Mycorrhizal fungi benefit their host plants in many ways - protecting roots from soil pathogens and diseases, helping build up populations of nutrient-processing bacteria, fluffing up hard-packed clay, as well as performing many other valuable underground functions.
The best-known functions are the seeking out of nutrients and moisture from the soil surrounding a plant, thereby allowing growers to use less fertilizer and irrigation water. This is a complex process and involves the fungi linking together different plant root systems into an underground network - sort of an underground spider web arrangement on a huge scale. A healthy undisturbed soil will have every square inch filled with root-threads (hyphae) of the beneficial fungi.
Through many millions of years of evolution, plants came to rely more and more on mycorrhizal fungi to do the seeking out of nutrients and moisture, to the point where many plants essentially stopped growing fine feeder roots. It is actually the fungi's vast network of interconnected hyphae throughout the soil that provides the surface contact needed for effective foraging. This is why a mycorrhizal plant can uptake a hundred times or more moisture and nutrients - in effect, it gains physical access to far greater soil areas because of the fungi.
A plant that is fortunate enough to take root in a fungi-rich soil has many advantages. Looking at the difference from a physical perspective, plants that lack mycorrhizae have only a small surface area where the roots are in actual contact with soil. Think about rope-like asparagus roots or the few thick roots of a rose - besides the minimal surface contact, these type of roots are not really designed to effectively draw in nutrients and moisture. The soil needs heavy amounts of fertilizer and water, much of it wasted by run-through, to maintain plants with inefficient root systems.
The popular view on plant roots seems to be that they have only two purposes - to anchor plants in soil and to suck in fertilizer and moisture. It would be helpful if growers could see roots more as "main arteries" with mycorrhizal fungi attached as the "foraging capillaries" that provide valuable access to surrounding soil.
A plant with mycorrhizae has no need for the large amounts of fertilizer and water that are being used by most farmers and gardeners. This may have important significance in the future, as growers explore the biological sciences for lower-input methods of producing food crops.
Cheers, and good growing,
President, BioOrganics, Inc.