There is no doubt that mycorrhizae fungi play an important role in plant growth. They help aggregate the soil which in turn provides plant roots with better access to water and oxygen. Their symbiotic relationship with plants helps them access water and nutrients. It is only natural that companies want to sell these fungi to you. Don’t fall for it.
Mycorrhizae Fungi
Mycorrhizal fungi (mycorrhiza) are found in all soil where plants grow. They form large networks of fine filamentous growth throughout the soil. They associate with plant roots; some even burrow into the roots to create an even greater association with plants. About 80% of all plant species form some type of association with these fungi.
Think of mycorrhizal fungi as a vast network of very fine plant roots. They are not plant roots, but they behave in similar ways to plant roots. They burrow into nooks and crannies in the soil and collect water and nutrients for the plants. In return the leaves of plants send sugars to the fungi as food. Given this important association it is natural for one to think that it would be beneficial to add more mycorrhizal fungi to the soil. For a more detailed description of mycorrhizal fungi see Microbe Science for Gardeners.
Arbuscular Mycorrhizal Fungi
There are two basic types: EM and AM.
EM fungi (ectomycorrhiza) form associations with only about 2% of plants, mostly the woody plants like trees and shrubs. Since woody plants make up a large portion of the plant community, they are important to natural ecosystems but they are not usually found in commercial products for the garden.
AM fungi, called arbuscular mycorrhizae (a type of endomycorrhiza), make up the largest group and form bonds with the majority of plants. They provide mostly phosphorus to the plant. These are obligate symbiotic fungi, which means they can not survive for long without a host, although they can produce spores (a types of seed) that can survive until a host root shows up. This is the type found in commercial products.
The fact that they can survive as spores is important because it makes it easier to produce them. Commercial products contain a combination of spores, small fragments of mycelium and small pieces of roots that contain bits of fungi. Together these are called arbuscular fungi propagules and the number is listed on some commercial packaging. For example one product claims to have “60,000 propagules per lb”.
Facts About Mycorrhizae
- Garden soil already contains vast quantities of mycorrhizae, or at least it does if any kind of plants grow in it. Adding more is a waste of money, since they are already in your soil.
- Manufactured mycorrhizae consists of 2 or 3 types of fungi. It turns out that there are hundreds if not thousands of different kinds in your soil and some of these are very specific to certain types of plants. How do you know your plants will benefit from the 2 or 3 types you buy?
- Manufactured mycorrhizae are heat sensitive. If the container is left in a greenhouse, or a transport truck too long, the heat kills the fungi. There is no way for you to know that the product you buy contains active fungi – they could all be dead.
- Manufactured mycorrhizae are of a specific species, almost certainly from an area that is foreign to your soil. It is possible that these foreign species inhibit the growth of your native species. You have no way of knowing but it is never a good idea to import foreign species of any kind into your garden.
There is just no good reason to buy mycorrhizal fungi for your garden.
Special cases
There are some special cases that might warrant the addition of mycorrhizal fungi.
Bare Soil:
Soil that has no plant growth probably has few natural fungi. It would seem that this would be a good case for adding them. The problem is that if the soil is not suitable for plant growth, it is probably not suitable for the growth of mycorrhizae. Adding them will simply result in them dying. If you want to grow plants in this soil you first need to solve the problem in the soil. As plants start to grow, native mycorrhizal fungi will also show up, naturally.
Sterile potting soil:
Sterile potting soil used in containers has no natural fungi. Studies have shown that adding mycorrhizal fungi to this type of environment can have some positive results. Keep in mind that the main value of the fungi is to provide the plants with water and nutrients. In a potted situation, many gardeners over water and over fertilize, negating the benefit of the fungi. If you water and feed your plants regularly, there is little benefit to adding mycorrhizal fungi.
Commercial Inoculants vs Compost
Most commercial products have a dozen or fewer species in the product. Some don’t even list the species. A recent analysis of a compost sample showed that it contains 305 fungal species and 360 bacterial specials.
In a recent review I checked with 5 commercial product manufacturer and asked them for proof that their product worked. You can read about the results in Mycorrhizal Inoculant Investigation – Do They Work?.
The Latest Science About These Fungi
The latest news investigates how tilling affects fungi in soil, and how effective commercial products have been in agriculture. Read all about it in Mycorrhizal Fungi – The Latest Scientific News.
The latest data clearly shows that claims such as “In soil that has recently been tilled/worked, mycorrhizae will be lacking” are not true.
