To assess soil health, microbiologists Tim Parkin (left) and
Peter Stahl check results of gas chromatographic/mass spectrometric analysis of
gaseous compounds from soil.
The earthy smell of healthy soil is linked to two filamentous
Fertile soil's distinctive earthy odor may someday give farmers and
gardeners something like a simple scratch-and-sniff test for assessing soil
"For years, farmers have relied on the smell of soil as a sign of its
health and vitality. We're developing an odor-based soil test that is a
barometer of organic activitya vital sign of healthy soil," says ARS
microbiologist Peter D. Stahl.
"Our tests show that of the many gases coming from soil, two make up
this earthy odoralthough other gases may contribute. The human nose is so
sensitive to geosmin and 2-methylisoborneol, it can detect these gases in soil
at parts-per-billion concentrations," he says. "That's about a drop
or two in a railroad tanker full of water."
It's the first time the two earthy-smelling gases have been used as
indicators of microbial activity in soil. Now, Stahl and colleagues are working
to find out what in soil is making these tell-tale gases.
He and microbiologist Timothy B. Parkin worked for over 2 years perfecting a
technique to purge, trap, and measure geosmin and 2-methylisoborneol. They
first screened more than 25 gases escaping from soils.
"We captured 15 percent of the geosmin and 24 percent of the
2-methylisoborneol. That may not seem like a lot, but it is the most yet
reported from extracting these gases from soil," Parkin says.
"Our studies indicate that the distinctive earthy odors are a gaseous
byproduct of two groups of soil microorganismsfungi and
actinomycetes," Stahl says.
These filamentous microbes are plentiful in healthy soil, forming up to a
mile or more of filaments in just a cubic inch.
Millions of these microbes per cubic inch of soil emit the earthy odors as
they decompose dead plants and animals and recycle their nutrients. Other soil
organisms produce a variety of additional gases and odors.
He says that besides waste by-products, he is not sure of the exact role of
these gases, although they may have antibiotic or stimulatory effects on other
After measuring the gases, the researchers counted the microbesfungal
hyphae and actinomycete filamentsin both a woodland and cropped soil.
Parkin says, "We found that the virgin woodland soil had higher amounts
of both gases, which agrees with these microbes' greater numbers." Virgin
soil produced over 20 times the gases that cropland did.
According to Parkin, since both gases are by-product of microbes'
metabolism, gas-production levels may be a better indicator of microbial
activity than their numbers or biomass. "Besides nutrient cycling, which
enriches the soil, these microbes keep soil stable and prevent erosion,"
A healthy soil ecosystem relies not only on microbes but also on fellow soil
workers like earthworms, insects, bacteria, and nematodes. They all live
together in soils to break down organic matter, Parkin adds.
"We're also using selective antibiotics to manipulate soil microbial
communities in the laboratory to find out which organisms are producing other
odor-causing compounds," he says. The scientists believe this information
may prove valuable in assessing the activity of other important soil organisms.
-- By Hank Becker, ARS.
Parkin is at the USDA, ARS National Soil Tilth Laboratory, Room 316, 2150
Pammel Dr, Ames, IA, 50011-3120; phone: (515) 294-6888, fax: (515) 294-8125
"Good Earth" was published in the
June 1995 issue of
Agricultural Research magazine.