Measuring Nutrient Buildup
With a GPS unit strapped
to his all-terrain vehicle,
agricultural engineer Roger
Eigenberg tows a sled with
an electromagnetic geoconductivity
meter attached to measure
Manure contains nutrients, such as nitrogen, that spur
growth in field crops. But excess nutrients can be harmful if they end
up in surface or subsurface waters. Agricultural
Research Service (ARS) agricultural engineer Roger A. Eigenberg
is pinpointing where the nutrients end up after being placed on a field.
In Clay Center, Nebraska, at the Roman L. Hruska U.S.
Meat Animal Research Center, Eigenberg studies nutrient movement in
several fields by use of electromagnetic induction (EI). He uses various
instruments to do this, including one that looks like an oversized sled,
which is pulled by an all-terrain vehicle or a human when the crop is
Using the EI information, a computer makes a multishaded
map, with light-shaded areas representing high electrical conductivityor
areas of high nutrient concentrationand dark areas low conductivity,
or low nutrient concentration.
Since 1999, Eigenberg has monitored nutrient movement
on a cornfield in Clay Center. The field was divided into sections that
either had or did not have winter cover crops and that were treated
with manure, compost, or commercial fertilizer. He took weekly measurements
to shed light on changing conditions in the top 6 to 8 inches of soil.
"Soil conductivity changes greatly throughout the
growing cycle," Eigenberg says. "From crop emergence to 1
foot tall, there is a gradual increase in soil conductivity and nitrate
content on this cornfield. But after this, there is a rapid decline
of conductivity, indicating that the corn is rapidly taking up the nutrients.
Once the crop is harvested, there is another gradual increase in conductivity."
The cover-crop region shows lower conductivity early until
midway into the growing season when cover and no-cover conductivity
values converge. Fields with cover crops retain some nutrients through
the winter and give them to the main crop in the spring.
From 1996 to 1999, Eigenberg studied a second site that
was used to process feedlot manure into compost in the early 1990s.
The old way of locating where compost rows had once been was to take
many soil samples, but with EI, similar results are faster and easier.
"The maps we make clearly show patterns of high conductivityand
thus high levels of nutrientswhere the rows of manure had been,"
Eigenberg conducted EI and soil tests periodically and
saw that the nutrients leached deeper into the soil every year. He hopes
EI maps will show farmers which areas of their fields may lead to leaching
because of high nitrate concentration. It's an effective management
tool to see where nutrients are and whether they are moving in a field.
It can also be used to help farmers decide where and how much manure
should be applied.By David Elstein, Agricultural
Research Service Information Staff.
This research is part of Manure and Byproduct Utilization,
an ARS National Program (#206) described on the World Wide Web at www.nps.ars.usda.gov.
"Measuring Nutrient Buildup With Electromagnetic Induction" was published in the September 2004 issue of Agricultural Research magazine.