Mycorrhizal fungi are beneficial soil organisms that form a symbiotic relationship with most plant roots, extending the root system’s reach to absorb water and nutrients like phosphorus more effectively. This partnership improves plant health, stress tolerance, and vitality. Determining the correct amount to use is often confusing because the fungi come in many different product formulations with widely varying concentrations. This guidance provides practical, quantitative information to ensure the fungi are applied effectively for maximum benefit.
Understanding Product Concentration and Formulations
Product potency is measured by the number of active fungal units, often termed propagules. A propagule is any fungal structure capable of colonizing a plant root. For endomycorrhizal fungi (which associate with about 85% of plant species), propagules include spores, colonized root fragments, and fungal hyphae. Propagule count is preferred over simple spore count because root fragments initiate colonization more quickly.
Ectomycorrhizal fungi (associated primarily with trees like pines and oaks) rely exclusively on spores for their active propagules. Understanding these terms is foundational, as the label’s listed propagule count per gram or pound is the actual measure of product strength and dictates application rate.
The inoculants come in three primary physical forms. Granular or powder formulations are mixed directly into growing media or sprinkled onto a root ball during transplanting. Liquid or water-soluble powders are dissolved to create a drench, allowing application through irrigation systems or as a root dip. Tablets or spikes are compressed granular material intended for deep-soil injection near the roots of established trees or shrubs.
Factors Influencing Application Rates
Application rates depend on soil condition and plant requirements, not just product concentration. Sterilized growing media, such as commercial potting mixes, require a higher rate to establish a functional fungal community. Native soil with existing microbial populations may only require a lower, maintenance-level dose.
Plant species also influence the required rate and fungal type. Most plants benefit from endomycorrhizae, but certain species (like blueberries and azaleas) require specific types, such as Ericoid mycorrhizae. Note that Brassicas (cabbage, kale) are non-mycorrhizal, making inoculation unnecessary.
The application goal also dictates quantity. Site restoration projects in degraded soil require a higher rate than simple landscape maintenance. Applying the inoculant early in the plant’s life cycle, such as at seeding or transplanting, maximizes early colonization and is the most cost-effective approach.
Practical Dosage Guidelines for Common Applications
All quantitative guidelines must be cross-referenced with the specific product label due to concentration variability, but general ratios offer a starting point.
Container and Potted Plants
A standard target is incorporating the inoculant into the growing media at approximately 0.4 pounds per cubic yard of soil. This is roughly equivalent to mixing one pound of granular product into every four cubic feet of potting mix. For smaller batches, use about one teaspoon per gallon of growing media, ensuring thorough mixing for uniform distribution.
Transplanting
The goal is direct contact with the roots, achieved by applying the product into the planting hole or as a root dip. For a one-gallon plant, sprinkle about one-half teaspoon of granular powder directly onto the root ball before planting. Liquid root dips require a high concentration slurry, following manufacturer instructions to coat the roots just before planting.
Established Trees and Field Applications
Application for established trees and shrubs is based on plant size, requiring deep-soil injections or tablets/spikes placed within the root zone. A common guideline is one ounce of granular product for every inch of the tree’s caliper. The caliper is the diameter of the trunk measured at breast height. For field applications or garden beds, broadcast the inoculant across the area and incorporate it into the soil. Rates typically range from one pound per 2,000 to 5,000 square feet, depending on product concentration.
Avoiding Common Application Errors
Successful inoculation requires proper application technique and correct quantity. The fungi must make physical contact with the plant’s growing roots, as propagules only colonize when close to the root tissue. Broadcasting granular products over the soil surface is inefficient and wasteful, especially for established plants, because the fungi are sensitive to sunlight and cannot travel far into the soil.
Moisture management affects viability. Fungi need water to become active, but they also require oxygen, so waterlogged soil is detrimental. Water in the inoculant immediately after application to settle the product around the roots, but avoid excessive saturation.
Chemical interactions can render the application ineffective. High concentrations of soluble phosphorus in fertilizers inhibit colonization, as the plant does not perceive a need for the symbiotic relationship. Certain fungicides, particularly systemic types or soil drenches, are detrimental and should be avoided in the application area. Finally, store the product in a cool, dark place to maintain the viability of the live fungal material.