Effective Microorganisms vs Beneficial Microorganisms – Should You Use Them in the Garden?

There is a lot of talk these days about Effective Microorganisms and Beneficial Microorganisms and at times the terms are interchanged. In this post I want to sort out the difference between them and also look at how they can be used in the garden.

Are these microbes something gardeners should be adding to their garden and potted plants?

What are Effective Microorganisms?

Dr. Higa extracted a consortium of microbes from natural composting systems, like forest leaf litter, and called the mixture Effective Microorganisms (EM). He later went on to form EMRO (EM Research Organization) to commercialize a variety of EM products. The specific formulation of EM is a trade secret and contains approximately 80 different microorganisms that are capable of positively influencing the decomposition of organic matter. Some of the key organisms are known and they include the following.

• Lactic acid bacteria (LAB): Lactobacillus casei
• Photosynthetic bacteria (purple non-sulfur bacteria or PNSB): Rhodopseudomonas palustris
• Yeast: Saccharomyces cerevisiae

A number of other companies now supply an EM mixture and there is a lot of information online for making your own concoction. The Lactic acid bacterium seems to be the main organism in the group. This friendly bacteria is an important part of your digestive system, is also found in your urinary and genital tracts and it’s a common probiotic. This bacteria is found on skin, in all types of soil and on plant material.

The term effective microorganism should only be used when referring to this set of organisms. Think of it as a commercial brand name.

What are Beneficial Microorganisms?

“Microbes run the world. It’s that simple.” Microbes make the essential elements of oxygen, carbon, nitrogen, and sulfur available for other life, decompose dead material, carry out half the photosynthesis on our planet and make nutrients available to plants. The human body hosts 10 microorganisms for every human cell. Most microbes are beneficial to us and plants.

I think gardeners sometimes use the term in place of Effective Microorganism, which are beneficial but the term beneficial microorganisms (BM) is a more general term. If we are talking about plants, BM are ones that benefit plants and include those that create symbiotic associations with plant roots (rhizobia, mycorrhizal fungi, actinomycetes, diazotrophic bacteria), make nutrients available, produce plant growth hormones, and are antagonists of plant pests, parasites or diseases. Most of these beneficial organisms are naturally present in the soil.

The EM were specifically selected to aid with decomposition making them a subset of BM, which includes a much larger range of organisms.

How Do Gardeners Use EM and BM?

EM is added to kitchen waste to carry out bokashi composting. Larger scale bokashi uses naturally occurring BM instead. Lactic acid bacteria are the most prominent organism involved and they are readily available from soil and plant material.

Rhizobium bacteria are applied to legume seeds to help create nitrogen-fixing nodules. This is a well established process that has been proven to work in soils that do not already host the bacteria. Once the bacteria is in the soil, seeds will form nodules without being treated.

The addition of mycorrhizal fungi to gardens and containers is becoming popular but the practice is not supported by science. The higher level of fertilizer used in pots and containers prevents plants from initiating a connection to the fungi and in garden soils native mycorrhizal fungi are already present.

In recent years a number of commercial products have been introduced to gardeners with names such as compost booster, soil probiotics, root probiotics, plant probiotics, soil conditioner, biotic booster, soil booster, the list goes on. Compost boosters speed up the composting process and are not needed. All of the other products add beneficial microbes to soil to improve soil health and plant growth, but do they work?

Do Effective Microorganisms Work?

The answer depends on how you define the word “works”.

EM occur naturally and are responsible for the decomposition of organic material. They certainly do that. They are active in hot and cold compost piles and they continue to decompose organic material in soil. The nutrients released certainly benefit plants. They definitely work.

Do you have to add them to compost piles to make compost? The answer is a clear no. They are already present on dead organic material and in soil. Gardeners do not have to add more and adding them does not speed up composting. Adding more to a compost pile does not work.

There are also claims that adding EM to soil will improve soil health and this has been studied quite extensively.

A well done field experiment in Switzerland, over a 4 year period, looked at crop yields and soil microbe parameters to test the effects of EM. They found “treatments with living EM compared with its sterilized control treatments showed no differences on any of the parameters.” A key point needs to be made about this study – they used a proper control. By comparing both living and dead EM, they eliminated other parameters.

