New Year's Greetings to you and may 2010 be filled with success.
It occurs to me that my newsletters have often spoken about the many benefits of mycorrhizal fungi and other soil organisms, but there has not been much mention of the details of how those "plant-friendlies" function.
Let's begin by planting a seed. As a root emerges and begins to extend downward into the soil, it actually starts a complex sequence of actions. If there have not been any living roots in that garden's soil, as during the winter months in northern climates, then the microbial organisms will have either gone dormant or deposited microscopic spores (fungi "seeds") during previous growing seasons. A healthy soil will be filled with countless billions of these sorts of tiny organisms patiently lying in wait for a new generation of plant roots to appear.
Also waiting along with the beneficials are viruses, harmful fungi spores, and other plant pathogens. There is always some competition for seeds and roots - good guys versus bad guys - going on underground. Indeed, if a seed takes too long to sprout, as when soil conditions are too cold, some decomposing types of fungi can enter and make mush of meaty seeds before they have a chance to get going.
A growing root has a lubricant coating that exudes chemical signals. Call it a "smell" for sake of simplicity. This smell lets all the soil critters know that a new growing season has arrived - and the effect is like ringing a dinner bell. Mycorrhizal fungi spores activate, quickly attach to the root, and begin fulfilling their dual role of protection/nourishment.
With mycorrhizae, the new root is able to uptake much greater amounts of nutrients than it could on its own, plus there is now a protective zone surrounding it. This "Mycorrhizosphere" consists of sticky fungi root-threads ("hyphae") that physically block many pathogens from gaining access to the root. There are also anti-viral substances generated by the hyphae.
As the plant roots develop and extend further out into the soil, many other soil organisms are supported. Bacteria populations explode into huge numbers, digesting organic matter and dissolving mineral elements into forms that the mycorrhizal fungi can deliver to their host plants. In return, the plant is kept busy gathering sunlight and sharing the resulting photosynthates (sugars and carbohydrates) through root exudates.
Note that endomycorrhizal fungi actually penetrate into the outer layer of plant root cells, where nutrient exchange takes place. This is a more direct approach than simply feasting on exudates outside the root. Ectomycorrhizal fungi (such as occur on oaks and pines) form more of a sheath around roots, but both types are critically important to the health and vigor of plants.
So a sprouted seed triggers all sorts of soil activity, with the growing tips of the roots being the "dinner bell" mechanism. As the oldest sections of roots nearer the plant age and harden, especially for trees and other perennials, they become more like conduit pipes rather than foraging tools. The real action is always out around the gel-covered new roots.
Plants and their soil associates are far more complicated than most realize. There are many forms of communication that take place, and subtle differences in appearance can prompt insects to destroy "unhealthy" plants, alarms are sounded when an attack occurs, and signals from a stressed plant spur increased efforts from their supportive mycorrhizal fungi. It's an ongoing battle between "the forces of good and evil" as far as plants are concerned. Luckily for us, the beneficials usually prevail due to superior numbers and/or powerful defensive weapons that can be used to protect their life-giving host plants.
All this is good reason to encourage bio-life in soil and to make sure that seedlings and transplant roots have the right kind of friendly fungi spores available very early in their life. Blending mycorrhizal powder into potting soils, or drenching trays of young plants with water-soluble inoculant, or dipping bareroot transplants (especially fruit trees, grapes, or berries) into a clinging spore slurry, or putting spores under garden seeds will ensure the immediate presence of "good guys." The new roots will be thrilled to see them waiting (in a silent plant version of thrilled, of course).
Good growing, my friends,