Better Ways to Build Soil Health.
10 Easy Soil Testing Methods For Measuring Soil Health
Preventing a Nitrogen Deficiency in Soil – How to Manage Nitrogen Levels
Soil Bacteria – The Myth of Identification & Management
10 Fertilizer Myths That Will Save You Money
Mycorrhizal: Reading ‘Mycelium Running’ by Paul Stamets is fascinating.
Hi Robert,
I love your site.
Do you have any insight on Hugelkultur.
Supposedly the rotting wood buried under the earth not only acts as a sponge for water but also all sorts of fungi develop which supply nutrients to the plants. Does it really work?
I have not studied Hugelkultur in detail. It is a combination of sound ideas, and folklore rolled up into a gardening system which has had very little scientific testing.
It has been discussed several times in the Facebook Group called The Garden Professors.
Many Thanks for your website. Could you please tell me how much of propagules can give one mycorhizae spore per generation?
Thanks and regards.
I don’t know.
I am no scientist but as a gardener of many years and currently a community food grower, I have spent the last five years using mycorrhizal fungi in many situations. I found roots were thicker and stronger when plants grown from seed were grown in compost inoculated with this fungi with an immense amount of rootlets and that plants were more resilient to drought. Growth is quicker and more lush. I couldn’t believe the results which is why I will continue to use mycorrhizal fungi. I live and work in Somerset in the UK.
I just read your article on anecdotal evidence….clearly I should conduct some experiments! I will see if I can find the time and a spare patch of land to do it!
Thank you ,,,,p.s. sorry Robert don’t agree with you……Sun & Earth Microbiology
I have met several mycologists recently Robert and they all say the same things as your goodself
Hi Robert,
Good post. People need to remember that dirt is indeed “dirty”. By it’s nature, life is tenacious and stubborn and I doubt you could find such a thing as a ‘sterile soil’ in any garden. I grow a lot in large raised beds made of commercially available, heat sterilised potting mix and experimented with innoculants as I often had poor initial growth until the plants had spent a month in the bed. For me, they seemed to work. I got better plant growth from my seedlings and I was happy. The only problem I had was the potting mix would become hydrophobic and difficult to easily water.
Then, by chance I got access to a large amount of free, fresh horse manure. So, I set out beds with the normal mix, layered the top with the manure and put in the seedlings as normal. I had huge success! A big difference – better than innoculant. The manure supply soon ran out and I took to laying a very thin (1 inch) layer of soil on top of the potting mix to prevent the hydrophobic behaviour of the potting mix. To my great surprise, I got results as good as I had with the manure – both better than innoculated sterile mix. I can’t speak to the details of the “why did this happen” (I have a PhD in Chemistry and have no clue). So I can only surmise that the hard clay pan local soil used as a topdressing had enough life in it to establish a good microflora in the beds all by itself and the fertilization effect of the manure was incidental if it mattered at all.
Additionally, at the end of each season we have to “top up” the beds as the potting mix breaks down. What I find is that the manure and soil topdressed beds need about 10% additional material added and the innoculated beds need about 5% in year one. All beds behave the same in year 2..roughly 10% top up.
Potting mix that is peat based is notoriously hydrophobic. So manufacturers add a wetting agent to the peat to make it easier to water. I’ve seen a few reports of the peat being OK at first, and then becoming hydrophobic. I suspect the wetting agent is either breaking down or washing away.
Putting manure or soil on top would prevent drying – so you might be seeing more consistent water content in the peat soil.
This issue is a huge one, that we can’t even begin to address here. Many sources claim that the largest threat to the rhizosphere (areas where roots and root symbiotic organisms interact) is actually soil disturbance. Since most people till their gardens annually it would make sense to add inoculants to the soil where it has been disturbed to hasten colonization of the soil. Also, some inoculants are worthless to some crops for example many plants won’t benefit from nitrogen fixing bacteria, and most leafy greens won’t interact with Glomus intraradices, a commonly used arbuscular micorrhizal fungi. This means that neither inoculant would benefit a garden of leafy greens from the Brassicaceae (Cabbage and lots more), Chenopodiae (Spinach and more), or Polygonaceae (Sorrel).
So… using gardening practices that promote micorrhizal fungi and healthy soil will negate the need for soil inoculants to some extent, but more traditional methods may greatly benefit from inoculants. I almost always inoculate my legumes, for example, with nitrogen fixing bacteria, but even in the areas that are not treated the legumes still produce nodules with nitrogen fixing bacteria.
