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Soil Microbes – Do You Need To Add Them To The Garden?

People are starting to understand that microbes (ie microorganisms) are very important to a healthy soil. This knowledge has led to a disturbing trend where people feel the need to add microbes to soil, and manufacturers are responding with an increasing number of products. In this post I will look at the misconceptions behind this trend.

Soil Microbes

Soil Microbes – do you need to add them to soil?

Soil Microbes

To start off, let’s define the word microbes. A microbe, also called a microorganism, is a generic term to refer to a wide range of microscopic life which includes things like fungi, algae and bacteria. Algae play a minor role but will not be discussed in this post.

Microorganisms digest organic matter, and in the process they provide nutrients to plants, and improve the structure of the soil. A gardeners job is to increase the number of microbes in the soil, and to provide the food they need to be productive. When this is done properly, plants grow well and soil is improved. There is no question that microbes are important to the gardener, the plants and the soil.

Each gram of soil, which is about the weight of a paperclip, contains anywhere from 100,000 to 1 million living microbes. That is a lot of microbes. Not only are there a lot of them, but there are also many different kinds. The kind of microbes present at any time depends very much on soil conditions such as moisture, temperature, pH, chemicals present, and available food. Even the kinds of plants growing in the soil will have an effect on the type of microbes present. So the microbes under tomato plants will be different from the ones under a zinnia.

How do these microbes get into the soil in the first place? Microbes are very small, and they are everywhere. Animals, insects, air and water move them from place to place.

The bottom line is that your soil contains thousands of different microbes already. If the conditions suit them, they will grow, reproduce and prosper.

You don’t have to add microbes to the soil, because it already has lots of microbes.

What About Poor Soil?

Some people make the argument that poor soil (heavily compacted soil, sand, extremely heavy clay etc) does not have a lot of microbes and therefore there is value in adding more. It is true that poor soil has fewer microbes, both in type and quantity. The soil conditions are such that microbes don’t grow well.

In this case does it make sense to add microbes?

The answer is NO. If you add microbes and the soil conditions are not to their liking, they simply die. If the conditions are acceptable to the microbes you add, they would already be there. Adding microbes to poor soil does not work. You need to first improve the soil, and as this process is taking place more microbes will come.

Sources of Microbes

This section reviews some common sources of microbes, both commercial and home made.

Compost Tea and Manure Tea

I have reviewed this topic in the past in Compost Tea. One of the ascribed benefits of these teas is that they increase the number of microbes in the soil. The problem with compost tea and manure tea is that you have no idea which microbes you are brewing. The home owner has no idea if the microbes are good for the soil or not. It is quite possible that you are brewing up a batch of pathogens that can damage the microbes already in the soil, or your plants, or even yourself. You could be brewing a batch of dangerous E.coli that will make you sick–you just don’t know.

Compost and manure already contain all kinds of microbes. Just add them to the soil as compost or manure and skip the brewing process.

Mycorrhiza

Mycorrhiza is a type of fungi that is very important for plant growth. Companies have started packaging them and promoting them to consumers. At first they were sold as an additive to soil, but now you can find them added to many soil and soiless products.

Your soil already has mycorrhiza so you don’t need to add them.

There are hundreds and maybe even thousands of different types of mycorrhiza, some of which are very specific to certain varieties of plants. Commercial products, at best, contain 4 types. Many products contain fewer types. You don’t know that the ones in the pail are the ones your plant needs!

Mycorrhiza are fairly sensitive to high temperatures. If the container holding them gets too warm, like sitting on a truck too long, they die. You have no way of knowing that the product you buy actually contains living mycorrhiza. It may just be a very expensive, useless white powder.

Probiotics

Probiotics for soil is the same idea as probiotics for your intestines. They are a combination of microbes that you buy and add to your soil.

How do you know if they are living? You don’t.

Will they live in your soil environment–remember microbes only grow in environments that suit them? You don’t know.

Probiotics for soil is just another way to fleece you of your money.

Keeping Soil Microbes Happy

Your soil already has lots of microbes. Don’t add more using commercial products or compost tea. The secret is to provide the microbes you already have with a home they love. How do you do that? Feed them.

Microbes eat and digest organic matter. Keep adding compost, manure, plant cuttings, wood chip mulch etc, to your soil. Just growing plants in the soil will provide organic matter for microbes to eat. Disturb the soil as little as possible. No rototilling–it destroys microbes. Hoe as little as possible for the same reason. Walk on the soil as little as possible–compaction kills microbes.

Note added January 2015: A comment was made below that made me think about the italicized statements. Does compaction kill microbes? It is something many people say, but is it true? Have I simply fallen into the myth trap and repeated a myth? I think that my statements are not completely true, but the subject is more complicated and needs it’s own post in the future. For now, I do feel confident in saying that compaction is not good for the garden because it destroys soil structure and that the microbe community will be happier without compaction. Rototilling and hoeing also destroys soil structure, and will disrupt fungi in the soil.

