In nature, these purple bacteria do exist and have a very versatile metabolism.

The real question is whether they are present and active in the finished product, a highly acidic liquid kept in the dark. A DNA analysis published in 2024 found none in the solution tested, and other analyses show that they are, at best, a very small minority. Their actual role in everyday use therefore remains unknown.

The "dominance principle": what Higa says, what science says

The manufacturer's theory

According to Professor Teruo Higa, the microorganisms in any environment fall into three groups: a small minority of "regenerative" ones, a small minority of "degenerative" ones, and a large majority of "opportunists" that side with whichever group is dominant. Applying EM regularly would tip the opportunists over to the right side, in soil, water, or air.

It is a vivid image, but it remains a theory. The proportions given actually vary from one of the manufacturer's documents to another, which clearly shows that they are not measurements. No independent study has validated this classification, and Higa himself acknowledged how hard it was to obtain reproducible results.

What is actually observed

Current knowledge allows us to distinguish between two situations:

• In a closed, fresh, sugar-rich environment (bokashi bucket, silo, activation container), a massive dose of lactic acid bacteria does indeed get ahead of the others. This is known as the priority effect or competitive exclusion: the first to settle in consume the sugars, acidify the environment, and take up the space. Here, the idea of "dominance" has a real basis.
• In an open environment that is already densely populated, such as garden soil, the opposite is true. A dose of EM adds a tiny quantity of microorganisms compared with the billions already present in every handful of soil. In a trial published in 2024, virtually no trace of the EM microorganisms could be found in the soil one week after application at the recommended dose; even at a dose a hundred times higher, they remained below 1% of the population. At the normal dose, the trial measured no effect. At a very high dose, soil activity rose briefly, but just as much with sterilized EM: the effect came from the added sugars, not from the living microbes.

Other trials, including a four-year field trial in Switzerland, compared live EM with sterilized EM: both gave the same results. In other words, what is attributed to the microbes often comes from what accompanies them (molasses, bran, bokashi). We cover these studies in detail in our guide what science says about EM.

Did you know?

The principle that works best was known long before EM: it is the principle of silage. Inoculating fresh, sugary, airless material with lactic acid bacteria is a standard farming practice that is well studied and well understood.

Why the effect depends on the context

The most cautious public assessments consider an effect of EM plausible mainly for composting, silage, and manure treatment, provided that temperature, moisture, and nutrient supply are controlled. In practice, four factors make the difference:

• The substrate. Without sugars or fresh material, lactic acid bacteria have nothing to ferment.
• Air. Lactic fermentation needs a closed, compacted environment. In an aerated compost pile, other microorganisms do the work.
• Temperature. Below about 59 °F (15 °C), everything slows down, and freezing kills microorganisms. To activate a solution, manufacturers aim for 77–86 °F (25–30 °C).
• Moisture. Too dry, and nothing happens; waterlogged, and the material may rot despite the EM.

Emy's tip

If a ferment disappoints, don't double the dose: first check that there is sugar, little air, a mild temperature, and material that is just moist. Fix these four points, then try again.

Gradual effects: from a few minutes to a few years

The manufacturer sums up how fast EM act as follows: a few minutes for a bad smell, sometimes several years for soil fertility. Here is what to take away from this:

• Odors: an acidic solution sprayed on animal bedding or a trash can quickly limits the release of ammonia. This is a plausible effect linked to acidity, and in a trial on composting toilets, sauerkraut juice worked just as well as EM.
• Fermentation of a bucket or silo: a few days to a few weeks, depending on temperature.
• Soil: long-term trials in temperate climates have shown no specific effect of the microorganisms on yields or soil life. What improves soil is first and foremost the regular addition of organic matter, whether fermented or composted.

To put these principles into practice, start with a well-prepared solution: our guide activating EM-1 step by step explains how to get a successful ferment, or you can start with Microferm EM-A, which is already activated. In the garden, where EM make the most sense is in managing organic matter: see speeding up your compost with EM.

In summary

EM work above all through lactic fermentation: their bacteria consume sugars, acidify the environment, and, away from air, keep organic matter from rotting. This mechanism is solid and has been known for centuries. The manufacturer's "dominance principle," on the other hand, has not been proven: it makes sense in a closed, sugary environment such as a bucket or silo, but much less so in open soil already rich in life, where trials attribute the observed effects to the organic matter added. EM therefore give their best results where the substrate, air, temperature, and moisture are under control.

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