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Compost Microbes – Good for the Soil?

Compost microbes are micro-organisms (mostly bacteria and fungi) that live in the compost pile and get transferred to your soil when you add the compost to the soil. Many people consider this to be a benefit of using compost. Compost manufacturers even advertise their “special microbes”, sometimes called Effective Microbes.

It is all a myth. Let’s dig deep and understand this important gardening topic.

compost Microbes

Compost Microbes

Does Compost Add Microbes?

Microbes are the living thing that helps plant material decompose in a compost pile. There is no doubt that compost contains a lot of different kinds of microbes especially bacteria and fungi. When you add the compost to your soil, you also add the microbes.

But that is not the important question. What you need to ask is, are these compost microbes beneficial to the soil?

Compost Microbes – Where do They Come From?

Lets first determine where these microbes come from in the first place. To build the compost pile you add various types of plant material. This material is covered in microbes. These microbes are in the air, water and soil of your garden. They are always there and they are present in large quantities–you just can’t see them.

As the organic material decomposes, different types of microbe communities develop. Each type has a preferred type of food (the plant material) and a preferred set of conditions (temperature and moisture) that they like. As the pile warms up, some microbes flourish, and others almost die off. If it rains and things get wet, the type of microbe population changes again. The compost pile is a dynamic living community of thousands of different species of microbes that is in constant flux.

The key point here is that the microbes in compost are, for the most part, native microbes. They already live in your garden. They are in the soil and were transferred to the compost pile attached to plant roots. They are on plants in your garden. They were blown in with the wind as it carried local microbes from garden to garden. The microbes you already have in the garden are now in your compost pile.

Compost Microbes Returned

When the compost is finished, you return it back to the soil in your garden. Sure you are adding microbes to the soil, but they are relatives of the ones that are already there.

Remember the dynamic microbe communities in the compost pile? Soil is no different. It also has dynamic populations and some types of microbes in the compost will do better than other types depending on environmental and food conditions.

But… the microbes are already in the soil before you add the compost. In good soil there can be 100 million microbes in a spec of soil (1 gram).

Compost contains huge amounts of microbes, mostly bacteria. But bacteria are very small. One estimate suggests that the weight of bacteria in finished compost accounts for just 0.02% of the weight of the compost. They are vital for decomposing the compost, but they don’t add much in the way of nutrients to the soil.

So to answer the question posed in this post, compost microbes don’t harm the soil, but they also don’t contribute very much to the soil.

Foreign Microbes

Some sources on the web, mostly manufacturers of products, claim that you need to add foreign microbes to your soil, but that is just dumb. Your soil will function just fine with the local populations.

More importantly, scientists don’t know what effect foreign microbes will have on your soil. Some are now concerned that we are moving too many types around the world via commercial products.

Consider the orange you bought at the local store which came from South America. It has S. American microbes on it. Did they survive the composting process? What effect will they have on your soil? Nobody knows. All imported frutis and vegetables contain foreign microbes–I am not picking on S. America here.

Dr. Elaine Ingham has done a lot of good work promoting the idea of life in the soil and the importance of microbes in the soil. Much of what she says is good advice, but unfortunately, she also promotes some ideas that are incorrect. In reference 1, she says the following about adding compost microbes:

“So, when trying to decide what compost is needed, understand your purpose in using compost

very clearly. The tests you need then should be come clear. For example,

if you want to know what organisms you need, you need to do a full foodweb analysis.

If you don’t know what part of the food web may or may not be

“out of whack”, you have to figure that out first”

This seems to make some sense. Your ‘food web’ is out of whack and so you need to figure out the problem before trying to fix it with compost.

First of all, how do you know the food web is out of whack? Scientists don’t really understand this complex system yet. They have not yet identified the vast majority of microorganisms that live in soil. Estimates for the number of species of bacteria in 1 gram of soil range from 10,000 to 800,000 (ref2). We don’t know when it is in whack or out of whack.

Even if we know that, you still have to be able to analyze for microbes. Scientists have trouble doing this, so how do gardeners do it??

Conceptually I have no problem with Dr. Ingham’s statement. In practical terms it’s nonsense. For a more detailed discussion of this topic have a look at Soil Bacteria – The Myth of Identification and Management.

Assume that the microbes you need are already in your soil, or on your plants or in your air. Don’t add new ones. Your job is to take care of the ones you have. Don’t dig your soil, and feed them with organic material like compost.

References:

1) Compost Foodweb Information: http://theearthproject.org/id64.html Note: Nov 2015, the web site for this link has been taken down, and i could not find the material anywhere else.

2) Soil ecology – What Lies Beneath: http://www.nature.com/news/2008/081008/full/455724a.html

3) Poto Source: BASF – We create chemistry

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.

6 Responses to 'Compost Microbes – Good for the Soil?'

  1. Sally Christenson Minnesota says:

    Do pesticides such as glyphosphate kill microbes in the soil when applied to plants? How harmful to life/soil life as well, are all the pesticides that growers apply to crops and decorative plantings?

    • Glyphosate does not harm bacteria in soil – they use it as a food source.

      You can’ talk about “pesticides” in these terms. You need to look at each one as a different chemical, which has different properties. Some will be harmful to soil.

  2. David White says:

    Robert, when you say “Even if we know that, you still have to be able to analyze for microbes. Scientists have trouble doing this, so how do gardeners do it,” I wonder if you’ve watched Ingham teach on how to analyze your soil for microbes, what to look for, what the different ratios of different categories of microbes mean to the soil, etc. Have you seen her work on the ratio of fungi to bacteria, and the progression of soil from being a promoter of weeds, to being a promoter of grasses, then to crops, and so on to the promotion of strong trees by the change of ratio? Have you seen her success in transforming farms and gardens, large and small, by her methods of inoculation? Have you disproved her work, or do you offer an alternative explanation of her success by changing the ratios of categories of soil biota? So that the question according to Dr. Ingham, is not does adding microbes work. Rather the question is can you manage the population of soil microbes by the addition of the right kind. And in what you say scientists don’t understand, it seems she has pioneered the research, namely soil microbiology.

    • I am quite familiar with Dr. Ingram. Her basic concepts of the importance of microbes is sound. She then goes on to present ideas that are not supported by science. No home gardener is going to be able to analyze the bacteria in soil. In fact even scientists have trouble doing this and many species of bacteria are still not described – new species are still being found.

      The problem with Dr. Ingram’s work is that she does not publish any of it in peer reviewed scientific journals. That means other scientists can evaluate it or critique the work. So it is not accepted by the scientific community and her work can’t be considered to be pioneering research.

  3. Roger Brook says:

    I am reading your recent posts very carefully as I am preparing my own on the subject of soil bacteria!
    What a lot of rubbish that lady writes about a soil being ‘out of whack’ and congratulations on debunking gardeners being so stupid as to buy microbes for their soil!
    I am having difficulty in combining the conflicting messages ‘bacteria are very significant’ and ‘most things gardeners try to do about it are a complete waste of time’.
    I won’t reveal the myth which is a lead into my as yet unwritten post!

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