Buying Soil Probiotics (Microbes) – Are They Beneficial, Effective or Nonsense?

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Robert Pavlis

The market is being flooded by new products offering all kinds of soil microbes. Many companies call them “beneficial” and others call them “effective”. Specialty products like mycorrhizal fungi, probiotics and bacteria for degrading lawn thatch are now available. There are even probiotics to speed up your compost pile.

The market is still sorting out the terminology for these products but I see the name probiotics used more. If they are good for your gut, why would they not also be good for your soil?

What do these products really do in the garden and are they worth buying?

Buying Soil Probiotic Microbes - Are They Beneficial, Effective or Nonsense?
Buying Soil Probiotic Microbes – Are They Beneficial, Effective or Nonsense?, credit Youth & Earth

Soil Microbes – A Quick Dive

This section alone could fill a book and in fact it does take up a significant part of my book, Soil Science for Gardeners. Here is a quick summary.

The two key groups of microbes in soil are bacteria and fungi. There is no doubt that these are critical for both soil and plant health and a gardeners job is to grow more microbes in soil. If they do that, they solve all kinds of gardening problems.

Plant Science for Gardeners by Robert Pavlis

Some facts:

There is no doubt that existing microbes are critical for the garden and that gardeners should take steps to grow more of them, but is there any benefit to adding them from a purchased bottle?

Microbe Populations are Dynamic

Microbes reproduce very fast when conditions suit them, and when conditions are no longer acceptable, they die or go into a state of hibernation, allowing them to wait until conditions improve. Population densities change very rapidly.

The species that are present at any given time are also very dynamic. If soil gets wetter or warmer, new species start to thrive, while others die off. Food sources have a great influence on populations. Most species can digest sugars, but only a few can survive on the lignin in woody plants. Oxygen levels and temperature are also important.

Science is just starting to get a handle on this, so as gardeners we really can’t tell what we have in our garden at any one time. We also have no way of knowing how to manipulate the populations to our advantage because we don’t know when and if it changes.

The fact that manufacturers of products claim to know exactly which microbes you should add to soil is quite surprising and should trigger all kinds of red flags. How can they know this when scientists don’t?

Bacteria grow very quickly when conditions are right
Bacteria grow very quickly when conditions are right, credit: Frontiers for young minds

Soil Microbes Are Always at Capacity

Thanks to Chuck, one of our members in the Garden Fundamental Facebook Group for putting this complex concept into such a simple term.

“Soil microbes are always at capacity.”

What this means is that in any type of soil the microbe population is always at maximum strength. Microbes populate an area so quickly that they are always filling any available space in soil, limited only by the conditions.

In very poor infertile soil, there is limited food so the population is smaller, but it will as large as the resources allow.

Really good fertile soil will have a much higher population, but again it will be at maximum capacity.

You simply won’t find soil where microbes have not grown to their fullest potential.

Soil is always full of microbes
Soil is always full of microbes

What Happens When Microbes are Added to Soil?

They will mostly likely die! A review of this topic concluded that, “Inoculation of soils has already been applied for decades, but it has often yielded inconsistent or disappointing results. This is caused mainly by a commonly observed rapid decline in inoculant population activity following introduction into soil, i.e., a decline of the numbers of inoculant cells and/or a decline of the (average) activity per cell. ”

There are two main reasons for this. The first one is that the space in soil is already occupied by native microbes. In order for the new ones to survive, they have to kill off the existing ones so that there is space and resources for the new ones. You can’t add another person to that phone booth without removing one.

The second reason is one of environment. Microbes are fairly fussy about where they live, what they eat and who their neighbors are. The chances are very good that the soil environment is not suitable for the new microbes, in which case they die. If the soil was suitable, they would already be there.

There is very little evidence that supports the idea that added microbes will prosper and colonize the soil.

What Are Beneficial Microbes?

Most academic discussions just talk about “just” microbes, but manufacturers of products like to use the term “beneficial microbes”. Gardeners seem to like the term too. By definition these are microbes that are beneficial to plants which includes anything that is not a pathogen. Even if a microbe does not help a plant directly, it usually helps indirectly, by providing nutrients to soil, or improving soil structure.

The term beneficial microbes tells the gardener nothing about the contents of a product – it is a marketing term that is used to make the product seem important.

What Are Effective Microbes?

The term Effective Microorganisms (EM) was initiated by Dr. Teruo Higa to describe a combination of about 80 different microbes which were capable of improving the decomposition of organic matter. He developed the idea that the right combination of “positive microbes” would improve any media, including soil. The initial product was called EM-1, which contained three groups of microbes: yeast, photosynthetic bacteria, and lactic acid bacteria.

Since the introduction of EM-1, many other formulations of effective bacteria have been produced by a variety of manufacturers. When you buy a product containing effective microbes, you are buying a combination of microbes that the manufacturer considers important.

Marketing departments and gardeners routinely mix up the terms, beneficial and effective.

