Mycorrhizal fungi are beneficial soil fungi that form a symbiotic association with the roots of most plants, essentially extending the plant’s root system and improving its access to water and nutrients. In return, the plant supplies the fungus with carbohydrates (sugars) produced through photosynthesis. This ancient partnership, dating back over 400 million years, is fundamental to the health of most terrestrial ecosystems and is increasingly important in agriculture and gardening.
The Symbiotic Relationship: How It Works
The word “mycorrhiza” comes from Greek, meaning “fungus-root.” The relationship is mutualistic: the fungus colonizes the plant’s roots, either growing between root cells or penetrating them, and then sends its thread-like hyphae out into the surrounding soil. These hyphae are much finer than root hairs and can explore a far greater volume of soil, effectively acting as a secondary root system.
The primary benefit to the plant is improved uptake of nutrients, especially phosphorus, which is often immobile in soil. The fungus also helps with nitrogen, zinc, copper, and other micronutrients. In exchange, the plant transfers 10-20% of its photosynthetically fixed carbon to the fungus. This exchange is not merely passive; both partners actively regulate it. The plant can limit carbon transfer if the fungus is not providing enough nutrients, and the fungus can signal its nutrient status.
Types of Mycorrhizal Fungi
There are two main types of mycorrhizal fungi, distinguished by how they interact with plant roots:
- Arbuscular mycorrhizal fungi (AMF): These fungi penetrate the cortical cells of the root, forming tree-like structures called arbuscules where nutrient exchange occurs. They are the most common type, associating with about 80% of land plants, including grasses, crops, and most horticultural plants. They are obligate symbionts, meaning they cannot complete their life cycle without a plant host.
- Ectomycorrhizal fungi (ECM): These fungi form a dense sheath (the mantle) around the outside of the root and grow between root cells, but do not penetrate them. They are typically associated with trees, especially in temperate and boreal forests, such as pines, oaks, birches, and beeches. Many ECM fungi produce mushrooms, including prized edibles like chanterelles and porcini.
A third, less common type, ericoid mycorrhizal fungi, is found in plants of the Ericaceae family (heathers, blueberries, rhododendrons) and helps them thrive in acidic, nutrient-poor soils.
Benefits for Plants and Soil
The advantages of mycorrhizal associations extend far beyond simple nutrient uptake. They are integral to plant health and ecosystem function.
- Improved nutrient uptake: The fungal hyphae can access phosphorus, nitrogen, and other nutrients from soil volumes far beyond the reach of plant roots.
- Enhanced water absorption: The hyphae also absorb water, helping plants tolerate drought stress.
- Increased resistance to pathogens: A well-colonized root system is less susceptible to soil-borne diseases. The fungus can physically block pathogens and also trigger the plant’s immune system.
- Better soil structure: The fungal hyphae bind soil particles together, creating stable aggregates that improve aeration and water infiltration.
- Heavy metal tolerance: In contaminated soils, mycorrhizal fungi can sequester heavy metals, reducing their uptake by the plant.
In forests, ectomycorrhizal fungi form vast underground networks that connect different trees, allowing them to exchange water, carbon, and nutrients. This has led to the popular term “Wood Wide Web,” though the extent and significance of these networks are still being studied.
How to Choose and Apply Mycorrhizal Inoculants
Mycorrhizal fungi are commercially available as soil amendments for gardens, landscapes, and agriculture. If you decide to use them, here are some key points to consider:
- Choose the right type: For most vegetables, flowers, and grasses, an arbuscular mycorrhizal (AM) inoculant is appropriate. For trees and shrubs, an ectomycorrhizal (ECM) inoculant is needed. Some products contain a mix of both.
- Check for viable spores: Look for products that list the number of propagules (spores, hyphae, or colonized root fragments) per gram. A higher count generally means a more effective product.
- Apply at planting: The best time to inoculate is at planting or seeding, when the roots are young and can be colonized quickly. You can dust the roots with the powder, mix it into the planting hole, or mix it with the seed.
- Avoid chemical fungicides: Many fungicides are toxic to mycorrhizal fungi. Avoid using them at the same time as inoculation.
- Be patient: It can take several weeks for the fungi to establish a visible effect. They are not a quick fix; they are a long-term investment in soil health.
Common Problems and Misconceptions
One common misconception is that mycorrhizal fungi are a fertilizer. They are not. They are a biological aid that helps the plant access nutrients already present in the soil. If your soil is completely devoid of nutrients, the fungi will not help. They work best in moderately fertile soils.
Another issue is that not all plants form mycorrhizae. Many plants in the Brassicaceae family (cabbage, broccoli, kale) and Chenopodiaceae family (spinach, beets) do not rely on these fungi. Inoculating these plants is a waste of money.
Also, excessive soil disturbance, such as heavy tillage, can destroy the fungal network. No-till or reduced-till farming practices help preserve existing mycorrhizal communities.
Cost and Availability
Mycorrhizal inoculants are widely available at garden centers and online. Prices vary depending on the type and quantity. A small container of powder sufficient for a few plants may cost around $10–$15, while larger bags for lawns or farms can be more expensive. The cost is generally low compared to the potential benefits in plant health and reduced fertilizer needs, but it is not a substitute for good soil management.
Safety and Environmental Considerations
Mycorrhizal fungi are native to most soils and are not considered harmful to humans, animals, or the environment. They are a natural part of the soil food web. However, it is important to use products from reputable suppliers to avoid introducing invasive or non-native strains. When handling any soil amendment, it is always wise to wear gloves and wash your hands afterward, but mycorrhizal fungi pose no specific health risk.
Frequently Asked Questions
Do mycorrhizal fungi really work?
Yes, but their effectiveness depends on the context. In soils that are degraded, compacted, or low in organic matter, adding mycorrhizal fungi can significantly improve plant growth. In healthy, undisturbed soils, native mycorrhizal fungi are likely already present and abundant, so adding more may have little effect. They are most beneficial when plants are under stress, such as drought or nutrient deficiency.
Can I use mycorrhizal fungi with all plants?
No. As mentioned, plants in the Brassicaceae (cabbage family) and Chenopodiaceae (beet family) do not form mycorrhizal associations. Also, some plants like rhododendrons and blueberries require ericoid mycorrhizae, which are different from AM or ECM. Always check the plant’s specific needs.
How long does it take for mycorrhizal fungi to colonize?
It can take anywhere from a few weeks to a few months for the fungi to establish a visible association. The speed depends on the plant species, soil conditions, and the viability of the inoculant. You may not see a difference in the first season, but over time, the benefits become apparent.
Are mycorrhizal fungi safe for pets?
Yes, mycorrhizal fungi are not toxic to pets. They are a natural soil organism. However, as with any soil product, it is best to prevent pets from digging in freshly inoculated areas until the product has been watered in.
Do I need to add mycorrhizal fungi every year?
No. Once established, mycorrhizal fungi can persist in the soil for years, as long as there are living plant roots to sustain them. Perennial plants will maintain the association. In annual vegetable gardens, the fungi may decline if the soil is left bare in winter, but they often survive in the soil. Re-inoculation may be beneficial if you rotate to non-mycorrhizal crops or if soil disturbance is high.
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