Group Title: Bulletin
Title: Soil moisture sensors
CITATION DOWNLOADS THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00008529/00001
 Material Information
Title: Soil moisture sensors
Series Title: Bulletin
Physical Description: 12 p. : ; 28 cm.
Language: English
Creator: Zazueta, F. S ( Fedro S )
Xin, Jiannong, 1961-
Florida Cooperative Extension Service
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1994
 Subjects
Subject: Soil moisture -- Measurement   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references.
Statement of Responsibility: Fedro S. Zazueta and Jiannong Xin.
General Note: Title from caption.
General Note: "April 1994."
 Record Information
Bibliographic ID: UF00008529
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6791
ltuf - AKA2280
oclc - 30692174
alephbibnum - 001926319

Downloads
Full Text




UNIVERSITY OF

SFLORIDA

Florida Cooperative Extension Service


Soil Moisture Sensors'


Bulletin 292
April 1994


Fedro S. Zazueta and Jiannong Xin2


INTRODUCTION

This bulletin is a survey and classification of the
general methods for determining soil moisture. The
techniques reviewed here involve the use of
gravimetric, nuclear, electromagnetic, tensiometric,
hygrometric, and remote sensing processes. Other
miscellaneous methods are grouped under the
heading Other Related Papers. Each of the soil
moisture measuring methods is presented by means of
(1) simple description, (2) measured parameter, (3)
estimated response time, (4) disadvantages, (5)
advantages, and (6) related papers.

GRAVIMETRIC TECHNIQUES

1. Description:

The oven-drying technique is probably the most
widely used of all gravimetric methods for measuring
soil moisture and is the standard for the calibration of
all other soil moisture determination techniques. This
method involves removing a soil sample from the field
and determining the mass of water content in relation
to the mass of dry soil. Although the use of this
technique ensures accurate measurements, it also has
a number of disadvantages: laboratory equipment,
sampling tools, and 24 hours of drying time are
required. In addition, it is a destructive test in that it
requires sample removal. This makes it impossible to
measure soil moisture at exactly the same point at a
later date. Eventually, measurements will become


inaccurate because of field variability from one site to
another.

2. Measured Parameter.

Mass water content (percentage of dry vs. wet soil
weight)

3. Response Time: a 24 hours

4. Disadvantages:

Destructive test
Time consuming
Inapplicable to automatic control
Must know dry bulk density and transform
data to volume moisture content

5. Advantages:

Ensures accurate measurements
Not dependent on salinity and soil type
Easy to calculate

6. Related Literature:

Erbach, D.C. 1983. Measurement of soil moisture
and bulk density. ASAE Paper No. 83-1553.

Gardner, W.H. 1986. Water content. In: Methods
of Soil Analysis. Part 1. Physical and
Mineralogical Methods (Klute, A., ed). Agronomy


1. This document is Bulletin 292, a series of the Agricultural Engineering Department, Florida Cooperative Extension Service, Institute of Food
and Agricultural Sciences, University of Florida. Publication date: April 1994. First published: June 1993 as Special Series AGE-27.
2. FS. Zazueta, Professor, Agricultural Engineering Department; Jiannong Xin, Graduate Assistant, Agricultural Engineering Department,
Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research,
educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap,
or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean

utinvl Y OF FLOSIBA L BRARIES






Soil Moisture Sensors


Series No. 9. Am. Soc. Agronomy, 2nd edition,
pp. 493-544.

Mckim, H.L., J.E. Walsh and D.N. Arion. 1980.
Review of techniques for measuring soil moisture
in situ. United States Army Corps of Engineers,
Cold Regions Research and Engineering Lab.,
Special Report 80-31.

Reynolds, S.G. 1970. The gravimetric method of soil
moisture determination part I: a study of
equipment, and methodological problems. J.
Hydrology. Vol. 11, pp. 258-273.

Reynolds, S.G. 1970. The gravimetric method of soil
moisture determination part II: typical required
sample sizes and methods of reducing variability.
J. Hydrology. Vol. 11, pp. 274-287.

Reynolds, S.G. 1970. The gravimetric method of soil
moisture determination part III: an examination
of factors influencing soil moisture variability. J.
Hydrology. Vol. 11, pp. 288-300.

Taylor, SA. 1955. Field determinations of soil
moisture. Ag. Engineering. 26:654-659.

