Along with reduced fertilizer, an important part of biological growing methodology is for nurseries to select a potting mix that will not inhibit soil bio life. Common bark-based potting products may have some fungicidal qualities, which is desirable when one is using a disease-prone chemistry-based approach, but not such a good idea when trying to encourage beneficial fungi on root systems.
In general, peat and sand mixtures work much better than wood-based products. Even pure sand can be used if weight is not an important factor. The sticky hyphae of AM fungi will quickly bind sand particles together into a moisture holding biomass, which will become an excellent environment for helpful bacteria.
Small amounts of gradual-release fertilizer will still be needed, but nothing compared to the continuous direct feeding required with a sterile potting medium. With the right biological activity in its root zone, a plant becomes nearly self-sufficient, making greater use of its own photosynthesis and the symbiotic relationships with nutrient-providing fungi and bacteria.
For nurseries that wish to adopt clean biological methods of growing healthier and more vigorous plants, the first step should be to experiment with various potting mixes. Through observation and microscopic exams, it will not take long to determine which mixture produces the best rate of mycorrhizal colonization.
Of course, the choice of potting mixtures is somewhat restricted by local availability and cost factors. The lucky nursery growers who have good affordable sources of bio-friendly mixes will enjoy an advantage over those with only wood-based options. It should be noted that some wood products may work OK - it will depend on decomposition levels, presence of fungi-inhibiting resins, addition of peat and/or sand, etc.
I expect that bio assays of soil will eventually be regarded as being far more valuable to growers than chemistry tests. Mycorrhizal fungi can fix various soil chemistry problems, shield their host plants from toxins or undesirable pH levels, and regulate the uptake of nutrients to individual plants on an as-needed basis.
For a grower the tricky part is learning how to provide good habitat for these valuable living organisms. Unlike chemicals, the AM fungi, beneficial bacteria, and other biological plant-helpers have specific media requirements.
Onward and upward, friends!
President, BioOrganics, Inc.