A lot of people till their gardens – but I am not sure I would agree it is most. Most people don’t own a tiller, nor do they have a garden big enough to till. To those who do till – it makes more sense to stop tilling, than to use a fungi inoculant – as you suggest.
Nitrogen fixing bacteria is a different product and one that has been shown to have beneficial effects on plant growth and production when used with the right kind of plant – – mostly legumes.
Hi Rob,
Many thanks for sharing your experience.
I agree with you on your understanding of adding “Mycorrhizae Fungi”, especially when we are working with fertile soil. Just want to know, should we even avoid adding any types of external fungi or bacterial inoculant to our soil. Right now i am myself is in testing phase & shall share my outcome.
Regards
Abhishek
Great post, Robert. I agree 100%.
I agree very much with jake, and I believe you are muddling the issue about species of mycorrhizae speaking of Ectos and not explaining the difference in AMF and the Ectos varieties with do not form with agricultural plants but instead form with trees and shrubs. I’m not sure why you are trying to frame mycorrhizae in a negative way as there has never any published data to suggest a negative effect as these organisms form symbiotic relationships with plants.
The purpose of the post was not to understand the difference between AMF and Ectos – different topic for a different post.
I also did not frame Mycorrhiza in a negative way. In fact what I said was “There is no doubt that mycorrhizae fungi play an important role in plant growth. They help aggregate the soil which in turn provides plant roots with better access to water and oxygen. Their symbiotic relationship with plants helps them access water and nutrients.” The point of the post, was that there is no good reason to add purchased product to your garden. I have talked to experts in the field and they agree that adding mycorrhizae to gardens is a waste of money.
Robert Pavlis: “The point of the post, was that there is no good reason to add purchased product to your garden. I have talked to experts in the field and they agree that adding mycorrhizae to gardens is a waste of money.”
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By experts, you mean the Garden Professors. I use to follow them on their older WSU page and in the Facebook pages. Not anymore. I also practiced and agreed with many things they published, but they have a strong propensity towards having a god complex. Let’s face it, their discussion forums are their own little universes and they have the power of life or death over anyone who disagrees with them. The hubristic insistence that truth comes only from something science-based and found in ONLY in peer-reviewed journals became way too much for me. Hence I quit my association over there. Their articles had too much name calling, insults, poking fun at people and generally their brazen double standard conduct towards readers who tried to comment or give their opposite experiences which worked for them , but didn’t accept the Garden Professor’s version of the scientific orthodoxy became annoying.
Now, I have heard this same myth about mycorrhizal spores are all over floating in the air. They are just out there somewhere and there is no reason to inoculate. Not true. I have heard this from so-called experts for years and it is an untruth, even the well educated people at the Southern California native plant nurseries I purchase from would religiously chant the same exact secular narratives on the “no need to inoculate” dogma. I had several acres of land up in Anza California above Palm Springs at one time where I experimented with plants and various mycorrhizas, even collecting my own long before numerous papers were available to the public on the subject. I fought the US Forest Service on many of their lousy bad scientific practices of reforestation. Like many, they never inoculated. Why ? Because it’s just out there. Their irresponsible practice where I lived was to chemically and mechanically kill off all the chaparral and perennials because these were consider an invasive hindrance to their successful tree planting program. They would even slightly till the soils, something that would insure the mycorrhizal network grid was obliterated. But the chaparral removal was what hurt their efforts the most. [this may even be hard for you to grasp since we are from extreme diverse ecosystems] I identified several extremely deeply rooted chaparral plants which actually facilitated seedling growth through a process they perform called hydraulic lift and redistribution which was further enhance through the mycorrhizal grid, if left intact. I actually proved this by establishing a forested wooded area at a lower elevation that they insisted would fail. The trees planted on this borderline high desert acreage still exist and have out performed every one of those US forest Service projects. In all of the forestry projects, they had to go out and weekly water those trees in the beginning. Most of my projects there was no need as water was acquired through mutualism with other plants.
I also collected plant specimens from around the southwest to establish on my land. One interesting side effect in inoculation was that the wild shrubs and trees close to where I placed the newer plants actually improved their health and lush foliage vigor that following season and truffle formation appeared underneath their canopies, especially the stunted scrub oaks which had only maintained survival each season with no real noticeable improvement ever over the first 15 years I lived there. Most all urban soils down there are devoid of much of the beneficial organisms found in the wild areas. New housing developments have a fairly sterile soil and inoculents are very necessary. The constant chemical dumping by homeowners and professional landscapers and gardeners has further degraded those soils because the mycorrhizae if it were even present will disconnect with to rich a nutrient soil. The plant will simply stop producing the chemical exudates which signal colonization and the fungi will disconnect and whither.
