They are far more valuable than most farmers realize. "What comes out the back end of an earthworm is up to 10 times more useful than what goes in the front."
A pretty strong statement? You bet. But Clive Edwards says it's absolutely true. So do Bill Becker, a Springfield, IL, crop consultant, and several other scientists.
"Earthworms break down organic matter and turn it into soluble forms of phosphorus and potassium, as well as nitrates and ammonium nitrogen that are re adily available to plants," declares Edwards, an Ohio State University entomologist.
Becker gets even more specific. Earthworms, he explains, can add nutrients worth $34 an acre to a field.
Here's how he calculates it. An earthworm population consisting of 25 worms per square foot equals one ton of worms per acre and produces 100 tons of casting (or 2/3" of manure on the surface of each acre).
He adds that earthworm burrows result in the macropore equivalent of 4,000' of 6" tiles per acre, a $4,800 value per acre.
Specifically, worms at that per-acre density add 4 lbs of nitrate nitrogen, 30 lbs of phosphorus, 72 lbs of potash, 90 lbs of magnesium and 500 lbs of calcium to an acre of soil annually. Becker says these nutrients, plus 3/4 ton limestone, have a value of $34.15 per acre.
Whether these figures would hold up in peer-reviewed research may be arguable. But many scientists would agree that most farmers, except perhaps some veteran no-tillers, don't realize how important earthworms are - or can be if the proper crop management is applied.
That's amusing in a way, says Eileen Kladivko, a Purdue University agronomist who has studied earthworms for 15 years.
She quotes from an 1881 book on the topic of worms, by Charles Darwin, the great biologist better-known for his theory of evolution. He wrote: "There are few animals that have played so important a part in the history of the world as these lowly organized creatures."
That respect and knowledge have been largely forgotten with the advent of the moldboard plow and later the chisel plow, notes Kladivko. Earthworms perform a mixing action of soil components, and with the advent of the plow, that function was largely taken over by steel.
"With no-till, earthworms and other soil organisms may be partially replacing the work of the steel that we had with the moldboard plow," Kladivko says.
Earthworms are important for conventionally tilled land, too, insists Edwards. But they become critical for reduced tillage and no-till acres.
There are more than 1,300 species of earthworms, according to Kladivko. But two common groups are the most important to crop growers: shallow-dwelling worms (called red worms, gray worms, fish worms and many other names) and deep burrowers (night crawlers).
Shallow-dwelling worms live primarily in the top 12" of soil and burrow randomly throughout the topsoil.
Night crawlers, on the other hand, build large, vertical, permanent burrows that may extend 5-6' deep, unless destroyed by tillage. The crawlers pull plant residues down their burrows, where the residues soften and are eaten later.
Night crawlers build middens over the mouths of their burrows. Middens, tiny tent-like structures, are a mixture of plant residues and casting (worm feces) and probably serve as protection as well as a food reserve, Kladivko explains.
Shallow-dwelling earthworms are found in most farm fields, but night crawlers may or may not be present, she says. Earthworms generally prefer residue from legumes as a food source over grass residue, hence populations are higher in soybean fields or corn-bean rotations than in continuous corn.
"Earthworms can increase the rate at which water filters into the ground by four times vs. fields that don't have earthworms," says Ed Berry, USDA entomologist in Ames, IA.
Ohio State's Edwards says that these deep, permanent (nightcrawler) burrows can markedly increase internal soil drainage, which can be especially helpful in fields that don't drain well.
The key to boosting earthworm populations is to reduce or eliminate tillage, most experts agree.
"When you do that, you create an effective environment for the earthworm," explains Berry. "Reduced tillage provides a food source - the microorganisms that grow on the crop residues."
Besides that, crop residue insulates the soil, so it freezes more slowly in the fall, giving worms time to get below the frost line. It also keeps the soil cooler and more moist in summer, which favors earthworms.
Berry says a good earthworm population can develop within five years if the environment is suitable.
That earthworm advantage for no-till has been confirmed big time in University of Missouri research. No-till plots have over three times as many worms as plots that were tilled, reports soil scientist John Stecker. Only one spring tillage with a chisel and disk, however, drastically reduces the worm population, Stecker notes.
Edwards agrees with Berry that earthworm populations will multiply if growers create the right conditions. He offers four tips for nurturing and hiking earthworm populations:
1) Maintain a decent organic content in the soil - at least 2%. For borderline fields, consider adding manure or growing crops that leave more plant residue.
2) If you must till - and some soils require tillage - use conservation tillage and avoid traditional moldboard plowing.
"Chisel plowing and light surface cultivators do less harm than deep plowing," Edwards notes. "Deep and frequent tillage will decimate earthworm populations, reducing their numbers by as much as 90%.
3) Choose insecticides carefully and avoid certain toxic products, such as carbamates.
"Almost all carbamates are toxic to earthworms," the scientist emphasizes. As for organophosphates, "Some are (toxic to earthworms), some aren't." On the other hand, he says, synthetic pyrethroids, such as Force, are harmless to earthworms in field use. John Wedberg, University of Wisconsin entomologist, says pyrethroids are safer for earthworms because they are not very water soluble and don't move much in the soil.
Also, Wedberg says, Force is applied at active ingredient rates that are significantly lower than many other insecticides on the market.
4) Avoid or at least minimize the use of triazine herbicides.
"Although most herbicides have little or no effect on the earthworms, triazine herbicides such as atrazine and simazine are slightly toxic to them," Edwards points out.