In trying to think of a good way to illustrate the good effects that mycorrhizal fungi have on soil, my own experience in a garden should work.
We had just moved to a new home in Camarillo, California, and the back yard that sloped downward had a relatively level area that could be transformed into a terrace about 15 feet wide by putting up a retaining wall of railroad ties. After the wall was built, I excavated the upper part of the terrace and moved that soil toward the wall, ending up with a level area.
Eventually, I would build several raised beds on that terrace and amend the black clay soil with sand and compost, but as it was already getting past planting season I did a rush job of starting various vegetables. (Side Note: Ignore anyone who tries to tell you that sand and clay make concrete. They don't. If you blend in enough clean sand and add some compost, you'll get beautiful loose garden soil to work with for years to come.)
At one end of my garden terrace, I pried open a few holes in the hard clay and planted potato seeds with a dusting of mycorrhizal inoculant - no fertilizer at all. A few weeks later, I was surprised to see that the potato plants were looking very healthy, but didn't really pay too much attention to them.
When they finished flowering, I went to "rob" a few new potatoes for dinner one night and was very surprised to discover that I didn't need a shovel. I could just poke my hand down into the soil near the plants and feel around in the loose soil for the potatoes. I also noted that this nice loose soil extended out in roughly an 18 inch circle from each plant. Beyond that area, the clay was as hard as a board.
I was seeing the effects of the mycorrhizal fungi. After colonizing the potato roots, the fungi sent their thousands of microscopic root-threads (hyphae) out into the surrounding soil to forage for nutrients and moisture, penetrating between the tightly-stacked clay platelets as they extended outward.
As the clay platelets were pried apart, oxygen was able to flow down into the soil and water drained away easily. Other benefical aerobic organisms were then able to multiply and produce nitrogen and solubilize phosphorus, which the mycorrhizal fungi transported back to the potato host plant. I had a huge crop of potatoes that season as the plants grew in the equivalent of great potting soil - always staying moist, but not soggy.
Interestingly, within a week or so after harvesting, the soil where the potato plants had been growing was nearly as hard-packed as the surrounding soil. The "friendly fungi" are obligate life forms and cannot remain active without a host plant. Their survival strategy is generally the same as plants that leave behind seeds for next spring, but these fungi leave tiny dormant spores in the soil. The spores will not become active until they receive a chemical signal from a new growing root nearby.
Other grow tests have shown that the fungi can also perform their magic on poor sandy soil - with the hyphae clumping the sand particles together to form the same sort of potting-type soil as from clay - only working from the opposite direction! This is simply the role of mycorrhizal fungi in nature - to improve soil for their host plants and, working with other microbial agents, to supply their plants with nutrients as needed. Clay gets loosened, sand gets clumped, plants thrive. So very simple.
But all it takes to mess up this elegant soil system is to add "fast-acting plant food" and systemic fungicides to your garden. Think about that the next time you are scraping sticky clay off your shovel.
Cheers and good growing, my friends,
Don Chapman
President, BioOrganics, Inc.