Maximizing the Benefits of Mycorrhizal Fungi: Understanding the Factors that Affect their Effectiveness

As we always explain to growers, the effectiveness of mycorrhizal fungi application is not always apparent. Soil microbes are living organisms, each with its own physiological and physical characteristics, much like plants, animals, and humans. They are influenced by nutrition, temperature, soil condition, symbiotic plant species, competition, and many other environmental factors. With so many factors affecting the quality and function of soil microbes, it can be challenging for growers to identify what applies to their specific case. However, as industry insiders, we feel obligated to provide a minimum guideline that can assist growers who may not have access to other resources.

[Soil Conditions That Diminish the Effectiveness of Endomycorrhizal Fungi]

The effectiveness of applying endomycorrhizal fungi can be diminished by soil conditions such as:

1. Soil pH: Endomycorrhizal fungi thrive in slightly acidic soils with a pH range of 5.5 to 7.5, while their performance may be limited in alkaline soils with a pH above 7.5. Nonetheless, it is important to keep the soil pH at a level that is optimal for the specific plants or crops you are growing, as this will benefit both the plants and the mycorrhizae.
2. Waterlogged soil: Endomycorrhizal fungi require oxygen to survive, and in waterlogged soils, oxygen is limited. This can lead to a decrease in fungal biomass and a reduction in their ability to form symbiotic relationships with plant roots.
3. Heavy metal pollution: High concentrations of heavy metals in soil can be toxic to endomycorrhizal fungi and can reduce their ability to form symbiotic relationships with plant roots. It's worth noting that some species of endomycorrhizal fungi may be more tolerant of these soil conditions than others
4. High levels of nitrogen and phosphorus: Endomycorrhizal fungi rely on plant roots to supply them with carbon in exchange for nutrients like nitrogen and phosphorus. In soils with high levels of macro nutrients, plants can take up enough nutrients without the help of fungi, reducing the demand for the symbiotic relationship.

Mycorrhizal fungi rely heavily on carbohydrates to fuel their growth and metabolic processes. Their efficient absorption of carbohydrates can be attributed to their high respiration rates and active enzyme systems.

In nutrient-limited soil, adding nutrients can boost mycorrhizal biomass until the plant host is no longer resource-constrained. However, in nutrient-rich soil where plants have sufficient nitrogen and phosphorus, mycorrhizal biomass may decline as plants allocate less carbon belowground, making fungi carbon-limited. Although both plants and mycorrhizal fungi have minimum requirements for nitrogen and phosphorus, plants have a higher total demand for these nutrients.

The availability of plant carbon allocated belowground limits the growth of mycorrhizal fungi. When plants have enough nitrogen and phosphorus, they allocate less photosynthate belowground, leading to a reduction in mycorrhizal growth.

When soil nitrogen and phosphorus levels are very low, both plants and mycorrhizal fungi are nutrient-limited, so adding these resources can increase mycorrhizal growth. Conversely, at very high levels of nitrogen and phosphorus, neither plants nor fungi are limited by these elements, resulting in reduced mycorrhizal biomass as plants allocate more carbon aboveground to shoots and less belowground.

[Understanding the Relationship between Mycorrhizal Fungi and Plants: Importance of Matching Fungi to the Right Plant Species]

More than 80% of plant species have a symbiotic relationship with mycorrhizal fungi. The majority of these plant-fungi partnerships are classified as endomycorrhizal, while the remaining partnerships fall into the categories of ectomycorrhizal, ericoid, or orchid mycorrhizae. A document is available that provides examples of plants that form endo- and ecto-mycorrhizal associations, which may be helpful for further reference: https://extension.okstate.edu/fact-sheets/print-publications/hla/mycorrhizal-fungi-hla-6449.pdf

If mycorrhizal fungi are applied to non-mycorrhizal plants, the fungi may not form a symbiotic relationship with the roots of those plants. Non-mycorrhizal plants lack the necessary structures, such as specialized root cells, to accommodate mycorrhizal fungi. Therefore, the fungi may not be able to colonize the root system of non-mycorrhizal plants, and the plants may not experience any benefits from the fungi. In some cases, applying mycorrhizal fungi to non-mycorrhizal plants can even have negative effects, such as competition for resources or inhibition of root growth. It is important to match the right type of mycorrhizal fungi to the specific plant species to ensure that a beneficial symbiotic relationship can be established.

