What Are the Best Ways to Increase Beneficial Mycorrhizal Fungi in the Soil?

Mycorrhizal fungi are beneficial fungi that live in close symbiotic association with the roots of most plants and trees. In this association, the plant provides the mycorrhizal fungi with sugars produced by photosynthesis. In return, the mycorrhizal fungi extend their network of long, threadlike filaments out into the soil to effectively extend the reach and root volume of the plant’s roots.

These mycorrhizal fungi play an important role ecologically in supporting the growth and health of plants by providing additional nutrients, increasing drought-tolerance and access to water, reducing stress in extreme environmental conditions, improving soil structure, and protecting plants from harmful pathogens.

For more information, see article – What are Mycorrhizal Fungi and How Do They Benefit Plants and Trees?

Increasing Mycorrhizal Fungi in the Soil

Mycorrhizal fungi are naturally found in undisturbed soils along with other beneficial soil organisms. There are several ways to increase mycorrhizal fungi in the soil, some of which include:

1. Reducing Tillage

Tillage is the agricultural practice of digging, plowing and turning over the soil, in order to break up the soil surface and soil clods, remove weeds and debris, in preparation for planting.

The action of tilling the soil has a disastrous effect on the mycorrhizal fungi in the soil, as it physically severs the networks of delicate hyphae (branching filaments or strands that make up the mycelium or body of the fungus) and breaks up the mycelium itself, reducing the mycorrhizal fungi populations.

Even though mycorrhizal fungi can grow at a range of soil depths, depending on the fungal species and the soil conditions, they are typically found in the upper layers, in the first 10-20cm (4-8″) of the soil, where they can easily access plant roots and the organic matter that supports their growth, but are also easily disrupted by digging.

Studies have shown that the abundance of arbuscular mycorrhizal (AM) fungi and their association with roots typically decreases with increasing soil depth, but over 50% of their total biomass can be found below 30cm (12″). Outside of agriculture, AM fungi have been reported to able to colonize roots and produce spores as deep as 8m (24′) within the soil.

Reducing or eliminating tillage by using no-dig gardening techniques can help maintain and enhance mycorrhizal fungi populations.

Avoiding tillage also has other benefits, studies have shown that the total earthworm populations in long-term no-tilled fields are typically at least twice those of clean-tilled fields!

2. Using Cover Crops

Mycorrhizal fungi can’t generate their own energy and are dependent on the host plants they associate with (attach to) for sugars, which they use as their source of energy.

These beneficial symbiotic fungi can’t survive for long periods of time without a plant host, which occurs when:

• Fields are cleared of all plants, such as when a field is left bare and fallow, letting it rest for a season or more to allow it to recover.
• Ony crops of non-host plants are planted in the crop rotation that the fungus cannot associate with, such as those from the Brassicaceae (cabbage) family.

Such conditions will cause the mycorrhizal fungi populations in the soil to decline over time.

Planting cover crops, which are crops that are grown to protect and improve the soil, also provide a host for the mycorrhizal fungi and increase their population during periods when the primary crop is not growing.

3. Growing Plant Polyculture, Not Monocultures

There are a few hundred species of arbuscular mycorrhizal fungi (that associate with non-woody plants), and different plant species host different types of mycorrhizal fungi.

Monocultures, which consist of a single crop species grown in a large area, can reduce the diversity of mycorrhizal fungi in the soil. The absence of plant diversity limits the varieties of mycorrhizal fungi present in the soil, which can reduce overall mycorrhizal fungi biomass and activity, and as a result, negatively impact plant growth and health.

In contrast, polycultures, which consist of multiple crop species grown together in the same area, can increase the abundance and diversity of mycorrhizal fungi in the soil, which can enhance nutrient availability and improve soil structure, benefitting plant growth and health.

4. Plant Mycorrhizal-Friendly Crops

Growing crops that support mycorrhizal fungi allow the fungi populations to increase in the soil.