NUCLEAR TECHNIQUES

Neutron Scattering

1. Description:

Neutron scattering is widely used for estimating
volumetric water content. With this method, fast
neutrons emitted from a radioactive source are
thermalized or slowed down by hydrogen atoms in the
soil. Since most hydrogen atoms in the soil are
components of water molecules, the proportion of
thermalized neutrons is related to soil water content.
This method offers the advantage of measuring a
large soil volume, and also the possibility of scanning
at several depths to obtain a profile of moisture
distribution. However, it also has a number of
disadvantages: the high cost of the instrument,
radiation hazard, insensitivity near the soil surface,
insensitivity to small variations in moisture content at
different points within a 30 to 40 cm radius, and
variation in readings due to soil density variations,
which may cause an error rate of up to 15 percent
(Phene, 1988). 1 0 I


: w.-;t


2. Measured Parameter:

Volumetric water content (percentage of volume)

3. Response Time: 1 to 2 min.

4. Disadvantages:

Costly
Dependent on dry bulk density and salinity
Radiation hazard
Must calibrate for different types of soils
Access tubes must be installed and removed
Depth resolution questionable
Measurement partially dependent on physical
and chemical soil properties
Depth probe cannot measure soil water near
soil surface
Subject to electrical drift and failure

5. Advantages:

Nondestructive
Possible to obtain profile of water content in
soil
Water can be measured in any phase
Can be automated for one site to monitor
spatial and temporal soil water
Measurement directly related to soil water
content

6. Related Literature:

Augustin, BJ. and G.H. Snyder. 1984. Moisture
sensor-controlled irrigation for maintaining
bermudagrass turf. Agron. J., 76:848-850.

Bavel, C.H.M., D.R. Nielsen and J.M. Davidson.
1961. Calibration and characteristics of two
neutron moisture probes. Soil Sci. Soc. Am.
Proc., Vol. 25. pp. 329-333.

Gardner, W.H. 1986. Water content. In: Methods of
Soil Analysis. Part 1. Physical and Mineralogical
Methods (Klute, A., ed). Agronomy Series No. 9.
Am. Soc. Agronomy, 2nd edition, pp. 493-544.

Gardner, Wilford and Don Kirkham. 1952.
Determination of soil moisture by neutron
scattering. Soil Sci., Vol. 73, pp. 391-401.


Page 2






Soil Moisture Sensors


Goodspees, MJ. 1981. Neutron moisture meter
theory. Soil Water Assessment by The Neutron
Method. Gsiro, Australia.

Klenke, J.M., A.L. Flint and R.A. Nicholson. 1987.
A collimated neutron probe for soil-moisture
measurements. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 21-28.

Lawless, G.P, NA. MacGillivray and P.R. Nixon.
1963. Soil moisture interface effects upon
readings of neutron moisture probes. Soil Sci.
Soc. Am. Proc., Vol. 27. pp. 502-507.

Mckim, H.L., J.E. Walsh and D.N. Arion. 1980.
Review of techniques for measuring soil moisture
in situ. United States Army Corps of Engineers,
Cold Regions Research and Engineering Lab.,
Special Report 80-31.

Rawls, WJ. and L.E. Asmussen. 1973. Neutron
probe field calibration for soil in the Georgia
Coastal Plain. Soil Sci., 110, pp. 262-265.

Simpson, J.R. and JJ. Meyer. 1987. Water content
measurements comparing a TDR array to neutron
scattering. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 111-114.

Stafford, J.V. 1988. Remote, non-contact and in-situ
measurement of soil moisture content: a review.
J. Agric. Eng. Res. 41:151-172.

Taylor, S.A. 1955. Field determinations of soil
moisture. Ag. Engineering. 26:654-659.

Tollner, E.W. and R.B. Noss. 1988. Neutron probe
vs. tensiometer vs. gypsum blocks for monitoring
soil moisture status. Sensors and Techniques for
Irrigation Management. Center for Irrigation
Technology, California State Univ., Fresno, CA
93740-0018. pp. 95-112.

Tyler, S.W. 1987. Application of neutron moisture
meters in large diameter boreholes. International
Conference on Measurement of Soil and Plant


Water Status. Centennial of Utah State Univ., pp. 41-
44.

Gamma Attenuation

1. Description:

The gamma ray attenuation method is a
radioactive technique that can be used to determine
soil moisture content. This method assumes that the
scattering and absorption of gamma rays are related
to the density of matter in their path and that the
specific gravity of a soil remains relatively constant as
the wet density changes with increases or decreases in
moisture. Changes in wet density are measured by
the gamma transmission technique and the moisture
content is determined from this density change.

2. Measured Parameter: Volumetric water content

3. Response Time: < 1 min.

4. Disadvantages:

Restricted to soil thickness of 1 inch or less,
but with high resolution
Affected by soil bulk density changes
Costly and difficult to use
Large errors possible when used in highly
stratified soils

5. Advantages:

Can determine mean water content with
depth
Can be automated for automatic
measurements and recording
Can measure temporal changes in soil water
Nondestructive measurement

6. Related Literature:

Gardner, W.H., G.S. Campbell and C. Calissendorff.
1972. Systematic and random errors in dual
gamma energy soil bulk density and water content
measurements. Soil Sci. Soc. Am. Proc. 36:393-
398.

Gardner, W.H. 1986. Water content. In: Methods of
Soil Analysis. Part 1. Physical and Mineralogical


Page 3






Soil Moisture Sensors


Methods (Klute, A., ed). Agronomy Series No. 9. Am.
Soc. Agronomy, 2nd edition, pp. 493-544.