Bacteria, the most important microbe, grows very quickly. They can double in number every 20 minutes. If you had only one bacterium and you provided food and other suitable conditions, you would have 250,000 in 10 hours. Who needs to buy them??

You might also be interested in this post about using a microscope to identify microbes: Soil Bacteria – The Myth of Identification and Management.

References:

1) Photo Source: EMSL

Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

15 Responses to 'Soil Microbes – Do You Need To Add Them To The Garden?'

  1. rick padgett says:

    Can you over compost?My soil situation; phosphorus, potassium, calcium, magnesium, on soil test all high to excessive.Have been collecting grass clippings in Summer and leaves in Winter by the truckload.Compost or till in every year.
    Do I need to stop composting.I do add nitrogen per soil test results.
    Central South Carolina.
    Advice??

  2. Josh M. says:

    Robert

    As someone who has spent hundreds of hrs studying Elaine Inghams work I can tell you that her work is backed up by facts. “I spoke to some experts” is not a convincing statement for you to make.
    Dr Ingham has also taught that compost tea shoold be brewed from locally made compost to introduce microorganisms that will thrive in your environment. She also advises you learn to use a microscope or send a sample in to a lab to have your brew/compost tested.
    Unfortunately there are many people selling compost tea and many products that have not been proven. However a biological approach to agriculture and gardening is gaining ground because of the results many are having after leaving conventional agriculture.
    I would urge everyone who wants to Learn more before making up their mind.
    LIsten to this presentation by Jeff Lowensfels, he has the longest running column on gardening in the U.S.
    http://youtu.be/vKTcbLbb2Wc

    • I’d like to see the facts that back up Dr. Ingham’s work. As far as I know she has not published any of her research. As I have said before some of her ideas make a lot of sense, unfortunately some other ideas she has are completely wrong.

      The idea that someone can use a microscope to evaluate the micros in compost tea is ridiculous. Soil contains thousands of species of bacteria, most of which the experts in the field have not even identified, and it is now suggested that a non-expert can learn to use a microscope to do this?

  3. Roger Brook says:

    As to your discussion about the effects of soil of cultivations on soil organisms the effects are huge but it is not so much that the organisms are killed as to that the balance of oxygen, perhaps moisture levels and a myriad of factors change and so does the dominant bacteria, fungi etc. There are vast changes in bacterial populations just depending on whether the soil is hot or cold, or flooded or dry. It is a mute point as to whether these changes are detrimental, beneficial or neutral in their effect.
    I do argue that rotavating or other methods of fluffing up the soil encourage those aerobic bacteria that oxidise organic matter. Good if you want a quick flush of nutrients, bad if you want to preserve your organic matter

    My friend Peter who I am always quoting or reproducing his articles on my blog is not only a skilled and dedicated gardener (whoops he digs too) is a microbiologist who has specialised in mycorrhiza and other soil organisms agrees with me that adding organisms from a packet is highly dubious

    • I’ve recently talked to some experts on mycorrhiza and they also confirm the fact that adding them to soil has little value. In fact they said that some scientists are now worried that adding these foreign strains may actually end up harming local soil.

  4. Roger Brook says:

    I have taken the easy way to reply by pasting my reply to your enquiry that I have made on my own blog!
    Thank you Robert for inviting me to join the discussion on your excellent blog. I will shortly make a comment on your post.
    (I agree with the thrust of what you say about soil organisms from a packet. In perhaps 95% of cases a complete waste of time. You will have noticed my wavering on the actual value of mycorrhiza at all in most garden situations where the soil is usually highly fertile. The other side of that coin is that there are situations in nature such as ancient bluebell woods that would not survive at all without mycorrhiza)

    Of course our UK soils are subject to structural damage. I blog about it all the time!
    Your correspondent is right however that our soils are generally more resilient than in most parts of the world and probably correct that our climate lacks some of the extremes that might for example facilitate widespread water and wind erosion when soil is loosened by cultivations. This contributes, in my opinion, for our farmers to lag behind the no till methods in other parts of the world.

  5. RayR says:

    Robert, I think you yourself are being a bit extreme in your views. Saying that Mycorrhizal and other microbial inoculants are “very expensive, useless white powder.” and the producers and sellers of these products just want to fleece you of your money?. And expensive? They are actually very inexpensive. Have you ever even experimented with inoculants yourself?

    • Robert Pavlis says:

      Let’s be clear about what I said.

      I never said “Mycorrhizal and other microbial inoculants are “very expensive, useless white powder and the producers and sellers of these products just want to fleece you of your money?.”. You have taken certain phrases and reworded them to present a different picture than I presented.

