Geoscience Reference
In-Depth Information
4 m. On the 15th and 25th of May, we measured the soil water content with a
neutron probe. This 10-day period was without rain. When we found a difference in
water content, it was due to the difference in the evaporation rate of water from the
soil, and we simply measured how much water evaporated. If there was a difference
between the two fi elds, it was due to the tillage and no tillage. Research running for
a long time during the growing season to study plant growth variations can easily be
done without repetitious drilling, frequent soil sample removal, and disturbing the
soil and its properties. And with the use of only one access tube, comprehensive
studies of the infi ltration, evaporation, and redistribution of rainwater are feasible
for both uniform and layered soils. Using more access tubes for large areas together
with spatial and temporal statistics, water balance studies can be linked to the causes
of specifi c soil-water-plant relationships without establishing traditionally repli-
cated plots with different levels of arbitrary treatments.
It is worth mentioning that the start of the neutron method was not easy. The
application of neutron scattering effect to measure soil moisture in fi eld soils was
developed in 1950 by John (Jack) F. Stone. He hand made a portable neutron meter
before such moisture meters were commercially available. He did so by following
up on the research of Wilford Gardner under the leadership of Don Kirkham, a pro-
fessor at Iowa State University. Jack's meter was powered by several kinds of dry-
cell batteries. At that time just after Jack had successfully tested his homemade
meter, Kirkham gave a seminar in the Physics Department on the application of the
neutron scattering to measure soil moisture. I (DRN) attended his seminar in the
Physics Department when all of the attending theoretical physicists criticized and
even laughed during the discussion of his presentation. I remember well Kirkham's
calm reaction and especially his fi nal remark at the end of the seminar - “I appreci-
ate your comments that such a method is theoretically impossible, but Jack's fi eld
measurements and those in large soil containers prove that neutron scattering can be
used in a very practical way to measure soil water content.” It was exciting for me
to interact with Jack in 1955-1956 while he tested the new instrument that was soon
commercially produced.
Lastly, but nevertheless essential and necessary, we mention that operators of a
neutron probe must be protected by a shield against the harmful radiation.
Time domain refl ectivity
(
TDR
) is based upon measurements of dielectric permit-
tivity
ʵ
. Permittivity is a characteristic that relates to the ability of a material to
transmit or “permit” an electric fi eld. Although it is sort of similar to electrical
conductivity, it is a measure of the charge separation, not the current, when a voltage
is applied under a specifi c condition. The charge that moves within the material is
bound to the positive charge of the nucleus and to the negative charge of the outside
electrons. Figure
8.3
illustrates a very simple model of what happens to molecules
within a material subjected to an electric fi eld. Since
ʵ
of water is about 80, that of
air is nearly 1, and that of soil particles ranges from 3 to 5 in sands and from 8 to
20 in clays, a measured value of
ʵ
for any soil is closely linked to its water content
provided that other conditions are not changed. This provision is fulfi lled since the
soil air humidity is always high and the composition of the soil solution remains
nearly constant.
