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Manage Soil Microorganisms for Healthier Soil

April 27, 2013
By: Darrell Smith, Farm Journal Conservation and Machinery Editor

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Manage soil microorganisms to pave the way to healthier soil and higher yields

You till (or don’t till), manage drainage and apply lime or fertilizer to grow higher yielding crops. But that’s the result, not the motivation. The reason you take those steps is to make your soil healthier by improving the habitat for microorganisms. You want those soil-dwelling creatures to be plentiful and active, decomposing residue and recycling its nutrients into forms your crops can use.

You can actually see a few of those valuable creatures, such as earthworms and night crawlers. But most are invisible to the naked eye and spend their lives beneath the surface of the soil. One estimate says there are 1,800 lb. of earthworms, 16,000 lb. of fungi, 9,000 lb. of bacteria and 5,000 lb. to 5,500 lb. of actinomycetes just in the top 6" of an acre of soil.

"No one knows for sure," says Farm Journal Field Agronomist Ken Ferrie. "We’re learning more and discovering new species all the time. The numbers vary based on the health of the soil."

Healthy soil contains more "critters of the dark," as Ferrie calls them, working to make the soil as productive as possible. Some soil organisms are carnivores, which eat other living organisms. Some are herbivores, which eat live plants. (Most of the herbivores, such as rootworms, are pests.) Fortunately, more than 80% of the underground dwellers are detritivores. They dine on dead plants, breaking down crop residue and recycling nutrients.

To those organisms, crop residue means carbon, which is their food source. You want to increase carbon, which is contained in organic matter. "The more carbon, or food, the more soil organisms," Ferrie summarizes.

Most of the organisms that improve soil health share one other trait: they are aerobic, meaning they breathe oxygen. "Oxygen is available in the upper portion of soil where fence posts rot off," Ferrie says. "The deeper the soil, the more area in which those air-breathing organisms can survive."

There also are lots of anaerobic species, who live in the absence of oxygen, but most of them do not contribute to soil health.

Key players in soil health. The most important microorganisms involved in soil health are fungi, bacteria and actinomycetes.

Fungi are involved in all stages of decay, from simple to complex material. Some (mycorrhizae) are involved in nutrient uptake by root systems.

Your Living Soil

Tillage and water management work together to create an optimal environment for soil microorganisms to grow and work. The ideal combination is 60% water and 40% air in soil macropores.

Viewed through a microscope, fungi look like long threads, with branching chains of cells growing through their food source. They sometimes are visible to the naked eye as white strands or filaments running through a crop residue food source.

"Fungi can handle slightly acid conditions better than other microbes," Ferrie says. "They don’t survive well in hot, dry conditions."

Bacteria are microscopic, single-cell organisms that come in various shapes. A gram of soil is estimated to contain 20,000 species of bacteria.

Several species turn full generations in hours, so populations increase or decrease rapidly. Two species, nitrosomonas and nitrobacter, although small in number, play a big role in the nitrogen cycle.

Many bacteria species move in water in the soil solution, propelled by whiplike tails. "Bacteria need moisture to move and to multiply," Ferrie says. "They go dormant when it’s dry. They are very sensitive to pH, preferring 6.3 to 7.0, with populations dropping off rapidly in acidic conditions."

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FEATURED IN: Farm Journal - Late Spring 2013

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