An acre's worth of earthworms can equal the weight of more than 20 hired men.
Earthworms are small, but productive, eat for free and work all night long. Most important, everything they do helps a farmer's crop. Replacing horsepower with worm power, earthworms can loosen up soil more effectively than a tractor pulling a deep ripper, and they never demand an ounce of diesel fuel or a spare part.
The diminutive earthworm could be a farmer's most important ally, in the long-term effort to improve soil tilth, fertility and quality. Understanding how earthworms help a farmer and how to give them a boost, could help the partnership blossom.
Earthworms can be divided into two categories, says soil scientist Eileen Kladivko, Purdue University. Shallow-dwellers-redworms, greyworms, fishworms and other small species--create shallow, random burrows near the soil surface, burrowing about three feet per week. Nightcrawlers dig vertical burrows that can reach five or six feet into the soil or more. Together, field populations of as many as half a million worms per acre can create 250 miles of tunnels each week, adds Dennis Linden, U.S. Department of Agriculture-Agricultural Research Service soil scientist, in St. Paul, MN.
Shallow-dwellers' tunnels create space for air and water in the soil, while nightcrawlers' burrows improve drainage and water infiltration. Though shallow-dwellers' burrows can fill with water during a rainstorm, they tend to drain quickly, notes Linden. "The soil will take a lot of water, but it won't stay long, so it won't get saturated and anaerobic," he says. For an illustration, watch ridge-tiller fields after a rain, Linden suggests: note the speedy drainage in the bottoms of the rows (the relatively undisturbed habitat where the earthworms live) compared to the ridges.
Earthworm burrows reduce the odds of off-farm flow of agricultural inputs, says Linden. Better infiltration means less chance of runoff, which is typically the main source of nutrients, chemicals and soil in surface water, he notes. And water flowing through worm burrows can travel deep into the ground without leaching through the soil, reducing the chance that it will pick up nutrients and other soluble compounds on its way to the water table.
If a major storm event closely follows application of fertilizer or crop protection products, some nitrates can flow deeper into the soil profile through nightcrawler burrows, says Clive Edwards, Ohio State University entomologist. However, he points out that the walls of the burrows are rich in organic matter, which binds crop protection products on their way down. As a result of that binding action, USDA studies show that little or no crop protection product ends up at the bottom of the burrow, Edwards notes.
Crop roots proliferate in nightcrawler burrows, adds Kladivko. Burrows offer more than just an easy path to the subsoil--the castings along their walls are rich in nutrients, and there also is some evidence that their slime coating could contain hormones that stimulate plant growth, she says.
"What comes out of the back end of a worm is up to 10 times more useful than goes in the front," notes Edwards. Worms excrete organic matter rich in phosphorus, potassium, calcium, magnesium, iron and sulfur; they also convert mineralized forms of nitrogen, phosphorus and potassium into more soluble, readily available forms for plant uptake.
As they feed on crop residues, worms distribute nutrients and humus through the soil profile and mix organic and inorganic soil components, says Kladivko. That is why soil with active worm populations tend to exhibit greater moisture holding capacity, better soil structure and more stable pH.
With all of that free--and priceless--labor, it pays to give earthworms a boost on the farm. Here are some pointers from the worm experts.
--Leave plenty of surface residue. Surface residue provides a critical food source for nigthcrawlers. It also insulates the soil from hot and cold extremes, lengthening the active periods for worms' burrowing and reproductive activities, says Kladivko.
--Reduce tillage. Frequent, deep tillage can reduce earthworm populations by as much as 30 to 60%, says Edwards. The problem isn't necessarily physical damage to the earthworms; it is a factor of burying the residue that nourishes them and protects them from temperature extremes. Tillage also destroys earthworm burrows, adds Linden. The worms may starve while trying to excavate their way back to food.
--Fertilize appropriately. Apply enough fertilizer to increase yields. A farmer will provide more food for the earthworms in the form of crop residues, says Kladivko. Manure or sewage sludge--high in partially decayed organic matter, which is rich worm food--directly benefit the worms, as well as the crop, she notes. Anhydrous ammonia can kill worms near the knives, but the increase in crop biomass makes it worthwhile, adds Edwards.
--Choose a soil insecticide carefully. The moist, high-residue environment that favors earthworms also can attract certain insect pests, such as army worms and cutworms, notes Edwards. Add that to the rootworm pressure Midwest corn growers face, and many fields will need a soil insecticide.
But it is important to choose the right soil insecticide. Ptyrethroids, such as Force, are harmless to earthworms. Force also is used at lower rates and quickly binds to the soil, so it presents little risk of contacting earthworms outside of the root zone.
This season, give some thought to the earthworms tilling beneath those fields. It could take a few years for earthworm populations to build up, but just a few changes in management could pay off in improvements that no equipment or hired hand could provide.