You might try Mike Amaranthus website and company up in Grant Pass Oregon. He has a wealth of information and he is one of the earliest mycologists to actually write scientific papers on the subject long before he created his own company, Mycorrhizal Applications Inc, to farm these spores for his blended products. Dr Donald Marx of PHC also wrote many of the earliest papers for the US Forest Service on the subject before becoming chief scientist at Plant Health Care Inc. Just be very careful putting faith in a people claiming to be the best experts because the follow the science. There is good and bad science and depending on who is researching and performing the science and what prejudices and biases they infuse into said research will determine whether or not it is a truth through replication and practical application.
The experts I talked to are not the Garden Professors, although they don’t disagree with my views on the subject. I have contacts who spent much of their lives studying mycorrhizal fungus and they have confirmed my conclusions.
The other thing I do is look for scientific evidence for support of an idea. In the case of adding mycorrhizal fungi to gardens, I have not found it. If you have such evidence I would be very interested in seeing it, but I doubt information from Mycorrhizal Applications Inc is going to be unbiased. I did look at their web site – they don’t provide information on any studies that would support their marketing claims.
I agree science is not perfect – but what else do we have? Anecdotal information? I’ve discussed that in Anecdotal Evidence – Not Worth The Screen It’s Displayed On.
As a micologyst I do disagree with some of your claims.
first there are only 240 +/- arbuscular mycorrhizal fungi (which are the ones that you are refering to), there are not thousands of them.
Even if you have some native species, they are competting for resourses with other microorganisms, so their populations are low, compared to a rich inoculum that you could add.
Even if you actually use an inoculum that contains 2 or 3 species, these usually are quite promiscuos, they to colonize most of the plants that you sow.
they have more than just nutritional benefits, they do tend to protect/ increase tolerance against pathogens and diseases, they increase tolerance to environmental stress, such as drought or too much radiation.
In gardens you barelly have some native arbuscular mycorrhizal fungi, since most of the soil that is in there, ussually comes from other parts, like mountains, forest or other fields, so maybe most of the arbuscular mycorrhizal fungi could be low in population and not very adapted to the plants that you might be sowing.
Companies that sells inoculum, usually test their fungi with some of the most used garden plants.
Stil,l I do have to mention that as it is a live organism, usually you can´t predict exactly that it will behave the same way all the time. But such a beneficial effect is generally observed.
There might be 240 +/- arbuscular mycorrhizal fungi, but the estimate for Ectomychorrizal fungi is in the order of 20,000 (Rinaldi, A.C.; Comandini, O.; Kuyper, T.W. (2008). “Ectomycorrhizal fungal diversity: separating the wheat from the chaff”. Fungal Diversity 33: 1–45).
If native resources are low and native species are having a problem growing to significant numbers, why would the added inoculum not also have limited growth due to lack of resources? I had a recent discussion with a researcher from the University of Guelph who studies mychorriza, and he confirmed that any added fungi would have the same struggles. He sees no point in adding a commercial product to garden soils.
In fact he indicated that the science community is now becoming concerned about the introduction of foreign mychorrizal fungi–nobody really knows how this will affect local communities.
Hi Robert,
To me it makes sense what you said.
After listening to a conference of Paul Stamets on TED about the miracles of those fungis I thought this could be the solution for my lack of success in my garden in Cista Rica. It’s a real challenge to get any edible green in this climate, so many plagues, predators…
I was about to buy some of those spores on line but after reading your sensible words on the risks of importing species to a different environment I stopped myself and I will continue doing some research around the issue. Thank you so much for your knowledge, I would appreciate some advises that could help me to produce some organic vegetables on the tropics.Greettings, Mahi.
I’ve never gardened in the tropics – so I can’t be of any help.
” If native resources are low and native species are having a problem growing to significant numbers, why would the added inoculum not also have limited growth due to lack of resources?”
That makes perfect sense but what if we add resources to the soil in the form of water and fertilizer, wouldn’t this support a larger fungi network including the introduced myco fungi?
If you add the right resources. But if you do that, then they will naturally start to grow on their own.