      In reference to mycorrhizal fungi, I said they were heat sensitive, and that, “It may just be a very expensive, useless white powder”. If the mycorrhizal fungi in the pail are dead due to overheating, then this statement is certainly true–you get nothing for your money. Is it “very expensive”, or “expensive” or “cheap”? A pound of the stuff sells for anywhere between $20 and $50. I personally find that “very expensive”, if I am getting nothing for my money–you might be OK with this.

      The problem is you don’t know if they are dead, or if they are working in your soil.

      The reference to “another way to fleece you of your money” was with respect to probiotics for soil, not mycorrhizal fungi. There is no scientific evidence that adding probiotics to soil does anything good for your garden. Selling a product that has no value is a way of fleecing you of your money. Is that extreme? I don’t think so. Anyone who is using false advertising to create a market for their product is trying to fleece you out of your money, and I think it is wrong.

      I have not experimented with inoculants myself. Before doing so, I reviewed the available research on the subject and found no reliable data to suggest that they work. I have been monitoring this topic for a number of years and some of these findings are presented in this post. Why would I spend money for something science can’t validate as being useful?? Even if I had tried them, I would not have performed scientific studies, so my findings would have no meaning. Besides, my plants are growing fine without inoculants. There is no problem to solve with new products.

      Show me some good science studies, using inoculants in the field, and I will certainly post the data on my blog, and try the inoculants.

  6. bio says:

    You might want to study a bit more.. Right now, I’m afraid, you sound like you don’t really know what a living soil is all about. I’d suggest you start with Elaine Ingham’s take on the soil food web.

    • Robert Pavlis says:

      Unless you provide some specific comment that can be discussed, your general statement is not of much help to the other readers.

      I am quite familiar with Elaine Ingham’s material. Some of it is quite good and I know she would agree with most of the facts in my post. However, she takes some of her ideas to extremes and presents information that is simply not correct, nor are all of herr opinions accepted by the scientific community. We don’t agree on everything.

      • Tony says:

        Humm. Most of what you say is fine Robert, however the final section seems to have some myths too. Is there any evidence that rototilling and hoeing kills microorganisms? They are tiny. I would suggest that air is being mixed into the soil when it is dug or hoed – adding air is something that we are encouraged to do with compost to keep it aerobic and increase the population of aerobic microorganisms. Elaine Ingham goes on and on and on about this. Why is it good for composts and not for soils?
        Compaction kills microbes? Are you sure? Compaction reduces the porosity of the soil reducing oxygen levels but both facultative aerobes and obligate anaerobic can live quite happily regardless. Extremophile bacteria are found in sulphur pools of volcanoes and deep in rock cracks of Antarctica. I would not be surprised if they found bacteria on Mars or an asteroid. A bit of squeezing wouldn’t worry them at all. In any case you would have to compact the soil quite a bit with heavy machinery to compact it seriously. A good job we no longer have dinosaurs.

        Like the site though…

        • Thank you for posting and asking your question–it is a good one.

          I’ve been trying to find a quick answer to the question but it’s not going to be easy. I think a complete post on this question is in order–some day.

          I tend to agree with some of what you say–I just repeated what others have said, perpetuating what is probably a myth. The word ‘kill’ is probably a poor choice of words, disruption of life might be better. I think fungi, with their long filaments will certainly be damaged–but are they killed–not sure. Bacteria are probably not killed by the mechanical actions, but the population mixture may change as you point out.

          I probably also need to review my choice of the word ‘microbe’. There are many types of organisms that are larger than bacteria, that are not strictly microbes, which will due to their size be more affected by compaction and the physical action of rototilling. maybe soil biota is a better term.

          I have updated the post with a comment and plan to do more posts on this subject. Thank you again.

          • Tony says:

            Hi Robert thanks for the reply.

            Let me first reiterate; I think the site is excellent.

            I usually call the soil animals larger than bacteria soil invertebrates.

            I think you are right when you say that soil organisms are disturbed by mechanical actions, however I would maintain that they are well adapted to cope with this. The action of boar and pigs, chickens and moles all contribute to the turning over of soil. I have little confidence that they sterilise it at the same time.

            I agree that rototilling or hoeing will alter the soil structure and this can lead to serious problems in some climates. However, this is not generic. I have the luxury of gardening in Britain where the climate is not severe and where digging does not cause too much damage.

            I think that fungi hyphae may well be seriously damaged – particularly those of mychorrhizal fungi. Which is why I add commercial mychorrhizal amendments to planting holes. Mychorrhizal fungi spores may well be ubiquitous, however my plants seem to benefit from the ‘generalist’ mychorrhiza fungi of commercial preparations.

          • I had not heard that climate might mitigate the destruction of soil structure. I’ll have to ask The No Dig Gardener (in the UK) about this.

            I was at a talk by a University of Guelph expert on mychorrizal fungi this week. He confirmed that there is little benefit to adding a commercial product to planting holes in the ground. In fact he stated that scientists are starting to worry about the ‘non-native’ varieties being spread globally through commercial products. They might upset local populations in a similar way to invasive plant species.

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