What Does the Science Say?

A lot of research has been done in this area and some studies show that bacteria or fungi added to soil improves soil and plant growth, while many others show no effect at all.

The problem with many of these studies is that they don’t design the experiments to separate the contribution of the actual organisms, from the media holding them. I have discussed this before with compost tea. You can’t compare tea containing microbes to water, because we know the tea contains nutrients that help plants grow. To prove the value of the microbes you have to compare compost tea containing live microbes to the same tea containing dead microbes. Almost no studies do that.

Many of the positive studies are lab experiments that are not using native field soil. When field soil is used results tend to be negative. For example, when four commercial products were added to tomatoes growing in field soil containing native populations of microbes, they did not improve plant growth and had limited effect on the native microbe population.

When studies are properly designed, most show no benefit when microbes are added to soil. There are some very specific cases where the addition of identified microbes reduce a specific plant disease. There are also well known inoculants like rhizobium bacteria for legumes. However, the science does not support the general idea that adding microbes to soil will improve soil health.

Quality of Commercial Products

If you buy some soil probiotics you will get a bottle, or package that contains a liquid or powder. A gardener has no way to know what is actually in the material.

Oregon Department of Agriculture has been testing products containing beneficial microbes. “Of the 51 products tested for bacteria, only nine met their guarantees. Of the 14 products tested for Trichoderma, a fungi, none met their guarantees. Of the 17 products containing mycorrhizal fungi, only three met the guarantees made on the product label.” DNA testing showed that in some cases the organism had never been in the product. In other cases the organism was no longer viable.

Remember that probiotics are living organisms. They have to be produced and stored correctly, or the product is worthless.

Probiotics in Agriculture

If probiotics worked, agriculture would be the most important customer for them because agriculture is driven by profit. Any product that can increase yield will be adopted.

A recent discussion, by an industry insider, about the lack of adoption of probiotics in agriculture highlighted two important reasons why they are not used more.

Probiotics are only successful when abiotic soil factors such as texture, pH, temperature, moisture content, and substrate availability are critical assessment to determine if they match the microbes needs. This is difficult for agriculture, but impossible for gardens.

The current science does not support their wide use in agriculture and there is almost no evidence they work in gardens.

Do You Have a Microbe Deficiency?

Companies are telling you that you need to add more microbes. Why not ask the question, “do I have a deficiency?”

How can you measure a deficiency? How can you tell which microbes you have and which ones are deficient?

The short answer is that you can’t find this out. Even labs specializing in this kind of testing can’t identify the microbes you have because scientists have not even named most of them. These labs can give you some general numbers for bacteria activity or fungi populations, but what do you do with this information?

There are no guideline numbers that indicate degrees of healthy soil. No one can test your soil and give you a list of the microbes you are missing.

And yet, manufacturers of these products are quick to tell you that their product will improve your soil. They have no way of knowing this. And you as a gardener have no way to verify that your microbe populations improve after applying the product.

Perhaps the most important point is that you never have a deficiency because microbe populations are always at capacity.

Probiotics Added to Soil

One of the claims by manufacturers 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, the 3,000 natural bacteria can be 6,000 in 20 minutes. What effect do you think one extra bacterium has on the population? None.

Also note that 1 teaspoon of lower quality soil has 5 times as many bacteria as one pack of this product.

Have Commercial Products Been Tested?

I have researched a dozen different manufacturers to find a scientific study that shows the efficacy of their product. I have contacted manufacturers and asked for their evidence. Most don’t even try to reply. I did get this reply, “This is a bacterial treatment. No information was published.”

I have not found one published scientific paper that shows these commercial products work. Customers have to start demanding this type of evidence before they buy these products.

If you find such studies please post a link to them in the comments below.

Should You Buy Soil Microbes

Although some very specific products may be beneficial for farmers in certain cases, home gardeners should not buy microbes except in rare cases which I will discuss in another post.

Your soil is already saturated with as many microbes as it will support. Any that you add will almost certainly die.

The only thing you can do is take steps to improve your soil health. As the soil gets better, microbe populations increase naturally.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

31 thoughts on “Buying Soil Probiotics (Microbes) – Are They Beneficial, Effective or Nonsense?”

  1. How do you feel about products that contain multiple strains of bacillus that use humic acid as a carrier? Wouldn’t the humic provide the organic matter needed for the microbes to survive and flourish upon application?

    • show me a study that concludes bacillus lives on humic acid?

      Let me expand your point. What happens if you add bacteria along with a food source – would they then live?

      We, yes, until the food runs out. Then you are back to the same starting point.

      Adding compost, for example, along with the microbes in it, gives them a chance to establish themselves in the soil. But if you add sterile compost (ie no microbes) you accomplish a similar end goal. Native microbes will grow in numbers. So it is the compost, not the microbes, that add the value.

      Humic acid is not a natural material found in soil. It is man-made.


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