Mycorrhizal fungi represent a mutually beneficial partnership between a fungus and the roots of a plant. This symbiotic association is widespread in nature, where the fungus colonizes the root system to extend the plant’s reach into the soil. The plant provides the fungus with essential carbohydrates, which are the products of photosynthesis and a direct energy source. In return, the fungal network increases the surface area available for the absorption of water and mineral nutrients, optimizing the efficiency of nutrient uptake, especially for elements immobile in the soil.
Understanding the Two Primary Categories
Determining the best product for a specific planting requires recognizing the two most common and commercially distinct types of fungi: Arbuscular Mycorrhizae (AM), also known as endomycorrhizae, and Ectomycorrhizae (ECM). These categories differ fundamentally in how they interact with the host plant’s root cells. The structural architecture of the association dictates the primary function and which plants can form the relationship.
Arbuscular Mycorrhizae are characterized by their intimate penetration of the plant’s root cell wall, forming specialized, highly branched structures called arbuscules inside the cortical cells. These structures are the primary sites for nutrient exchange, while other globular structures, called vesicles, serve as storage organs for the fungus. This endosymbiotic relationship is ancient and is primarily tasked with the enhanced delivery of immobile nutrients, most notably phosphorus, as well as nitrogen, zinc, and water. AM fungi associate with most plant species.
Ectomycorrhizae do not penetrate the host cell walls, instead forming a dense, external sheath known as a mantle that completely encases the root tips. From this mantle, a hyphal network called the Hartig net grows into the spaces between the root’s cortical cells, where the nutrient exchange occurs. ECM fungi are known for their ability to mobilize and transfer nitrogen and phosphorus from complex organic matter, a role linked to their prevalence in nutrient-poor forest environments.
Matching Fungi to Specific Plant Groups
The most effective mycorrhizal product contains the appropriate fungal type for the plant species being inoculated. Since AM fungi partner with approximately 80% of all plant species, they are the appropriate choice for almost all vegetables, grains, fruit trees, grasses, and ornamental plants. This compatibility makes them the default selection for general gardening and agriculture.
Ectomycorrhizae, while less common overall, are the exclusive requirement for many woody species. This category includes conifers (such as pine, spruce, and fir) and specific hardwood trees (including oak, beech, birch, and willow). Choosing an AM product for an oak tree, or an ECM product for a tomato plant, will result in failure because the plant and fungus are biologically incapable of forming the necessary symbiotic structures.
Some commercial inoculants are sold as combination products, containing spores from both AM and ECM types. These can be useful for mixed plantings or when the exact plant species’ needs are unknown. However, for a single type of crop, a product focusing on the dominant required type is more targeted and cost-effective for maximizing colonization success.
Optimizing Soil Conditions for Success
The introduction of mycorrhizal fungi through an inoculant is only the first step; their successful establishment depends heavily on the existing soil environment. The single most important factor that inhibits colonization is high levels of available phosphorus in the soil. Since the primary function of the fungi is to scavenge for this immobile nutrient, plants will actively suppress the symbiotic relationship when phosphate is abundant. High-phosphate fertilizers can essentially signal to the plant that the costly partnership is unnecessary, leading to minimal or no root colonization.
Soil acidity and alkalinity also influence fungal activity, with colonization performing best in a pH range of 5.5 to 7.5. Extremes, particularly soil pH below 4.0 or above 8.0, can hinder the growth and survival of the fungi. Furthermore, the application of certain broad-spectrum fungicides can impede the development of the fungal network. Systemic fungicides, which are absorbed by the plant, and non-systemic fungicides applied as a soil drench are particularly detrimental, with compounds like benomyl and carbendazim known to suppress arbuscular mycorrhizal colonization.
Practices that minimize soil disruption tend to favor the fungal network. This is because the extensive hyphal threads that reach far beyond the root system are delicate and easily damaged by aggressive tillage. Low-till or no-till methods preserve the continuity of the fungal network, allowing the symbiosis to establish and persist through successive growing seasons.