Conversely, growing plants that don’t form associations with mycorrhizal fungi cause a drop in the soil fungi numbers. The plant groups that do not form associations with mycorrhizal fungi are the Brassicaceae (cabbage, brocolli, mustard, canola) family, the Chenopodiaceae (spinach, silverbeet/chard, beetroot, Malabar spinach, saltbush) family and the Proteaceae (banksia, macadamia) family. A more extensive list of these plants can be found here – Which Plants Don’t Benefit from Mycorrhizal Fungi?

5. Increasing Soil Organic Matter

Adding organic matter to the soil, such as mixing compost into the soil, or adding a layer of mulch on the soil surface, can increase the number and diversity of mycorrhizal fungi.

Adding organic matter to the soil improves soil structure by increasing soil porosity, which allows better air and water movement thorough the soil. It also creates a more favorable environment for mycorrhizal fungi by providing more space for them to grow through, to extend their network of hyphae into the soil, increasing their ability to acquire more nutrients and transfer them to plants.

Soil organic matter contains nutrients that are slowly released over time through breakdown by soil microorganisms. This steady supply of nutrients can be taken up by the mycorrhizal fungi to support their growth and activity, and to be passed onto the plants.

Organic matter can also provide a source of food for the fungi. Even though mycorrhizal fungi lack the capacity to act as saprotrophs (organisms that feed on decaying organic matter) like common mushrooms can, they are able use several strategies to access nutrients locked in soil organic matter and promote its decay.

Some can produce enzymes which break down soil organic matter, use an oxidative chemical process (oxidation via Fenton chemistry) that generates highly reactive hydroxyl radicals (•OH) that attack organic matter, or stimulate heterotrophic microorganisms to perform the task by producing carbon-containing compounds and releasing them into the soil.

6. Avoiding Overuse of Fertilizers

The addition of high levels of nitrogen or phosphorus fertilizers can decrease mycorrhizal association with plant roots, as can excessive applications of zinc. These inhibit the colonisation of roots by the mycorrhizal fungi, preventing the symbiotic attachment, which has the resultant effect of suppressing mycorrhizal populations.

In soils with high nutrient availability, plants may rely less on mycorrhizal fungi to acquire nutrients, which will reduce the need for the plant to form symbiotic relationships with these fungi.

For this reason, it is best not to use phosphorous-rich fertilisers when utilising mycorrhizal fungi products to inoculate the soil.

In agroecosystems, mycorrhizal fungi provide more benefits when farmers use complex organic fertilizers rather than synthetic mineral fertilizers, as beneficial fungi help slowly release the nutrients bound in complex organic molecules and aggregates.

Even when using organic fertilisers such as manures, use the correct manure application rate for growing crops, don’t over-fertilise. Some manures may contain high levels of salts, especially if they’re fresh and haven’t been left aside to compost for three months, as high salt levels can be toxic to mycorrhizal fungi and reduce their abundance in the soil.

Another matter to be aware of is the use of soil amendments that change the soil pH to make it more alkaline. Mycorrhizal fungi generally prefer slightly acidic soil conditions for optimal growth and functioning. Adding too much garden lime, dolomite lime or mushroom compost can make the soil too alkaline which can decrease the colonization of plant roots by mycorrhizal fungi. For more information see – How to Increase Soil pH to Make Soil More Alkaline.

7. Avoiding Toxic Fungicides, Herbicides and Pesticides

Synthetic fungicides designed to control fungal diseases in plants should be avoided, since they are toxic to mycorrhizae fungi, which can be affected as they’re a type of fungi. These chemicals can reduce mycorrhizal fungi in the soil or affect their ability to colonise plant roots.

Some herbicides, such as glyphosate (Roundup) have been shown to reduce mycorrhizal colonization of plant roots. Glyphosate can affect the growth and activity of mycorrhizal fungi by disrupting the shikimate pathway, which is involved in the production of amino acids and other compounds needed for fungal growth.

