Soil Secrets Probed
While technician Nancy Nubel checks temperature and humidity,
plant physiologist Tom Kaspar measures the height of oat plants on one of the
large soil columns in the upper level of the NSTL controlled environment
Massive, state-of-the-art rhizotrons help solve the mysteries of
Thanks to futuristic technology, Agricultural Research Service scientists
can now observe plant and soil processes from below ground, as well as above.
The National Soil Tilth Laboratory (NSTL)a new, state-of-the-art
laboratory in Ames, Iowahouses three sophisticated, fully instrumented
and monitored, controlled-environment chambers called rhizotrons.
"Most rhizotrons are outdoor facilities where roots and shoots of
plants growing in soil columns can be observed without disturbing plant
growth," says Jerry Hatfield. He is a plant physiologist and director of
the Ames laboratory.
Hatfield says the major limitation of most facilities is that the soil is
exposed to ambient weather conditions. Thus, root systems being studied are
affected by the temperatures of soil surrounding the facility and of nearby
access areas or walkways.
"Unlike those rhizotrons, the NSTL facility has a controlled
environment with separately controlled chambers for shoots and roots,"
These two-story chambers, the only ones of their kind in the world, can
house various-size soil columns in their lower chamber, while the upper part is
large enough for plants as tall as corn to grow to maturity. Each chamber is
served by an independent computer system that automatically collects all the
data generated during an experiment.
"Rhizotrons are ideal facilities for studying the interaction of plant
roots with soil, as well as soil processes and soil organisms," says ARS
plant physiologist Tom Kaspar. He is beginning a long-term experiment on cover
cropsthe first of its kindusing the new rhizotrons.
"These chambers will give us unique opportunities to make observations
and measurements that would be difficultif not impossiblein the
field," he says. "One of the biggest advantages is that any
experiment can be conducted year round, regardless of the changes in the season
The chambers' controlled environments will make it possible for scientists,
under regulated, repeatable environmental conditions, to compare processes that
occur in soil samples from different geographical and climatic regions. This
unique ability is in keeping with the NSTL's national scope and mission: to
understand soil processes and components as they relate to plant productivity
and water quality.
"We can simulate just about any growing condition experienced by plants
and roots in any soil and environment in the United States," Kaspar says.
And we will be able to study how soil and biological processes, like those of
earthworms and important soil microbes, respond to changing environmental
These chambers are capable of answering some age-old mysteries like, does
soil temperature influence how deep plant roots will grow?
The lower chamber can house cylinders, of soil from 15 to 20 inches in
diameter to monolithic columns 3.3 feet square by 5 feet deep.
For Kaspar's current experiment, the largest, 7,000-pound soil columns are
being used. These are actually pieces of Iowa farmland taken from the Deep
Loess Research Station near Treynor, Iowa, about 150 miles from Ames.
In the lower level of the NSTL rhizotron, Kaspar collects a
water sample for nitrate and herbicide analysis from the bottom of the soil
columns. This allows scientists to estimate the soluble elements lost in
The columns have been kept as undisturbed as possible. To collect the soil,
steel boxes were pushed into the soil, excavated, and then lifted by cranes
onto a truck and transported to the NSTL. There, they were hoisted from the
truck and placed in the lower chambers.
The surface of the soil columns is level with the floor of the upper
chamber. Once columns are in place, the opening between the upper and lower
chamber is sealed and insulated. Except for the surface of the columns, which
is the only exposed area shared by both chambers, the upper and lower parts are
separated and sealed from each other. The only access to the full depth of the
soil column is gained from inside the lower chamber.
In the lower chamber, temperature, water, and drainage can be controlled and
monitored. Chemicals and minerals dissolved in waterapplied to the
surfacecan be traced as they move down the column.
Scientists also measure water balance and uptake, as well as the water given
off by plants and soils (evapotranspiration) and solute movement. And they
measure and monitor root growth at certain depths using a fiber optic scope and
horizontally installed transparent tubes (mini-rhizotrons) at various depths.
Soil gases can be measured by periodic sampling and analysis using a
The rhizotron's upper half resembles a standard controlled-environment plant
chamber. But it is 11-½ feet high, nearly 7-½ feet wide, and about
12-½ feet long. Microprocessors control temperature, moisture, and
relative humidity, allowing scientists to program daily, weekly, and seasonal
"Seasonal changes in light, temperature, and humidity can be simulated
to mirror actual growing conditions," says Kaspar.
Taking It Out for a Spin
Last December, Kaspar, along with ARS micrometeorologist John H. Prueger and
soil scientist Sally D. Logsdon, started a 2- to 3-year experiment looking at
the effects of planting small grains as cover crops after soybeans.
"We wanted to find out how growing oats, a crop that doesn't
overwinter, and rye, which does, affects soil moisture and nitrate leaching
from soil," Kaspar says.
"Both grains are overseeded into standing soybeans. They are relatively
inexpensive, grow well in cool weather, and take up a lot of nitrogen that they
store in their leaves, stems, and roots."
The idea is that the small grains will fill the gap between soybean maturity
and corn planting. This will make the rotation more like a natural prairie
"In nature, some plant is always growing when the weather is warm
enough," says Kaspar. "So some plant is always using the nitrogen and
water in the soil, which reduces the leaching of nitrate into water
The researchers planted soybeans in the rhizotron and simulated May 1
temperatures and day length. Since then, the temperature and day length have
been changed weekly to match normal conditions for Ames from May onward.
The upper chamber will be set to mimic daily 30-year-normal temperatures.
Enough water will be applied to simulate a wet year, so drainage can be
collected from the cover crop treatment. The photoperiod simulated will follow
normal summer, fall, and spring patterns. Lower chambers will have temperature
settings to mimic changes in soil temperatures observed in the field.
During the summer of 1994, three more large columns were taken from the
field. They will be installed in the second chamber so the two chambers can run
simultaneously at different crop stages to repeat and confirm the original
After adjusting temperature to "winter kill" the cover crops, the
researchers will grow corn before starting the crop cycle over again.
"We are using the rhizotron for this experiment," says Kaspar,
"because it allows us to eliminate the effect of weatherwhich varies
so much from year to yearand concentrate on the effect of the cover crop.
-- By Hank Becker, ARS.
C. Kaspar are at the USDA-ARS
Soil Tilth Laboratory, 2150 Pammel Drive Ames, IA 50011; phone (515)
294-5723, fax (515) 294-8125.
"Soil Secrets Probed" was published in the
October 1995 issue
of Agricultural Research magazine.