[Critical Factors Affecting the Effectiveness of Mycorrhizal Products]

There are several critical factors that can affect the effectiveness of a mycorrhizal product, regardless of its form (powder, granular, or liquid). These factors include:

1. Concentration and purity of the product: The concentration and purity of the mycorrhizal product can affect its effectiveness. A higher concentration of mycorrhizal propagules (spores, hyphae, or root fragments) can increase the likelihood of successful colonization and establishment. Additionally, the product should be free of contaminants, such as pathogens or competing fungi, which can interfere with the establishment of the beneficial mycorrhizal fungi.
2. Compatibility with fertilizers and pesticides: Some fertilizers and pesticides can be harmful to mycorrhizal fungi. It is important to choose a mycorrhizal product that is compatible with the fertilizers and pesticides used in the specific agricultural or horticultural system.
3. Application method: The method of application can also affect the effectiveness of a mycorrhizal product. For example, broadcasting a granular product on the soil surface may not result in as effective colonization as applying a liquid or powder product directly to the root zone.
4. Excessive watering in the root zone can lead to the displacement of recently applied mycorrhizal fungal spores or propagules.
5. Know the difference between Spores vs. Propagules: Mycorrhizal fungal spores and propagules are both helpful to plants, but they serve different roles in the soil. Spores are reproductive structures that can remain dormant until the right conditions occur for them to germinate and form new fungal threads that can attach to plant roots. Propagules are actively growing and spreading fungal structures that can quickly colonize plant roots when conditions are favorable. 

   Propagules have better survival rates than spores as they are already acclimatized to the soil conditions and can readily adapt to fluctuations in the environment and plant requirements. Additionally, propagules have higher infectivity rates than spores since they are in direct contact with the host plant and can overcome certain obstacles that spores need to overcome to form a symbiotic relationship.

6. The greater the diversity of soil microbes included in a product, the more favorable the outcome and effectiveness can be expected.

Trichoderma, Bacillus, and other beneficial microbes can help mycorrhizal fungi propagate and prosper by improving soil biodiversity and soil health. These beneficial microbes can work in synergy with mycorrhizal fungi to enhance nutrient uptake, promote plant growth, and suppress plant pathogens.

Trichoderma and Bacillus are examples of soil bacteria that are known to have a positive effect on mycorrhizal fungi. These bacteria produce enzymes that can break down organic matter, releasing nutrients that mycorrhizal fungi can use for growth and development. In addition, these bacteria can help to create a more favorable soil environment for mycorrhizal fungi by suppressing plant pathogens and promoting the growth of other beneficial microbes.

The presence of mycorrhizal fungi and beneficial microbes in the soil can also help to improve soil health in general. Mycorrhizal fungi can form extensive networks of fungal hyphae that can improve soil structure and porosity, increase water-holding capacity, and reduce soil erosion. Beneficial microbes can also help to improve soil fertility by fixing nitrogen, cycling nutrients, and breaking down organic matter.

In general, the use of mycorrhizal fungi and beneficial microbes can help to create a more diverse and resilient soil ecosystem. This can lead to improved plant growth and health, increased crop yields, and a reduction in the need for synthetic fertilizers and pesticides. Additionally, by promoting soil health and biodiversity, the use of these beneficial microbes can contribute to the overall sustainability of agricultural and horticultural systems.

If you haven't had success with the applications of mycorrhizal fungi, you can refer to the checklists provided here to identify potential solutions. If you still need assistance, please contact us at info@mikrobs.com.