There are also some insecticides, such as imidacloprid, a systemic neonicotinoid insecticide (the class of pesticides implicated with bee colony collapse) that have been shown to reduce mycorrhizal root colonization.

It is important to note that toxic synthetic fungicides, herbicides and pesticides can have broader impacts on soil health and create more harm to ecosystems beyond the effects on mycorrhizal fungi. Therefore, it is wiser to use environmentally safe alternatives whenever possible.

It is good gardening practice to use the lowest feasible amounts of manure, fertiliser, fungicides, and cultivation. This will not only save gardeners time and money, but also helps towards protecting the environment.

Does Adding Mycorrhizal Fungi Products to the Soil Work?

Most healthy, natural, undisturbed soils have ample populations of living mycorrhizal fungi or dormant spores that will awaken when a host crop is grown.

When soils are damaged through regular tillage and digging, inoculation of the soil with mycorrhizal fungi is beneficial to re-establish soil health.

Soil inoculation is a technique used in agriculture to introduce beneficial microorganisms such as bacteria or fungi into soil in very large numbers to improve soil fertility and plant growth as well as to restore degraded or contaminated soils.

Mycorrhizal fungi products are used by homeowners and commercial growers, and are utilised in orchards, vineyards, home gardens, nurseries, landscapes, and in land reclamation projects. The use of mycorrhizal fungi is also popular in organic production systems.

Another use for mycorrhizal fungi products is for container growing, as potting mixes generally do not contain mycorrhizae, and they can be incorporated into these growing mixes to support plant growth and health.

Mycorrhizal fungi products are available in various forms, as granules, powders or a concentrated solution, and vary in the fungi types they contain, and spore counts of fungi used. They can be purchased at garden centers, nurseries or from online companies and retailers.

These products can be applied to plugs, seedlings of plants during transplanting, incorporating into the media or the soil, or in the case of liquid products, applied through the irrigation.

Most commercial mycorrhizal fungi products only need to be applied once, no reapplication is necessary, while some recommend an additional application after several weeks. After application, some mycorrhizal fungi can colonize new roots within a week, while others may take as long as a month.

Mycorrhizal Fungi Product – Biostim MycoGold Complete Mycorrhizal Fungi

One such product is Biostim MycoGold Complete Mycorrhizal Fungi that is pictured below, which I tested in my garden.

This product is a very fine, light-grey powder that comes in a foil protected resealable bag, in various package sizes. It’s best stored in a cool dry place and if possible, the fridge (not the freezer).

The ‘best before date’ is one year after purchase if stored in a cool dry area, though it can still be used after then, by increasing the application rates by 30% per 6 months. It can be used at any time of year.

It may not look alive, but this Inoculant contains dormant spores of mycorrhizal fungi that when hydrated will come out of dormancy and start to associate with plant roots within 24-36 hours.

This product not only contains both kinds of mycorrhizal fungi (endomycorrhizal fungi and endomycorrhizal fungi), it also contains a range of beneficial bacteria and other additives.

• 4 species of Endomycorrhizae (Glomus intraradices, Glomus mosseae, Glomus aggregatum and Claroideoglomus etunicatum) – 160 thousand propagules/spores per kilogram minimum.
• 4 species of Ectomycorrhizae (Pisolithus arhizus, Scleroderma cepa, Rhizopogon roseolus and Laccaria laccata) – 240 million propagules/spores per kilogram minimum.
• 2 species of Trichoderma (Trichoderma harzianum and Trichoderma viride).
• 2 species of beneficial Bacillus (Bacillus subtilis and Bacillus licheniformis).
• 5 species of beneficial Bacteria (Azospirillum brasilense, Azospirillum lipoferum, Pseudomonas fluorescens, Pseudomonas putida and Streptomyces cellulosae). – 5 billion cfu per kilogram minimum (Trichoderma, Bacillus and Bacteria total count).
• Also contains humic acid / vitamins and other selected natural ingredients to enhance growth/survival.

