I've often written about how mycorrhizal fungi colonize the root zones of plants and forage for nutrients, but let's take a closer look at this relationship.
Some many, many, millions of years ago (for sake of simplicity, I'll just say "lotsa" millions), a connection was made between plants and a particular type of fungi. As so often happens in nature, two completely different organisms discovered that there was value in helping each other survive.
The fungi found that plant roots contained a particularly tasty substance - the result of photosynthesis, which takes place above ground in the leaves of the plant. As plants are able to produce an abundant supply of these photosynthates, the small amount consumed by the fungi was not harmful. In time, the fungi became fully dependent on this root-exude diet. Without a host plant, the fungi died - but left behind spores in the soil that would remain dormant for years if necessary.
The fungi, which might have originally been a parasite to the plant, also spread miles of tiny root-threads throughout the surrounding soil, foraging for nutrients in yet another cooperation, this time with bacteria and decomposing-type fungi. These other soil organisms are able to convert mineral elements and organic matter into forms that plants can uptake. Mycorrhizal fungi, in effect, became the delivery mechanism for these converted elements.
Over more "lotsa" years, the mycorrhizal fungi was able to tune into the needs of their host plants and began foraging for the specific nutrients and moisture required to support the plant. Research has shown that the fungi became particularly adept at foraging for bacteria-produced nitrogen and phosphorus, elements essential to plant health, growth, and fruit production.
After again "lotsa" years, it seems that a form of communication developed between the two parties, most likely a chemical signal of some sort, that sent the fungi "need more-need less" signals from the plant. For example, we know that a moisture-stressed plant causes the fungi to go into a state of emergency activity, sending hyphae out faster and deeper to seek water. After all, if the plant dies, so does the fungi.
After "lotsa" more years, this mutually beneficial relationship was fine-tuned to the point where many plants stopped growing their own fine feeder roots and turned over nearly all underground foraging activities to the fungi, which became more and more adept at plant husbandry. With the fungi's skilled efforts, a plant needed only a few "pipeline" type thick roots.
Then, in the blink of an eye time-wise, man began growing plants with synthetic petro-fertilizers. Seemed like a great idea, except virtually no one realized that high-analysis NPK plant foods destroyed mycorrhizal fungi. Suddenly, plant roots were expected to uptake nutrients by themselves, a job that they were no longer evolutionary suited for.
Some plants, such as grapes, melons, tomatoes, and many types of fruit trees now grew only non-foraging thick roots, so as the mycorrhizal fungi were eliminated, growers had to apply larger and larger doses of NPK to get enough into their plants - so abnormally large that runoff amounts routinely contaminate water supplies. A plant might make actual use of 1% of commonly used liquid fertilizers - the rest runs through to groundwater or steams, or evaporates into the atmosphere.
If leaf analysis shows a lack of some macro element, the usual recommendation is to apply greater amounts of that chemical element to the soil. If a plant shows some purple on the underside of its leaves, then the recommendation is to apply P in larger amounts. The plants that evolved a near-complete dependence on mycorrhizal fungi are called "heavy feeders" and the strategy is to push plant food at them in enormous and wasteful dosages, even side-dressing during the growing season.
Every "soil-chemistry problem" is typically met with a fast-action over dosage response, which only exacerbates the fungi-destroying cause of the problem. Even today with all we have learned about soil biology, this is rarely recognized, but maybe "lotsa" years from now, we'll learn how to use the living organisms in the soil instead of killing them off. Fortunately, even the most depleted, salt-contaminated, compacted soils can be restored to productivity, but it's a shame that soils have to ever reach that level of ruin.
Extra question: The Eastern U.S. had severe problems with a tomato virus last year. I'm curious as to whether our mycorrhizal inoculant customers had the same problem as their neighbors, or were your plants able to survive longer? Lab tests have shown that mycorrhizal plants have greater resistance to many diseases - but how did it work out in real life? If you have any observations on this, please let me know.
Good growing, my friends,