A 5-year study
points out that the neutron probe, being used here by ARS researcher David
Nielsen, is still the gold standard for soil moisture sensors.
In light of recent
findings about the weaknesses of soil moisture sensors, farmers who have access
to field weather station networks may have the upper hand in knowing when to
turn the water on. In photo, ARS researchers maintain a weather station as part
of the North Plains Evapotranspiration Network at Bushland, Texas. Click the
images for more information about them.
New Soil Moisture Sensors Assessed
By Don Comis
February 23, 2007
Five years of trials around the world have conclusively shown the
strengths and weaknesses of commercial soil moisture sensors used for
Evett, a soil physicist with the Agricultural Research Service (ARS)
and Water Management Research Unit in Bushland, Texas, and colleagues had a
major role in the research, which was sponsored by the
Food and Agriculture Organization of the
United Nations and the International Atomic
The Bushland researchers found that only a field-calibrated neutron
probe gave consistently accurate soil water content data. Use of the neutron
probe is limited to researchers because of cost and regulatory issues related
to the radioactive source used to count water's hydrogen atoms.
The study compared the neutron probe with several commercial soil
moisture sensing systems, including four based on the electromagnetic
properties of soil as influenced by its water content. The study also tested
tensiometers and electrical resistance blocks, including gypsum blocks.
Tensiometers use vacuum pressure to sense soil water potential, which is
related to how difficult it is for plants to take up water from soil.
While most of the devices worked well some of the time, the scientists
found that most also performed poorly in some circumstances. In fact, the
blocks and tensiometers proved to be the only sensors that could consistently
fill the gap for irrigation scheduling while improvements are made in the
Now the researchers have put their results together in a practical
guide for irrigators and researchers, to show them which probes work best under
different circumstances and how to get the most accuracy out of each probe. It
can also be a guide to manufacturers pointing the way to improved sensors that
will be practical. The United Nations will publish the guide.
Evett will present his findings at a
Honolulu on March 19-21, which ARS is co-sponsoring with the
University of Hawaii. Attendees will
include major agricultural sensor manufacturers. The numerous publications
resulting from this work include a paper Evett will present at the
Irrigation Conference set for Feb. 27-28 in Kearney, Neb.
ARS is the U.S. Department of
Agriculture's chief scientific research agency.