Applying Mycorrhizal Products

Mycorrhizal fungi products can be applied at any time of year, and are best applied directly near sown seeds or root zone of plants, but can be applied as a top dressing and gently mixed or watered in.

• When planting from seed, apply a small sprinkle directly into where the seeds will be placed.
• When transplanting, apply the inoculant near the roots of the plant.
• Alternatively, slightly mix the inoculant with the top soil near the roots of established plants.

Once the product is applied, the beneficial fungi will naturally multiply without any further need for reapplication. If conditions are adverse, re-application is recommended at the start of each spring season.

Incidentally, the mycorrhizal fungi inoculant is also compatible with the legume nitrogen-fixing rhizobium bacteria inoculants. They serve different functions and can be used together.

Water in the inoculant well so it gets washed into the soil and is not left sitting on the surface. Use rainwater if possible. The chlorine or chloramine in tap water may harm and inhibit mycorrhizae when it’s first applied, but won’t severely affect mycorrhizal fungi once they’re established.

That’s all there is to it. There’s nothing more to be done! In a couple of days, improvements in growth and health of plants will become visible.

There is no need to feed the mycorrhizal fungi, as they are symbiotic, the plants feed them through sugars produced by photosynthesis, and in turn the mycorrhizal fungi supply the plant with nutrients that they extract from the soil.

Application Rates for Biostim Mycogold Mycorrhizal Fungi

To give readers an idea of how much of the product is required for different uses, I’ve included a copy of the application rates below:

• Turf / Lawns: Apply 1 – 2 g per 1 m² and water into the soil profile. In high value or problem areas the rates may need to be increased to 10 g per 1 m².
• Orchard / Vineyard: Apply 10 g per 1 m² around the active root zone (area irrigated normally about 1m² per plant) and water into the soil profile.
• Vegetables: 0.25 g per plant and water into the soil profile.
• Home Garden: Apply 10 – 50 g per 1 m² and water into the soil profile. A level teaspoon (5 ml) of MycoGold powder is about 3.0 grams.
• Roses (Bare Rooted): Cover damped roots with 1 teaspoon of MycoGold powder.
• Nursery: Apply 10 g per 1 m² and water into the soil profile.
• Potting Mix (General): 2.5 g per 25 L bag or 30 g per m³.
• Seed or Seedling/Cutting Raising Potting Mix: 25 g per 25 L bag or 300 g per m³.
• Hydroponics / Aquaponics: Mix into reservoir with regular feeding at a rate of 5 grams per 40 litres of water.
• Cuttings: Dip cutting into rooting gel/solution then lightly dip directly into MycoGold powder. Alternatively soak rooting media (cubes) in a MycoGold solution at 1 g per litre of water for up to 24 hours before taking cuttings.
• Transplanting: Apply/Dip to water damped roots until covered. Try to apply at least ¼ – 1 teaspoon per plant depending on transplant size.
• Seedlings: 0.5 – 2 g per 10 seedling cells. Apply to surface and water into potting mix or apply via watering can.
• Tree Planting: 5 g per tree placed directly under the root ball in the planting hole.
• Tree Saplings (up to 1 year): 5 – 20 g per tree watered in around the root zone. Apply enough water to carry the mycorrhizal to the root zone.
• Trees (Established): 5 g per 1 m². Apply enough water to carry the mycorrhizal to the root zone.
• Pasture: 1 kg per hectare per year.
• Agriculture Crops: 1 kg per hectare per year.
• Agriculture Crops Liquid Inject: 50 – 150 grams per hectare.
• Horticulture Crops: 1 kg per hectare per year.
• Mycorrhizal Seed Inoculant: 500 grams (small seed) – 1kg (large seed) per tonne of seed. Can be applied as a seed coat or dry powder. To coat seed, dampen seed with a fine mist of water while mixing in the powder. Apply no more than 3 litres of solution per tonne of seed for large size seed (grain legumes, cereals) and up to 1 L per 100 kg for small seed (clover, lucerne, ryegrass). Do not wet the seed so it sticks together.

