Effective Microorganisms (EM): what the science really says, studies and results

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Effective microorganisms, or EM, have their enthusiasts and their detractors. Some credit them with almost universal effects: in the garden, around the home, on health and even in rivers. Others see them as just another ferment, sold at a premium. We sell EM products and even make our own bokashi activator with them: that is precisely why we want to tell you what the studies really show, including when the results are disappointing.

This guide covers all uses of EM. For the bokashi bucket in particular, see our guide Bokashi and EM: what the science really says.

Why EM is controversial

The blend was developed in the 1980s by Professor Teruo Higa in Japan. Since then it has spread to many countries, and there is a wealth of literature on the subject. However, much of it comes from the manufacturer, its partners or journals with low standards. Professor Higa himself acknowledged, in a 1994 paper, that the main limitation of this research was the lack of reproducibility and consistency of the results.

Three difficulties complicate the evaluation:

• The composition varies between products and countries. Not all labels declare the same strains.
• EM is almost always applied together with food: molasses, bran, bokashi, fermented extracts. An observed effect may come from this organic matter rather than from the microbes.
• The uses are very different: a result on compost tells you nothing about swimming pool water or human health.

Key points

A good study on EM compares three situations: nothing added, the EM product, and the same product sterilised. If the sterilised product performs as well as the live one, the effect does not come from the microorganisms.

In the garden and the field

This is the most studied use, and the one where results are most mixed.

• A four-year trial in Switzerland, under organic farming, compared live EM, sterilised EM and a control. Sprayed on their own, EM had no effect on yield or on soil microbial life. With bokashi, a few differences appeared, but inconsistently, and sterilised EM performed as well as live EM: the small effects came from the nutrients in the bokashi.
• Three-year trials in Bavaria measured an 8 % gain on wheat with activated EM, with no significant effect on field beans or barley. Once the cost of the product was deducted, it was not profitable. EM ceramic, spread at 150 kg per hectare, did not improve any yield.
• A meta-analysis published in 2015, pooling 39 papers, found an average positive effect on growth, but it was small and highly variable between crops, sometimes negative. In the field trial run by the same researchers, treated maize did not yield more. In the greenhouse, however, it was 51 % more palatable to a caterpillar, because its chemical defences were weaker. In the field, the diversity of predatory insects was also 26 % lower.
• On soil-borne diseases, Dutch trials found no general improvement in soil resistance, with even more diseased plants in one test, and a favourable effect in only one soil out of three for another disease.

Conversely, several favourable reviews claim that most studies are positive. But they often simply count publications, without taking into account their quality or the absence of a sterilised control. The most encouraging results come mainly from hot climates or pot and greenhouse trials, and do not necessarily transfer to a European vegetable garden. To judge for yourself, run a trial with a control bed, as we explain in our guide Using EM in the vegetable garden.

Compost, fermentation, odours and silage

This is where EM is most at home, because it is about fermentation, a field in which lactic acid bacteria have proved their worth for centuries.

• Lactic acid fermentation is an established mechanism: lactic acid bacteria convert sugars into acids, the pH drops and the material is preserved instead of rotting. This is the principle behind bokashi, sauerkraut and silage. How EM goes about it is explained in our guide How does EM work?
• For conventional compost, the available trials have shown no clear difference: a well-balanced heap already contains all the microorganisms it needs.
• For silage, the benefit of lactic acid bacteria inoculants is well demonstrated. Data specific to EM products are scarcer.
• For odours and ammonia from litter, a few poultry farming trials conducted in Asia measured a clear drop in ammonia when an EM solution was sprayed on the litter, with no effect on the animals' growth. The results are encouraging, but the trials remain few, in hot climates, with varying products and doses.
• For septic tanks, biological additives in general have shown no lasting benefit: a tank already contains enormous quantities of bacteria.

Water and the environment

Releasing EM balls into rivers is the most publicised use. Laboratory trials claiming cleaner water often have weak protocols. The most rigorous evaluation, carried out over two years in Hawaii with a control area, found no measurable effect in the field. In tanks, the balls actually degraded the water by adding phosphorus and bacteria, and at ten times the recommended dose they depleted the oxygen. In a large European reservoir, a liquid EM lowered phosphorus one week after application, but most measurements had returned to their starting levels after two weeks. We go into these results in detail in our guide on bokashi balls for ponds.

As for claims about cleaning up contaminated soils or reducing radioactivity after nuclear accidents, they rely on documents produced by the manufacturer itself, without independent validation. We do not repeat them.

Health, drinks and ceramics

• Drinks: published research on EM-X concerns cells or rats, with an old formula based on rice, papaya and seaweed. We have found no rigorous clinical trial in humans. The rules governing how these products may be presented are detailed in our guide EM drinks, salt, soap and toothpaste.
• Ceramics: fired at high temperatures, they no longer contain any live microorganisms. The claimed effects on water are not supported by any independent study, and the explanations put forward do not match any known physical or chemical mechanism. Our full analysis is in the guide EM ceramics.

Did you know?

In an independent evaluation, genetic analysis of an EM product found only lactic acid bacteria. The photosynthetic bacteria, often presented as the heart of the blend, were absent. The actual content may therefore vary from one product to another and from one batch to another.

Our position

Having read these studies, this is how we see EM.

• A fermentation tool, useful for starting and securing lactic acid fermentation: bokashi bucket, fermented bran, grass clippings or manure under a tarpaulin, silage, odour control.
• An optional aid in the garden, to be tried on just one area, without expecting a bigger harvest.
• Not a fertiliser, not a disinfectant, not a treatment for plants, animals or people. These categories also have a precise legal meaning, explained in our guide EM and regulations.
• Not a miracle potion: we always present unproven effects as manufacturer's claims, and we remove anything pseudo-scientific from our guide.

Emy's tip

Before adopting an EM product for good, split a bed, a bin or a litter area in two: treat one half, leave the other as it is, and note your observations over a whole season. Your own comparison will be worth more than any promise.

In summary

Science confirms the core of the matter: lactic acid fermentation preserves organic matter and limits putrefaction. However, rigorous trials in temperate climates have not shown that EM increases harvests or enriches the soil in the long term, and the observed effects often come from the organic matter applied with it. The promises about rivers, health and ceramics are unproven. We therefore see EM as a practical fermentation tool, to be used for what it does well, without believing in miracles.

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Other topics in the guide

Vermicomposting30 sheets →Composting25 sheets →Bokashi24 sheets →