Gurr, C.C. 1959. Use of gamma rays in measuring
water content and permeability in unsaturated
columns of soil. Soil Sci. pp. 224-229.

Mckim, H.L., J.E. Walsh and D.N. Arion. 1980.
Review of techniques for measuring soil moisture
in situ. United States Army Corps of Engineers,
Cold Regions Research and Engineering Lab.,
Special Report 80-31.

Nofziger, D.L. 1978. Errors in Gamma-ray
measurements of water content and bulk density
in nonuniform soils. Soil Sci. Soc. Am. Proc.,
Vol. 42. pp. 845-850.

Nuclear Magnetic Resonance

1. Description:

With this technique, water in the soil is subjected
to both a static and an oscillating magnetic field at
right angles to each other. A radio frequency
detection coil, turning capacitor, and electromagnet
coil are used as sensors to measure the spin echo and
free induction decays. Nuclear magnetic resonance
imaging can discriminate between bound and free
water in the soil.

2. Measured Parameter: Volumetric water content

3. Response Time: < 1 min.

4. Disadvantages: Same as for neutron scattering

5. Advantages: Same as for neutron scattering

6. Related Literature:

Anderson, S.H. and CJ. Gantzer. 1987.
Determination of soil water content by X-ray
computed tomography and NMR imaging.
International Conference on Measurement of Soil
and Plant Water Status. Centennial of Utah State
Univ., pp. 239-246.

Paetzold, R.F., A.D. Santos and GA. Matzkanin.
1987. Pulsed nuclear magnetic resonance
instrument for soil-water content measurement:
sensor configurations. Soil Sci. Am. J. 51:287-
290.


Stafford, J.V. 1988. Remote, non-contact and in-situ
measurement of soil moisture content: a review.
J. Ag. Eng. Res. 41:151-172.

Tollner, E.W., J.M. Cheshire, Jr. and B.P. Verma.
1987. X-ray computed tomography and nuclear
magnetic resonance for soil systems.
International Conference on Measurement of Soil
and Plant Water Status. Centennial of Utah State
Univ., pp. 247-254.

ELECTROMAGNETIC TECHNIQUES

Resistive Sensor (General)

1. Description:

Electromagnetic techniques include methods that
depend upon the effect of moisture on the electrical
properties of soil. Soil resistivity depends on
moisture content; hence it can serve as the basis for
a sensor. It is possible either to measure the
resistivity between electrodes in a soil or to measure
the resistivity of a material in equilibrium with the
soil. The difficulty with resistive sensors is that the
absolute value of soil resistivity depends on ion
concentration as well as on moisture concentration.
Therefore, careful calibration is required for these
techniques.

2. Measured Parameter:

Soil water potential aided by electrical resistance
measurements

3. Response Time: Instantaneous

4. Disadvantages:

Calibration not stable with time and affected
by ionic concentration
Cost of equipment to generate signal and
readout system is high but could decrease
with new solid-state technology

5. Advantages:

Theoretically, can provide absolute soil water
content
Can determine water content at any depth
Sensor configuration can vary in size so
sphere of influence or measurement is
adjustable


Page 4







Soil Moisture Sensors


Relatively high level of precision when ionic
concentration of the soil does not change
Can be read by remote methods

Resistive Sensor (Gypsum)

1. Description:

One of the most common methods of estimating
matric potential is with gypsum or porous blocks.
The device consists of a porous block containing two
electrodes connected to a wire lead. The porous
block is made of gypsum or fiberglass. When the
device is buried in the soil, water will move in or out
of the block until the matric potential of the block
and the soil are the same. The electrical conductivity
of the block is then read with an alternating current
bridge. A calibration curve is made to relate
electrical conductivity to the matric potential for any
particular soil. Using a porous electrical resistance
block system offers the advantage of low cost and the
possibility of measuring the same location in the field
throughout the season. The blocks function over the
entire range of soil water availability. The
disadvantage of the porous block system is that each
block has somewhat different characteristics and must
be individually calibrated. The main disadvantage of
the gypsum block is that the calibration changes
gradually with time, limiting the life of the block
(Phene, 1988).

2. Measured Parameter: Soil moisture tension

3. Response Time: 2 to 3 hours

4. Disadvantages:


Each block requires individual calibration
Calibration changes with time
Life of device limited
Provides inaccurate measurements


5. Advantages: Inexpensive

6. Related Literature:

Armstrong, C. Fletcher, J.T. Ligon and M.F. Mcleod.
1987. Automated system for detailed
measurement of soil water potential profiles using
watermark brand sensors. International
Conference on Measurement of Soil and Plant
Water Status. Centennial of Utah State Univ.,
pp. 201-206.


Bloodworth, M.E. and J.B. Page. 1957. Use of
thermistor for the measurement of soil moisture
and temperature. Soil Sci. Soc. Am. Proc., Vol.
21. pp. 11-15.

