Mycorrhizal fungi (MF) are symbiotic partners that form associations with a plant’s root system. They act as an extension of the roots to enhance nutrient and water uptake. In exchange for carbohydrates from the plant, this interaction significantly boosts plant health and resilience. It is possible to add mycorrhizal fungi after planting, even if the plant is already established. However, success depends heavily on proper application technique and ensuring the right environmental conditions for the fungi to thrive.
The Requirement for Direct Root Interaction
The success of adding mycorrhizal fungi after planting hinges on a fundamental biological requirement: the fungal inoculum must make physical contact with the plant’s active root tissue. Simply sprinkling the product on the soil surface is ineffective. The fungi are live organisms that colonize roots by penetrating the cell structure or forming a sheath around them. This colonization cannot happen if the fungal spores or hyphae remain in the topsoil layer, far from the root zone.
The fungus must recognize and respond to chemical signals, called strigolactones, exuded by the plant’s roots to initiate the symbiotic relationship. Once a spore germinates near a root, the fungal hyphae grow toward the plant and adhere to the root surface. For this exchange to occur, the plant must be actively growing. The fungal propagules must be delivered directly to the part of the root system that is actively absorbing water and nutrients.
Effective Techniques for Applying Fungi to Established Plants
Since surface application is ineffective, inoculation methods focus on bypassing the topsoil to deliver the inoculum directly into the root zone.
Deep Soil Drenching
This method is effective in porous soils. It involves mixing a water-soluble or fine powdered inoculum into a suspension. The soil around the plant’s base is then soaked, allowing the liquid to carry the spores directly down to the roots.
Targeted Delivery
For larger plants or trees, more targeted methods are necessary to reach the deeper root mass. Soil injection uses specialized equipment to place the liquid fungal suspension several inches below the surface, ensuring placement within the active root zone. Another technique involves drilling small holes around the plant’s drip line, where active roots are typically found. These holes are then filled with a granular inoculum and watered in thoroughly to help the spores migrate closer to the root surfaces.
Environmental Factors Impacting Colonization Success
Even with perfect application, the success of colonization is regulated by the environment surrounding the roots.
A high concentration of available phosphorus (P) in the soil is one of the most significant inhibitors. Plants naturally reduce the signals that initiate colonization when P is readily available. The plant restricts the colonization process to avoid the energy cost of the symbiosis, perceiving it does not need the fungi’s help. To maximize success, avoid applying high-phosphate fertilizers in the weeks leading up to and immediately following inoculation.
Several other factors influence establishment. Fungal spores require water for germination and the subsequent growth of hyphae towards the root. A dry soil environment during the initial weeks of colonization will severely limit the fungi’s ability to establish the partnership. The soil pH also plays a role, as each fungal species has an optimal range for growth. Additionally, the presence of competing soil microbes can influence colonization rates.