The soil beneath our feet is a living system, housing organisms that influence plant health and growth. Among these, certain fungi form beneficial partnerships with plant roots, creating a symbiotic relationship known as a mycorrhiza. A mycorrhizal inoculant introduces these specific beneficial fungi into the growing environment. Its purpose is to establish or enhance this natural underground network, improving the plant’s ability to thrive.
Defining Mycorrhizal Inoculants
A mycorrhizal inoculant is a biological soil amendment formulated to deliver fungal propagules directly to a plant’s root zone. These products are typically a blend of one or more species of beneficial fungi that naturally form root associations. Inoculants vary in physical form, commonly appearing as a powder, granular material, or liquid suspension.
The active components are fungal structures capable of initiating symbiotic colonization of the plant root. These structures include dormant spores, hyphae fragments, or pieces of already colonized root. The formulation ensures that sufficient propagules are placed directly in contact with young roots or seeds, which is necessary for the fungi to establish the mutualistic relationship.
The Mechanics of the Symbiotic Relationship
The relationship between the plant and the fungus is a mutual exchange, benefiting both organisms. The fungus extends its microscopic filaments (hyphae) far beyond the plant’s root hairs, creating a massive extension of the root system. This vast network, known as the extraradical mycelium, allows the fungi to scavenge water and mineral nutrients from a much larger volume of soil.
Fungi are efficient at absorbing immobile nutrients, primarily phosphorus, but also micronutrients like zinc and copper. The fungus transports these captured resources back to the plant’s root cells. In return for enhanced nutrient and water acquisition, the plant supplies the fungus with carbon compounds (sugars) produced during photosynthesis, which the fungus needs to grow since it cannot photosynthesize.
Major Categories of Mycorrhizal Fungi
Mycorrhizal fungi are classified into two major groups based on how their hyphae interact with root cells: endomycorrhizae and ectomycorrhizae. This distinction is important because different plant species partner with only one type of fungus. Endomycorrhizae, specifically Arbuscular Mycorrhizae (AM), are the most common type, associating with about 85% of all plant species, including most vegetables, grasses, and agricultural crops.
AM fungi penetrate the plant’s root cell walls to form specialized, highly branched structures called arbuscules inside the cortical cells. These arbuscules are the primary sites for nutrient exchange between the fungal hyphae and the plant cytoplasm. Ectomycorrhizae do not penetrate the host cell wall; instead, they form a dense sheath (mantle) around the root tips. From the mantle, a network of hyphae (the Hartig net) grows between the root’s epidermal and cortical cells. Ectomycorrhizae are typically associated with woody plants, such as most species of pine, oak, and birch trees.
Using Mycorrhizal Inoculants
Successful application depends on ensuring direct physical contact between the fungal propagules and the plant’s developing roots.
Application Methods
For seeds, the inoculant powder is often mixed directly with the seed before sowing, or a granular form is banded with the seed in the furrow. For transplants, effective methods involve dusting the powder onto the root ball, dipping the roots in a liquid suspension, or sprinkling granules into the planting hole. When growing in containers, the inoculant can be blended directly into the potting media before planting. For established plants, a liquid suspension can be applied as a soil drench, or a granular form can be worked into the soil around the root zone. The application is generally a one-time treatment for the life of the plant, as the fungus spreads once established.
Environmental Considerations
Environmental conditions significantly affect the fungus’s ability to colonize the roots and thrive. High levels of available phosphorus (above approximately 70 parts per million) can inhibit spores from germinating or colonizing the root. This occurs because the plant senses an abundance of phosphorus and does not signal the fungus to establish the partnership. Maintaining adequate soil moisture and a neutral to slightly acidic pH range also supports fungal establishment and growth, maximizing the benefits of the inoculant.