Bouyoucos, G.J. and AH. Mick. 1948. A
comparison of electric resistance units for making
a continuous measurement of soil moisture under
field conditions. Plant Physiology. pp. 532-543.

Bouyoucos, G.J. and R.L. Cook. 1961. Humidity
sensor: permanent electric hygrometer for
continuous measurement of the relative humidity
of the air. Soil Sci., Vol. 100. pp.63-67.

Carlson, T.N. and J.E. Salem. 1987. Measurement of
soil moisture using gypsum blocks. International
Conference on Measurement of Soil and Plant
Water Status. Centennial of Utah State Univ.,
pp. 193-200.

Cary, J.W. and H.D. Fisher. 1983. Irrigation
decisions simplified with electronics and soil water
sensors. Soil Sci. Soc. Am. J., 47:1219-1223.

Collins, J.E. 1987. Soil moisture regimes of
rangelands: using datapods to record soil
moisture. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 193-200.

Erbach, D.C. 1983. Measurement of soil moisture
and bulk density. ASAE Paper No. 83-1553.

Fowler, W.B. and W. Lopushinsky. 1987. An
economical, digital readout for soil moisture
blocks. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 215-218.

Fowler, W.B. and W. Lopushinsky. 1989. An
economical, digital meter for gypsum soil
moisture blocks. Soil Sci. Am. J. 53:302-305.

Freeland R.S. 1989. Review of soil moisture sensing
using soil electrical conductivity. Trans. of ASAE,
Vol. 32(6):2190-2194.

Freeland, R.S., L.M. Callahan and R.D. Von Bernuth.
1990. Instrumentation for sensing rhizosphere
temperature and moisture levels. Applied
Engineering in Agriculture. 6(1):106-110.


Page 5







Soil Moisture Sensors


Gardner, W.H. 1986. Water content. In: Methods of
Soil Analysis. Part 1. Physical and Mineralogical
Methods (Klute, A., ed). Agronomy Series No. 9.
Am. Soc. Agronomy, 2nd edition, pp. 493-544.

Henson, Jr., W.H., G.M. Turner, M. Collins and OJ.
Yeoman. 1987. Electrical measurement of the
moisture content of Baled Alfalfa Hay. Paper
No. 87-1073, ASAE, St. Joseph, MI 49058.

Mckim, H.L., J.E. Walsh and D.N. Arion. 1980.
Review of techniques for measuring soil moisture
in situ. United States Army Corps of Engineers,
Cold Regions Research and Engineering Lab.,
Special Report 80-31.

Rose, M.A. and J.M. Russo. 1987. Integrated system
for evaluating performance of soil moisture units
in field capacity conditions. International
Conference on Measurement of Soil and Plant
Water Status. Centennial of Utah State Univ.,
pp. 207-214.

Taylor, S.A. 1955. Field determinations of soil
moisture. Agr. Engineering. 26:654-659.

Thomson, SJ. and C.F. Armstrong. 1987.
Calibration of the watermark model 200 soil
moisture sensor. Applied Eng. in Agr. Vol. 3. pp.
186-189.

Tollner, E.W. and R.B. Noss. 1988. Neutron probe
vs. tensiometers vs. gypsum blocks for monitoring
soil moisture status. Sensors and Techniques for
Irrigation Management. Center for Irrigation
Technology, California State Univ., Fresno, CA
93740-0018. pp. 95-112.


Wheeler, PA. and G.L.
Electromagnetic detection
ASAE Paper No. 84-2078.


Duncan. 1984.
of soil moisture.


Capacitive Sensor

1. Description:

Soil moisture content may be determined via its
effect on dielectric constant by measuring the
capacitance between two electrodes implanted in the
soil. Where soil moisture is predominantly in the
form of free water (e.g., in sandy soils), the dielectric
constant is directly proportional to the moisture
content. The probe is normally given a frequency
excitation to permit measurement of the dielectric


constant. The readout from the probe is not linear
with water content and is influenced by soil type and
soil temperature. Therefore, careful calibration is
required and long-term stability of the calibration is
questionable.

2. Measured Parameter:

Volumetric soil water content

3. Response Time: Instantaneous

4. Disadvantages:

Long-term stability questionable
Costly

5. Advantages:

Theoretically, can provide absolute soil water
content
Water content can be determined at any
depth
Sensor configuration can vary in size so
sphere of influence or measurement is
adjustable
Relatively high level of precision when ionic
concentration of soil does not change
Can be read by remote methods

6. Related Literature:

Bell, J.P., TJ. Dean and AJ.B. Baty. 1987. Soil
moisture measurement by an improved
capacitance technique, Part II. Field techniques,
evaluation and calibration. J. of Hydrology.
93:79-90.

Dean, TJ., J.P. Bell and A.J.B. Baty. 1987. Soil
moisture measurement by an improved
capacitance technique, Part I. Sensor design and
performance. J. of Hydrology. 93:67-78.

