By Dan Crummett
Progressive Farmer Contributing Editor
A little field diplomacy can go a long way toward maintaining healthy, resilient crops -- and that means taking care of billions of helping hands busy at work in the topsoil of your farm.
Kristine Nichols, chief scientist at the Rodale Institute, said naturally occurring mycorrhizal fungi that live in and around roots supply crops with nutrients and water, and serve as the plant's extended root system. In addition, they secrete soil-building glycoproteins and defend against soil-borne pathogens that otherwise could attack the crop. In exchange for all this, the plant shares 40% to 50% of its sugars and starches through its roots to feed the fungi.
Nichols is a veteran soil microbiologist, formerly with USDA at the Northern Great Plains Research Laboratory. She said mycorrhizal fungi are probably the most critical organisms in crop production because they form supplemental pipelines for water and nutrients mineralized by other bacteria and fungi from the soil.
"The rapid-growing mycelium of these fungi extend well beyond the plant's root system and provide a means to gather water that otherwise would be out of the roots' depletion zone," she explained. "Also, the hair-like structures, or the mycelium, transport mineralized phosphorus, zinc, copper, etc., that wouldn't be available to the plant otherwise."
INTO THE CELLS
Arbuscular mycorrhizal fungi (AMF) penetrate the cell walls of the root and reside there, preventing pathogens from harming the root system. They also function symbiotically with specialized bacteria that fix nitrogen (N) on the roots of legumes -- in the case of natural prairie, forests or in fields managed with leguminous cover-crop species.
Research shows mycorrhizal networks provide significant reductions in the need for applied fertilizer in no-till fields grown with cover crops, Nichols said.
"In undisturbed conditions, these networks can include miles of fungal hyphae in a handful of soil," Nichols explained. "In an annual crop system, the networks won't be as complex, but a large part of them will survive from season to season if conditions are right.
"Anytime the soil is disturbed by a plow or other tillage operation, the networks are disconnected, forcing them to regrow instead of tapping into the soil's moisture and nutrient reserves," she continued. "This is particularly true when a moldboard plow is used to invert the soil. With plowing, many segments of the networks are buried so deeply they cannot reattach to roots. This results in a population decline in beneficial fungi."
DO NOT DISTURB
Nichols said minimizing tillage with no-till, strip-till or occasional rotational tillage is the best way to ensure healthy populations of soil fungi. Also, managing with cover crops allows farmers to make the most of the help the fungi provide.
"With a multispecies cover crop, you have grasses and legumes which live symbiotically with nitrogen-fixing bacteria and all are linked through hyphal networks," she said. "Through this network, the grass [corn, wheat, grain sorghum, etc.] can signal the fungus it needs N and P [phosphorus] from the soil. The fungus then signals a legume that it needs N. The legume says, 'I'll do it because I have excess N, but I need your phosphorus.'
"Fungus and bacteria need carbon provided by the plant, while plants need nitrogen and phosphorus. So everybody wins," she said.
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