Questions with Jozsef Racsko

What types of soil microbial inoculants exist?

1. Biofertilizers. This group of inoculants contain living microbes that enhance plant nutrition by either mobilizing or increasing nutrient availability in soils. Various microbial taxa including beneficial bacteria and fungi are currently used as biofertilizers, as they successfully colonize the rhizosphere, rhizoplane or root interior. Plant growth promoting rhizobacteria (PGPR) and mycorrhizal fungi are the most common biofertilizers in the agriculture and horticulture.
2. Biocontrol agents. They protect plants from pests and diseases. These inoculant products are specific to harmful organisms and do not kill beneficial organisms present in the soil. For commercial use, bacterial and fungal biocontrol agents are available for growers.

What mechanisms do mycorrhizal fungi employ to provide benefits to their host plants?

1. Increased physical exploration of the soil. Within the soil, plant roots are limited to a small area for absorption of nutrients; hyphae of arbuscular mycorrhizal fungi grow out beyond the depletion zone, where the plant roots have already removed the available nutrients and water. It has been reported that the root absorptive surface area can increase up to 50 times in mycorrhizal plants compared with non-mycorrhizal plants.
2. Enzymatic activity. A number of studies have demonstrated the ability of arbuscular mycorrhizal fungi to solubilize otherwise insoluble phosphate and nitrogen sources through the production of enzymes.
3. Indirect solubilization of insoluble minerals. Soil organisms, working together with arbuscular mycorrhizal fungi, attracted by the hyphal exudates of the fungi or the remnants of their food include phosphate-solubilizing bacteria that help break chemical bonds and make it easier for the fungi to take up.
Access to storage reserves of absorbed minerals. Hyphal vacuoles and vesicles of mycorrhizal fungi
4. can store larger amounts of absorbed minerals and water than plant roots. These stored nutrients act as reserves and become accessible by the host plant when their supply is limited.

What does “propagule” mean in mycorrhizal inoculant products?

Propagules are structures of mycorrhizal fungi that are capable of forming/re-establishing the symbiotic association with plant roots. They can be mycorrhizal spores and colonized root fragments. They are the “active ingredients” of mycorrhizal inoculant products.

What application methods exist for soil inoculant products?

Mycorrhizal fungal propagules can be applied in various ways. It is very important to provide direct contact between the mycorrhizal propagules and the plant roots. When inoculation fails the most common problem is that the propagules did not get in direct contact with the roots.

1. Seed treatment. Seeds can be coated (e.g., dusted with a powder formulation) with soil microbes before seeding. This way, mycorrhizal propagules are right next to the roots when seeds start to germinate in the soil.
2. Incorporated in the growing medium: Most commonly powders and granular formulations are used for this purpose. Due to the low-medium application rates of the microbes, a good mixing is essential, particularly for plant cell trays where only 1-2 propagules should be distributed in each cell.
3. Drench, applied through irrigation: Popular with mycorrhizal fungi. Propagules can be applied through both drip and sprinkler irrigation systems. For best efficacy, growth medium needs to be pre-wet to facilitate propagule movement.
4. Root dipping: At transplanting, roots can be dipped in powder and granular formulations or in a water solution containing the soil microbial inoculum.

For more: mycorrhizae.com