Gardner, W.H. 1986. Water content. In: Methods
of Soil Analysis. Part 1. Physical and
Mineralogical Methods (Klute, A., ed). Agronomy
Series No. 9. Am. Soc. Agronomy, 2nd edition,
pp. 493-544.

Halbertsma, J., C. Przybyla and A. Jacobs. 1987.
Application and accuracy of a dielectric soil water
content meter. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 11-16.


Page 6







Soil Moisture Sensors


Malicki, M.A., E.C. Campbell and RJ. Hanks. 1987.
Investigation on power factor of the soil electrical
impedance as related to moisture, salinity and
bulk density. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 233-238.

Malicki, M.A. and RJ. Hanks. 1989. Interfacial
contribution to two-electrode soil moisture
sensors reading. Irrig. Sci., 10:41-54.

Mckim, H.L., J.E. Walsh and D.N. Arion. 1980.
Review of techniques for measuring soil moisture
in situ. United States Army Corps of Engineers,
Cold Regions Research and Engineering Lab.,
Special Report 80-31.

Varallyay, G. and K. Rajkal. 1987. Soil moisture
content and moisture potential measuring
techniques in Hungarian soil survey.
International Conference on Measurement of Soil
and Plant Water Status. Centennial of Utah State
Univ., pp. 183-184.

Time-Domain Reflectometer (TDR)

1. Description:

Time-domain reflectometer (TDR)
determinations involve measuring the propagation of
electromagnetic (EM) waves or signals. Propagation
constants for EM waves in soil, such as velocity and
attenuation, depend on soil properties, especially
water content and electrical conductivity. The
propagation of electrical signals in soil is influenced
by soil water content and electrical conductivity. The
dielectric constant, measured by TDR, provides a
good measurement of this soil water content. This
water content determination is essentially independent
of soil texture, temperature, and salt content.

2. Measured Parameter:

Volumetric water content aided by propagation of
electromagnetic wave measurements.

3. Response Time: = 28 sec.

4. Disadvantages: Costly

5. Advantages:

Independent of soil texture, temperature, and
salt content


Possible to perform long-term in situ
measurements
Can be automated

6. Related Literature:

Baker, J.M. and R.R. Allmaras. 1990. System for
automating and multiplexing soil moisture
measurement by time-domain reflectometry. J.
Soil Sci. Soc. Am., 54(1):1-6.

Dalton, F.N. 1987. Measurement of soil water
content and electrical conductivity using time-
domain reflectometry. International Conference
on Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 95-98.

Dasberg, S. and F.N. Dalton. 1985. Time domain
reflectometry field measurements of soil water
content and electrical conductivity. Soil Sci. Soc.
Am. J., 49:293-297.

Dasberg, S. and A. Nadler. 1987. Field sampling of
soil water content and electrical conductivity with
time domain reflectometry. International
Conference on Measurement of Soil and Plant
Water Status. Centennial of Utah State Univ.,
pp. 99-102.

Drungil, C.E.C., K. Abt and TJ. Gish. 1989. Soil
moisture determination in gravelly soils with time
domain reflectometry. Transaction of ASAE,
Vol. 32(1), pp. 177-180.

Heimovaara, TJ. and W. Bouten. 1990. A
computer-controlled 36-channel time domain
reflectometry system for monitoring soil water
contents. Water Resource Research, Vol. 26, pp.
2311-2316.

Herkelrath, W.N., S.P. Hamburg and Fred Murry.
1991. Automatic, real-time monitoring of soil
moisture in a remote field area with time domain
reflectometry. Water Resour. Res., Vol. 27, pp.
857-864.

Reeves, T.L. and S.M. Elgezawi. 1992. Time domain
reflectometry for measuring volumetric water
content in processed oil shale waste. Water
Resource Research, 28:769-776.

Simpson, J.R. and JJ. Meyer. 1987. Water content
measurements comparing a TDR array to neutron
scattering. International Conference on


Page 7







Soil Moisture Sensors


Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 111-114.

Stein, J. and D.L. Kane. 1983. Monitoring the
unfrozen water-content of soil and snow using
time domain reflectometry. Water Resour. Res.,
19:1573-1584.

Topp, G.C. 1980. Electromagnetic determination of
soil water content: measurements in coaxial
transmission lines. Water Resources Research,
16:574-582.

Topp, G.C., J.L. Davis and A.P. Annan. 1982.
Electromagnetic determination of soil water
content using TDR: I. Application to wetting
fronts and steep gradients. Soil Sci. Am. J., Vol.
46, pp. 672-677.

Topp, G.C. and J.L. Davis. 1985. Measurement of
soil water content using time-domain
reflectometry (TDR): A field evaluation. Soil Sci.
Soc. Am. J., 49:19-24.

Topp, G.C. 1987. The application of time-domain
reflectometry (TDR) to soil water content
measurement. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 85-94.

