By James Hoorman, Hoorman Soil Health Services, and Dale Strichler, Green Cover Seeds
Beneficial soil fungus called mycorrhizae fungi (MF) can optimize crop yields. MF use to be abundant be MF must have a live root as a host. Plowing soil, fallow periods, and annual crops caused many beneficial MF to died off. Long fallow periods, 14-16 weeks; greatly reduce (85-98%) MF population levels while shorter fallow periods, 3-6 weeks; reduce MF populations 30-70%. Some hardy MF species survive in tilled crop land but using cover crops with a live root, can gradually increase MF populations over time (maybe 5-10 years). Inoculating a crop with MF spores speeds up the process and crops respond quickly.
A full rate of MF inoculant, depending on formulation, costs about $12-15/acre. This rate is designed to provide 150,000 propagules (spores and root fragments containing MF) or more per acre. MF research on crops is extensive with over 155,000 published research articles at this time with 10X more research articles on the use of corn (maize MF) than corn using anhydrous ammonia!
About 95% of all plants associate with MF with at least 200 different MF species. Multiple MF species infect each plant, giving different benefits. Ideally, at least 30% of each root segment needs to be infected with various MF species to optimize crop yields. Almost all of our crops, fruits, and vegetables use MF including oats, corn, sorghum, wheat and other small grains, and all other grass crops including forage grasses. Sunflowers, tomatoes, potatoes, sweet potatoes, strawberries, onions, and carrots, are highly dependent on MF. Alfalfa, clovers, soybeans, and all other legumes (except lupine, which is non-MF) including most fruit crops and berries.
Some plants are non-MF, and do not benefit from inoculation. These include Brassicas like radish, turnip, kale, and rape and canola; Pigweed family (amaranth); Chenopod family (weeds like lambsquarters, spinach, sugar beets); and the Polygonacea family, (smartweed, buckwheat). The lack of MF association means these species should be added to cover crop mixes in small amounts. Non-MF species have developed alternate nutrient uptake methods. Brassicas have large root systems while buckwheat uses extensive root exudates to free minerals. Add at least one or several MF hosts in a cover crop mix so the MF do not die off. If only non-MF cover crop species are planted, expect a 5-10 bushel decrease in corn yields until MF recover.
Since the primary benefits of MF are improved uptake of water and nutrients (phosphorus and micro nutrients), most benefits occur when water or fertility are below optimal. Dryland or drought conditions, sandy soils, heavy clay, salty soils, high pH soils with iron chlorosis, fumigated soils, extensively fallowed soils, eroded or highly tilled soils, and soils that have a history of growing non-MF hosts in monoculture (like canola) will have the best response. Some crops also respond more than others. Perennial crops like fruit trees or alfalfa respond very well economically to MF, because one inoculation lasts for years.
MF inoculant can be applied to cover crop seed mixtures or to crops at seeding. Some MF comes as a powder, others as a liquid. On corn seeded with a planter, powder can be blended into a small amount of water then added to liquid starter fertilizer. Unlike Rhizobium bacteria, MF spores are very durable. They can last as long as two years on seed without much loss of viability. They are not bothered by ultraviolet light, drying out, freezing, fertilizer contact, or normal storage temperatures. MF are killed above 140 degrees F.
What about fungicides? Most seed treatment fungicides are either nontoxic to mycorrhizal fungi, or stay adhered to the seed and do not follow the root out away from the seed where the fungicide can contact the MF spores. Foliar fungicides generally cause minimal damage because the fungicide has to translocating from the leaf down to the roots. Nearly all foliar fungicides in common use on field crops are nontoxic to mycorrhizal fungi. The worst fungicides for MF are the soil applied highly toxic fungicides used to sterilize (fumigate) the soil.
Mycorrhizal fungi last as long as the host plant they are attached to remains alive and can live for a couple weeks or after that plant matures or dies. Usually, if a cover crop or double crop is planted immediately after the harvest of another crop, the fungi can be kept alive from one crop to the next. It takes 6 to 9 months for most MF to reproduce so adding a MF cover crop increases MF populations. Mycorrhizae fungi (MF) can be a great yield booster for most farms if managed correctly.