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

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Robert Pavlis

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 - do you need to add them to soil?
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.

Soil Science for Gardeners book by Robert Pavlis

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 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 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.


1) Photo Source: EMSL

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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!

37 thoughts on “Soil Microbes – Do You Need To Add Them To The Garden?”

  1. As always you make some good points about the limits of at home soil microscopy that anyone trying to use it should know, I think you are making another straw-man argument here. Dr. Ingham explicitly states in here soil microscopy videos that are freely available that the goal is not to identify individual species, which she also states is not possible without much more complex processes and equipment. My understanding of her is that she is more suggesting looking at the sample like a forest ecologist would look at any given spot in a forest ecosystem, and one can great insight by doing so without identifying any given species.

    As a forest ecologist that has specialized in the Pacific NW, I could go to a different forest in New Zealand or some other foreign place and be pretty decent at identifying their ecological health simply by looking at its diversity, biomass, and structure. I can tell if I’ve seen 30 different birds, or dozens of different trees, ferns, flowers, etc. If I see predators, or sign thereof, I know something is supporting them. Diverse predators of all shapes and sizes are even better. I can identify predators by their morphological characteristics (skull, footprint, gait) or their scat.

    Similarly, even basic first impressions by a relative novice in soil microscopy (like myself) can be valuable and informative for making decisions as a gardener. For instance, “wow that homemade compost has an abundance of diverse fungi and protozoa compared to the bagged and bought compost we just looked, which had mostly bacteria and fewer humates and aggregates, and even those bacteria were much less numerous.” We can also see the differences in similar factors between soils with differing disturbance histories, and then make decisions with better insight as to where in the soil ecology succession process we are, what we want to avoid doing again, and provide the habitat for what we want to grow below and above ground.

    I appreciate your insights, but I often find your articles deflating straw man arguments that are frustratingly myopic.


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