Some studies report positive results. One review concluded, “In this investigation, among 22 reports on the effects of EM on the yields of vegetables, 84% were positive, 4% were negative, and 12% showed no significant influence”. That sounds very positive until you look at the details.

• There is no selection criteria for the 22 reports – they clearly cherry picked them.
• They say, “Research has shown that EM must be applied together with organic matter.” So EM alone does not work, and yet their conclusion is that it works 84% of the time. Their own findings contradict their conclusion.

This last point is critical. Many of the EM studies compare the addition of EM + organic matter to the addition of only water. We know that adding organic matter helps plants grow so it clearly affects the results. A properly designed study will use the same material with living EM and with dead EM, as in the Swiss study above. This allows you to see only the effects of EM.

Another review looked at the available information about EM and concluded that, “most of the information found has not been published yet or has been published in journals with a low reliability factor. Very well known and (higher) rated scientific journals related with soil quality have been used in our work and in none of them EM technology was mention.” This review also found that many reports used poorly designed experiments and included no statistical analysis.

Science does not support the idea of adding EM to soil to improve soil or plant health.

Do Beneficial Microorganisms Work?

There are some clear cases were adding them to the garden works. I have discussed the rhizobium bacteria above. They can definitely help plant growth if the right one is added for the legume being grown and if the legume has not been grown in that soil before.

The addition of mycorrhizal fungi does not seem to add value in a garden, in containers or in potted plants.

Proponents of vermicompost make a big deal about the beneficial microbes in worm castings. They even worry about drying out the finished vermicompost because, it will “kill” the biology. The reality is that the value of vermicompost is in the organic matter and nutrients. The living biology does not matter because the soil is already saturated with microbes.

There have been many studies looking at the use of ACCT compost tea which is brewed to increase the number of BM. These studies also suffer from poor design. Many compare compost tea to water and conclude it grows better plants. No surprise there – nutrients grow better plants. There are almost no studies that compare compost tea to compost tea where the biology has been killed. Such a control would show the benefits of the biology.

There have been studies that show the addition of specific organisms can decrease diseases, but these are very specific cases.

Science does not support the addition of a general microbe cocktail to soil, like the soil probiotics being sold to gardeners.

Beneficial Microorganisms Added to Soil

One of the claims by manufacturers of commercial BM is that they are adding large numbers of microbes and that allows them to overpower existing microbes, thereby eliminating pathogens and modifying natural populations.

How many microbes are actually added?

I looked up Plant Probiotics by Micra Culture. It was the first one I found that provided the contents of the product. A 57 g pack is enough to cover 100 sq ft and it contains 20 x 10⁷ bacteria per gram. This adds 10⁸ bacteria/sq ft.

Lower quality soil contains 10⁸ bacteria per teaspoon, or 6 x 10¹¹/cu ft.

That means the product will add 1 bacterium for every 3,000 bacteria already in the top 6 inches of soil, assuming all of the bacteria in the product are still alive. I don’t consider that a large number! Remember that bacteria can multiply very quickly. If given extra food, that 3,000 natural bacteria can be 6,000 in 20 minutes. What effect do you think one extra bacterium has on the population? None.

Should Gardeners Add Microbes to Soil?

Microbes are critical for soil and plant health – there is no debate about that. This has led organizations and companies to promote the idea that adding more microbes to soil is a good thing and that gardeners should be doing it. That is simply not true.

Except in a few rare cases, mentioned above, adding microbes to garden soil or potting soil is a waste of time, money and resources.

This includes adding other sources such as compost, vermicompost, compost tea and bokashi ferment. There is value in adding these but the value is in the organic component and the plant available nutrients, and not in the living microbes. Soils are saturated with biology – there is no room for more microbes.

Do You Disagree?

I know that some of the ideas presented here go against common gardening advice. If you disagree with this post, don’t add a comment telling me you disagree. Instead, add a link to a scientific study that shows my conclusions are wrong. Then we have something to discuss.

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Soil Borne Diseases – Does Soil Make Gardeners Sick?

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Does Fertilizer Kill Soil Bacteria?