TENSIOMETRIC TECHNIQUES

1. Description:

Theprimary method for measuring matric
potential (capillaric tension in soil involves the use of
the tensiometer, which Idirectly- measures matric
-otentia." Tefisiometers are commercially available
from several different sources and in numerous
configurations. The main disadvantage of the
tensiometer is that it functions only from zero to
about -0.8 bar, which represents a small part of the
entire range of available water. The lower moisture
limit for the good growth of most crops is beyond the
tensiometer range. It is apparent, therefore, that the
use of the tensiometer to schedule irrigation can
cause overirrigation, unless tensiometer readings are
combined with information on soil water content
(Phene, 1988).

2. Measured Parameter:

/Soil water potential (capillary potential)


3. Response Time: 2 to 3 hours

4. Disadvantages:

Limit range of 0 to -0.8 bar not adequate for
sandy soil
Difficult to translate data to volume water
content
Hystersis
Requires regular (weekly or daily)
maintenance, depending on range of
measurements
Subject to breakage during installation and
cultural practices
Automated systems costly and not
electronically stable
Disturbs soil above measurement point and
can allow infiltration of irrigation water or
rainfall along its stem

5. Advantages:

Recommendation for irrigation policy
developed with the aid of tensiometers
Inexpensive and easily constructed
Works well in the saturated range
Easy to install and maintain
Operates for long periods if properly
maintained
Can be adapted to automatic measurement
with pressure transducers
Can be operated in frozen soil with ethylene
glycol
Can be used with positive or negative gauge
to read water table elevation and/or soil water
tension

6. Related Literature:

Augustin, BJ. and G.H. Snyder. 1984. Moisture
sensor-controlled irrigation for maintaining
bermudagrass turf. Agron. J., 76:848-850.

Cassell, D.K. and A. Klute. 1986. Water potential:
tensiometry, Methods of Soil Analysis, Part 1.
Physical and Mineralogical Methods (Klute, A.,
ed.). 2nd edition, Madison, Wisconsin.

Erbach, D.C. 1983. Measurement of soil moisture
and bulk density. ASAE Paper No. 83-1553.

Lowery, B., B.C. Datiri and BJ. Andraski. 1986. An
electrical readout system for tensiometer. Soil
Sci. Soc. Am. J. 50:494-496.


Page 8






Soil Moisture Sensors


Marvil. J.D., A.L. Flint and WJ. Davies. 1987.
Tensiometer-transducer system: calibration and
testing. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 151-155.

Mckim, H.L., J.E. Walsh and D.N. Arion. 1980.
Review of techniques for measuring soil moisture
in situ. United States Army Corps of Engineers,
Cold Regions Research and Engineering Lab.,
Special Report 80-31.

Pogue, W.R. and S.G. Pooley. 1988. Tensiometric
management of soil water. Sensors and
Techniques for Irrigation Management. Center
for Irrigation Technology, California State Univ.,
Fresno, CA 93740-0018, pp. 175-180.

Rogers, E.P. 1988. Care and Checking of
Tensiometers. Sensors and Techniques for
Irrigation Management. Center for Irrigation
Technology, California State Univ., Fresno, CA
93740-0018, pp. 111-112.

Snyder, G.H., BJ. Augustin and J.M. Davidson.
1984. Moisture sensor-controlled irrigation for
reducing N leaching in bermudagrass turf. Agron.
J. 76:964-969.

Taylor, S.A. 1955. Field determinations of soil
moisture. Ag. Engineering. 26:654-659.

Tollner, E.W. and R.B. Moss. 1988. Neutron probe
vs. tensiometers vs. gypsum blocks for monitoring
soil moisture status. Sensors and Techniques for
Irrigation Management. Center for Irrigation
Technology, California State Univ., Fresno, CA
93740-0018, pp. 95-112.

Wierenga, PJ., J.L. Fowler and D.D. Davis. 1987.
Use of tensiometer for scheduling drip-irrigated
cotton. International Conference on Measurement
of Soil and Plant Water Status. Centennial of
Utah State Univ., pp. 157-161.

SHYGROMETRIC TECHNIQUES

1. Description:

The relationship between moisture content in
porous materials and the relative humidity (RH) of
the immediate atmosphere is reasonably well known.
Since thermal inertia of a porous medium depends on
moisture content, soil surface temperature can be


used as an indication of moisture content. Electrical
resistance hygrometers utilize chemical salts and acids,
aluminum oxide, electrolysis, thermal principles, and
white hydrosol to measure RH. The measured
resistance of the resistive element is a function of
RH. The main application for this technology seems
to be in materials where RH is directly related to
other properties.

2. Measured Parameter: Soil water potential
3. Response Time: < 3 min.

4. Disadvantages:

Sensing element deteriorates through
interaction with soil components
Each material to be tested requires special
calibration

5. Advantages:

Wide soil matric potential range
Low maintenance
Well suited for automated measurements and
control of irrigation systems

6. Related Literature:

Brown, R.W. and J.C. Chambers. 1987.
Measurements of in situ water potential with
thermocouple psychrometer: a critical evaluation.
International Conference on Measurement of Soil
and Plant Water Status. Centennial of Utah State
Univ., pp. 125-136.