Growing Trials Showing Benefits of Using Mycorrhizal Fungi Product

Mycorrhizal fungi product work surprisingly fast, producing visible results within weeks. In my own garden, in a rose garden bed with a row of ten different varieties of roses, two were always struggling. After applying the inoculant around the root zones of all the roses and watering it, despite being in mid-autumn season with some warm weather persisting, the stunted roses put on new growth and some new shoots after a week or two, and started looking much healthier.

I’ve managed to source some more striking before and after photos from the product website comparing various plants treated with the inoculant agains the untreated control.

The first example is the common turf grass, couch grass, also known as Bermuda grass (Cynodon dactylon). The inoculated sample shows a much denser and stronger root system.

The image below shows a twelve-week trial of Cytisus plants growing in tube trays, with much stronger growth displayed by the inoculated plants.

Seedlings treated with inoculant clearly look more vigorous, with stronger root development, and more extensive growth.

A second example using a different plant shows the same results.

Lavender growing in tube trays, with the inoculated plants on the left showing much stronger growth and brighter green foliage than the untreated plants on the right.

Two apple tree root systems compared side-by-side, with the inoculated tree showing a much larger, stronger and healthier looking root system with much thicker roots.

In this final comparison, the growth of geranium plants was compared when their fertiliser was reduced by 40%. The inoculated plant on the right appears twice as large and in much better condition.

Association with mycorrhizal fungi permits plants to access more nutrients and thrive in much harsher environments where they would normally struggle to grow.

This root association with mycorrhizal fungi is the natural state of existence of nearly all plants and trees, and has been in place for up 460 million years, when plants first began growing on land!

The only reason why this association is broken is because of our destructive agricultural practices which damage the living soil ecosystem.

When we restore this connection of plants and trees to mycorrhizal fungi, it should be no surprise that they actually grow better, because all we’re doing is returning things to their natural order, it’s that simple!

References

• Guzman, A., Montes, M., Hutchins, L., DeLaCerda, G., Yang, P., Kakouridis, A., Dahlquist-Willard, R.M., Firestone, M.K., Bowles, T. and Kremen, C. (2021), Crop diversity enriches arbuscular mycorrhizal fungal communities in an intensive agricultural landscape. New Phytol, 231: 447-459. <https://doi.org/10.1111/nph.17306&gt;
• Managing Soil Health: Concepts and Practices. Pennsylvania State University, PennState Extension, Updated: July 31, 2017. <https://extension.psu.edu/managing-soil-health-concepts-and-practices&gt;
• Sosa-Hernández Moisés A., Leifheit Eva F., Ingraffia Rosolino, Rillig Matthias C. Subsoil Arbuscular Mycorrhizal Fungi for Sustainability and Climate-Smart Agriculture: A Solution Right Under Our Feet? Frontiers in Microbiology, Volume 10, 2019. DOI:10.3389/fmicb.2019.00744. <https://www.frontiersin.org/articles/10.3389/fmicb.2019.00744&gt;
• de Araujo Pereira, A. P., Santana, M. C., Bonfim, J. A., de Lourdes Mescolotti, D., and Cardoso, E. J. B. N. (2018). Digging deeper to study the distribution of mycorrhizal arbuscular fungi along the soil profile in pure and mixed Eucalyptus grandis and Acacia mangium plantations. Appl. Soil Ecol. 128, 1–11. doi: 10.1016/j.apsoil.2018.03.015
• Mycorrhizal Fungi as Mediators of Soil Organic Matter Dynamics, Serita D. Frey. Annual Review of Ecology, Evolution, and Systematics 2019 50:1, 237-259. <https://www.annualreviews.org/doi/full/10.1146/annurev-ecolsys-110617-062331&gt;
• Biostim, MycoGold product description <https://biostim.com.au/shop/myco-gold/>