Advice about boosting the health of arbuscular mycorrhizal fungi is probably best started with the list of three "do-not" commandments.
1. Do not till. Tillage breaks up the myriad of hyphae/mycellium, most of which can never reconnect. Once broken, they must start all over, says Wendy Taheri, an expert on arbuscular mycorrhizal fungi. Dramatically shorter mycellium equals dramatically reduced plant food for your crops or forages. One source says it is not uncommon to have 20 meters of fungal root in one cubic centimeter of soil. Calculate that out to just the top inch of soil in an acre and you get a length that would stretch about 80% of the way to the moon; roughly 195,000 miles of food collectors per acre, all interconnected, all sharing stuff.
2. Do not provide much phosphorus. Taheri says too much phosphorus fertility in the soil, especially with crops and farm ground, can be almost as damaging as tillage. The plants latch onto the inorganic phosphorus source when young and never develop much of a trading relationship with the AMF. Remember that phosphorus is so necessary to plant life and so generally unavailable in a natural setting, that plants are programmed to make most of the trade deals with AMF based first on the need for phosphorus. You want the plants to be a bit hungry for phosphorus, Taheri says. For example, 200 ppm of phosphorus in a soil test is usually considered adequate for crop growth. If you want AMF to successfully colonize and trade with your plants, 50-100 ppm is a more AMF-encouraging range for phosphorus fertility.
3. Avoid pesticides as much as possible. Although AMF have a lot of resistance to even such compounds as fungicides, Taheri says most pesticides can be hard on AMF. The more you put on and the more frequently and the more you rotate pesticides, the more you will damage your AMF.
4 good things to do
1. Diversity is key. Manage for diversity above ground to build mycorrhizal and microbial diversity below ground. Although varied species of AMF tend to favor certain species or types of plants, their interactions directly and/or through microbial relationships, will add benefits at a compounding rate.
Plant roots become colonized by many species of AMF. Some are better at taking up water, others may be good defenders against certain pathogens, some relieve other environmental stresses such as soil compaction or excessive salinity. More is better.
Virtually every other trace mineral plants need for good health is provided by AMF, and those nutrients pass up through the food chain. Research has shown that plants associating with AMF have a higher nutrient density than the same plants without AMF.
2. Graze right. Use multi-paddock, adaptive grazing to alternate the natural pulse of grazing and defoliation with fairly lengthy recovery periods. This was a common behavior in the drier environments where large herds of ruminants grazed for hundreds of thousands of years. If done well, such forages can develop deeper, more highly functioning root systems. Also remember that cattle speed up the nutrient cycling by turning forage into urine and manure, which are more available forms of nutrients. Early scientific examination of these practices suggests this kind of controlled grazing will build soil fungal populations and overall productivity. Further, a study in North Dakota showed recovery time between grazing periods increased AMF.
3. Keep the soil covered. Leaving ample plant material on the soil surface at all times keeps it cooler and captures more rainfall. This is good for all soil life, which needs moderate temperature and moisture to function.
4. Develop a long growing season. The longer you can keep things growing in the soil, the more opportunity to feed the microorganisms. Diversity helps a lot in pastureland, and multi-species cover crops do that in crop ground.