Campbell, G.S. and W.H. Gardner. 1971.
Psychometric measurement of soil water
potential: temperature and bulk density effects.
Soil Sci. Soc. Am. Proc., Vol. 35. pp. 8-11.

Campbell, G.S. 1979. Improved thermocouple
psychrometers for measurement of soil water
potential in a temperature gradient. J. Physics. E:
Sci. Instr., 12:739-743.

Mckim, H.L., J.E. Walsh and D.N. Arion. 1980.
Review of techniques for measuring soil moisture
in situ. United States Army Corps of Engineers,
Cold Regions Research and Engineering Lab.,
Special Report 80-31.

Phene, CJ., GJ. Hoffman and S.L. Rawlins. 1971.
Measuring soil matric potential in situ by sensing
heat dissipation within a porous body I. Theory


Page 9






Soil Moisture Sensors


and sensor construction. Soil Sci. Soc. Am. Proc.,
Vol. 35. pp. 27-33.

Phene, CJ., GJ. Hoffman and R.S. Austin. 1973.
Controlling automated irrigation with soil matric
potential sensor. Trans. of ASAE, Paper No. 71-
230.

Phene, CJ. and T.A. Howell. 1984. Soil sensor
control of high-frequency irrigation systems.
Trans. of ASAE, pp. 392-396.

Rawlins, S.L., W.R. Gardner and F.N. Dalton. 1968.
In situ measurement of soil and plant leaf water
potential. Soil Sci. Soc. Am. Proc., Vol. 32. pp.
468-450.

Rawlins, S.L. and F.N. Dalton. 1967. Psychrometric
measurement of soil water potential without
precise temperature control. Soil Sci. Soc. Am.
Proc., 31:297-301.

Savage, MJ., J.T. Ritchie and I.N. Khuvutlu. 1987.
Soil hygrometers for obtaining water potential.
International Conference on Measurement of Soil
and Plant Water Status. Centennial of Utah State
Univ., pp. 119-124.

Shaw, B.S. and L.D. Baver. 1939. An electrothermal
method for following moisture changes of the soil
in situ. Soil Sci. pp. 78-83.

Wiebe, H.H., R.W. Brown and J. Barker. 1977.
Temperature gradient effects on in situ
hygrometer measurements of water potential.
Agron. J. 69:933-939.

REMOTE SENSING TECHNIQUES

1. Description:

This method includes satellite, radar
(microwaves), and other non-contact techniques. The
remote sensing of soil moisture depends on the
measurement of electromagnetic energy that has been
either reflected or emitted from the soil surface. The
intensity of this radiation with soil moisture may vary
depending on dielectric properties, soil temperature,
or some combination of both. For active radar, the
attenuation of microwave energy may be used to
indicate the moisture content of porous media
because of the effect of moisture content on the
dielectric constant. Thermal infrared wavelengths are
commonly used for this measurement.


2. Measured Parameter:

Soil surface moisture, through the measurement
of electromagnetic energy

3. Response Time: Instantaneous

4. Disadvantages:

System large and complex
Costly
Usually used for surface soil

5. Advantages:

Method allows remote measurements to be
taken
Enables measurements to be taken over a
large area

6. Related Literature:

Blanchard, BJ. and A.T.C. Chang. 1983. Estimation
of soil moisture from Seasat SAR data. Water
Resources Bulletin. Vol. 19, No. 5, pp. 803-810.

Carlson, T.N., F.G. Rose and E.M. Perry. 1984.
Regional-scale estimates of surface moisture
availability from GOES infrared satellite
measurements. Agron. J., 76:972-978.

Estes, J.E., M.R. Mel and J.O. Hooper. 1977.
Measuring soil moisture with an airborne imaging
passive microwave radiometer. Photogrammetric
Eng. and Remote Sensing, 43(10):1273-1281.

Gutwein, BJ., EJ. Monke and D.B. Beasley. 1986.
Remote sensing of soil water content. ASAE
Paper No. 86-2004, St. Joseph, MI 49085-9659.


Jackson, TJ. 1980. Profile
surface measurements.
Drainage. 106(IR2):81-92.


soil moisture from
J. Irrigation and


Jackson, TJ., TJ. Schmugge and P. O'Neill. 1984.
Passive microwave remote sensing of soil
moisture from an aircraft platform. Remote
Sensing of Environment, 14:135-151.

Jackson, TJ., M.E. Hawley and P.E. O'Neill. 1987.
Preplanting soil moisture using passive microwave
sensors. Water Resources Bulletin. Vol. 23
No.l, pp. 11-19.


Page 10






Soil Moisture Sensors


Myhre, B.E. and S.F. Shih. 1990. Using infrared
thermometry to estimate soil water content for a
sandy soil. Trans. ASAE 33(5):1479-1468.

