One of the most effective things that can be done in a garden or farm is to use a living mulch. The idea is to seed "nurse" plants, such as crimson clover or vetch, in advance of a main crop such as tomatoes or melons. The seeds of the nurse crop are inoculated with mycorrhizal spores, which are brought out of dormancy by sensing the sprouting roots nearby. (I recommend using our micronized BEIM product, which clings to slightly dampened seeds very well.)
As the mycorrhizal fungi attach to the nurse plant roots and multiply, they send out millions of tiny root threads (hyphae) to explore the surrounding soil for nutrients to bring to their host plants, forming an interconnected web that soon fills every cubic inch of the topsoil. These hyphae separate clay platelets to loosen tight soils and bind together sand particles to form a moisture-retaining biomass - either type of soil is greatly improved.
The beneficial fungi also protect roots from various pathogens, stimulate root development, enhance rooting of cuttings, prevent diseases, and reduce drought stress. All of these benefits are important, but the fungi must be active and have a chance to colonize the entire root zone before the good effects can happen.
This is the function of the nurse plants - to get the soil filled with mycorrhizal fungi (and their accompanying beneficial bacteria) before the main crop goes in. For example, in my garden I scatter crimson clover seeds about six weeks in advance where my tomato plants will go. Then using a scuffle hoe, I scrape off the clover plants in a 12" circle and put a tomato transplant in that small bare area. I let the other surrounding clover continue to grow, as the mycorrhizal fungi will move nutrients between all the plants in their network, including the tomatoes.
The fungi only activate when there are growing roots nearby, so by pre-activating them with the clover there is no lag time before the tomato transplants begin to be protected and nourished. Also, by leaving the "companion-plant" clover growing nearby, I am providing a continuous source of free nutrients as-needed to the tomatoes. Before the clover matures and scatters seeds, I scrape off the tops of those plants, leaving their roots to decay, release many more nutrients, and provide fodder for the now-plentiful earthworms.
This same sort of method can be applied on a larger scale by market-vegetable farms that wish to get maximum value out of their acreage. A small amount of mycorrhizal inoculant on any seeded short-term nurse crops will quickly multiply, and fertilizer inputs can be reduced by half or more if the soil is not damaged by over tillage or high-analysis plant foods.
The fungi are particularly sensitive to heavy doses of P. If chemical fertilizers must be used, use only slow-release types. An 18-6-12 Osmocote type formulation at half the recommended dosage should be sufficient and non-harmful - use nothing stronger. But, dry organic types of fertilizer are much preferred. Occasional light applications of volcanic minerals to ensure the presence of all minor and trace elements is a good idea as well.
Note that no-till or limited-till methods are advised for maintaining top soil health, and that main crops should be planted as quickly as possible after the nurse crop is knocked down - while the mycorrhizal fungi are still fully active.
Pretty simple, really - build up the microscopic little critters to big populations in the soil, then just put your crop plants right in among them.
Good growing, my friends,