Price, J.C. 1980. The potential of remotely sensed
thermal infrared data to infer surface soil
moisture and evaporation. Water Resources
Research, 16(4):787-795.

Rasmussen, V.P. and R.H. Campbell. 1987. A
simple microwave method for measurement of
soil moisture. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 275-278.

Schmugge, TJ., J.M. Meneely, A. Rango and R. Neff.
1977. Satellite microwave observations of soil
moisture variations. Water Resources Bulletin,
13(2):265-281.

Shih, S.F., D.S. Harrison, A.G. Smajstrla and F.S.
Zazueta. 1990. Infrared thermometry to estimate
soil water content in pasture areas. Soil and
Crop Sci. Soc. of Florida, Proc. 50:158-162.

Wallender, W.W., G.L. Sackman. K. Kone and M.S.
Kaminaka. 1985. Soil moisture measurement by
microwave forward-scattering. Transactions of
the ASAE, Vol. 28(4), pp. 1206-1211.

Wang,. J.R., P.E. O'Neill, TJ. Jackson and E.T.
Engman. 1983. Multifrequency measurements of
the effects of soil moisture, soil texture, and
surface roughness. IEEE Trans. on Geoscience
and Remote Sensing, Vol. GE-21, No. 1, pp. 44-
51.


Wheeler, P.A. and G.L.
Electromagnetic detection
ASAE Paper No. 84-2078.


Duncan. 1984.
of soil moisture.


OPTICAL METHODS


1. Description:

Optical methods rely on changes in the
characteristics of light due to soil characteristics.
These methods involve the use of polarized light,
fibre optic sensors, and near-infrared sensors.
Polarized light is based on the principle that the
presence of moisture at a surface of reflection tends
to cause polarization in the reflected beam. Using


this device, an achromatic light source is directed at
the soil surface. Fibre optic sensors are based on a
section of unclad fibre embedded in the soil. Light
attenuation in the fibre varies with the amount of soil
water in contact with the fibre because of its effect on
the refractive index and thus on the critical angle of
internal reflection. Near-infrared methods depend on
molecular absorption at distinct wavelengths by water
in the surface layers; therefore, they are not
applicable where the moisture distribution is very
nonhomogeneous.

2. Measured Parameter: Soil water content

3. Response Time: Instantaneous

4. Disadvantages and Advantages:

These methods are still in developmental and
experimental stages.

5. Related Literature:

Bowman, G.E., A.W. Hooper and L. Hartshorn.
1985. A prototype infrared reflectance moisture
meter. J. Ag. Eng. Res. 31:67-79.

Kano, Yoshio, W.F. McClure and R.W. Skaggs. 1985.
A near infrared reflectance soil moisture meter.
Transactions of the ASAE, Vol. 28(6), pp. 1852-
1855.

Price, R.R., Ximing Huang and L.D. Gaultney. 1990.
Development of a soil moisture sensor. Paper
No. 90-3555, ASAE, St. Joseph, MI 49058.

Prunty, L. and R.S. Alessi. 1987. Prospects for fiber
optic sensing in soil. International Conference on
Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 261-265.

Stafford, J.V. 1988. Remote, non-contact and in-situ
measurement of soil moisture content: a review.
J. Ag. Eng. Res. 41:151-172.

OTHER RELATED PAPERS


Collins, J.E.
rangelands:
moisture.


1987. Soil moisture regimes of
using datapods to record soil
International Conference on


Page 11






Soil Moisture Sensors


Measurement of Soil and Plant Water Status.
Centennial of Utah State Univ., pp. 193-200.

Gardner, W.R. 1987. Water content: an overview.
International Conference on Measurement of Soil
and Plant Water Status. Centennial of Utah State
Univ., pp. 7-9.

Hutmacher, R.B. 1988. Infrared thermometry for
canopy temperature measurements: applications
and limitation in irrigation scheduling. Sensor
and Techniques for Irrigation Management, Proc.
Center for Irrigation Technology. California
State Univ., Fresno, CA 93740-0018. pp. 19-22.

Merriam, J.L. 1988. Soil moisture deficiency and
fell/appearance technique for irrigation control.
Sensor and Techniques for Irrigation
Management. Center for Irrigation Technology,
California State Univ., Fresno, CA 93740-0018.
pp. 113-116.


Phene, CJ. 1988. Soil water relations. Sensor and
Techniques for Irrigation Management. Center
for Irrigation Technology, California State Univ.,
Fresno, CA 93740-0018. pp. 127-138.

Schmugge, TJ., TJ. Jackson and H.L. McKim. 1980.
Survey of methods for soil moisture
determination. Water Resources Research,
16:961-979.

Stafford, J.V. 1988. Remote, non-contact and in-situ
measurement of soil moisture content: a review.
J. Ag. Eng. Res., 41:151-172.

Wobschall, Darold. 1978. A frequency shift dielectric
soil moisture sensor. IEEE Trans. on Geosci.
Electronics. 16:112-118.


Page 12




University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - - mvs