This paper presents review of the different eight methods for measurement of soil moisture and describes the principle used, methodology, advantages, disadvantages and comparison. Described methods are Gravimetric method, Neutron moderation (NM), Time Domain Reflectometer (TDR), Frequency Domain Reflectometer (FDR), Amplitude Domain Reflectometer (ADR), Phase Transmission (PT), Time Domain Transmission (TDT) and Tensiometer method.

1. PRESENTED BY
Bhushan N. Patil
M.E. Scholar (DE-E&TC)
A REVIEW OF VARIOUS SOIL MOISTURE
MEASUREMENT TECHNIQUES
S.S.B.T’s COET, Bambhori, Jalgaon
GUIDED BY
Prof. P. H. Zope
Prof. K. S. Patil

2. CONTENT
1. INTRODUCTION
2. SOIL MOISTURE MEASUREMENT TECHNIQUES
i. Gravimetric Technique
ii. Neutron Scattering
iii. Time Domain Reflectometer (TDR)
iv. Frequency Domain Reflectometer (FDR)
v. Amplitude Domain Reflectometer (ADR)
vi. Phase Transmission
vii. Time Domain Transmission (TDT)
viii. Tensio-meter Method
3. ADVANTAGES & DISADVANTAGES
4. OPERATING RANGE & ACCURACY
5. MEASUREMENT VOLUME & COST
6. CONCLUSION
REFERENCES

3. 1. INTRODUCTION
NEED OF SOIL MOISTURE MEASUREMENT
 In agriculture & Plant science field to determine best time to Sow & plow the field.
 Various physical & chemical properties of soil changes with amount of moisture
present in soil.
 To measure changes in infiltration, irrigation.
 To study ground water recharge & Evapo-transpiration.
 It is also important in the fields like Hydrology, Forestry, Agrology.
 To study & determine the parameters like soil profile, surface tension related with
civil & soil engineering.

4. 2. SOIL MOISTURE MEASUREMENT TECHNIQUES
I. GRAVIMETRIC TECHNIQUE
It is the most oldest, satisfactory & widely used soil moisture measurement
technique. Used for calibration of other techniques.
Principle- Based on calculations of the weighing of the soil sample before & after
the drying
Methodology- Measurement is carried out in following steps :
1) Collecting soil sample- Using Sampling Augers, Sampling Tubes, Core Barrels
or Open-drive Sampler.
2) Weighing original soil sample
3) Drying of soil- Oven drying or Radiation drying
4) Weighing dried sample of soil
5) Calculating amount of moisture content
% Moisture Measurement = * 100

5. II. NEUTRON SCATTERING
Principle- High energy is lost in Neutron collision with atoms having low atomic
weight like hydrogen atoms. Water content these atoms on very large scale.
Methodology-
1) Uses a long & narrow cylindrical probe containing source & detector for
neutrons.
2) Probe is then inserted in soil & source is then activated
3) The effect of collisions changes a fast neutron to a slow neutron.
4) The number of slow neutrons detected by a counter tube electronically
5) Measuring Gauge is calibrated in terms of moisture content
.
Fig-1: Schematic of Neutron Scattering [18]

6. III. TIME DOMAIN REFLECTOMETER (TDR)
Principle- Dielectric constant of soil is the function of content of moisture present
in soil. Traveling time of a EM wave changes as velocity of traveling wave is
affected by the dielectric constant of soil.
Ka = (c/v)2 = [(c × t)/(2 × L)]2
Methodology-
1) An open ended sensor transmission waveguide is designed & inserted in soil.
2) A pulse of EM is transmitted through it. A part of signal reflected back which
then observed using Sampling Oscilloscope.
3) Using Time delay between transmitted & reflected signal soil moisture
measurement is carried out.
Fig-2: Operation of TDR measurement [1] Fig-3: TDR system [10]

7. IV. FREQUENCY DOMAIN REFLECTOMETER (FDR)
Principle- The electrical capacitance of a capacitor that uses the soil as a dielectric
depends on the soil water content. When connecting this capacitor (made of
metal plates or rods imbedded in the soil) together with an oscillator to form an
electronic circuit, changes in the circuit operating frequency detects changes in
soil moisture.
Methodology-
1) Probes are designed using two or more electrodes (i.e., plates, rods, or metal
rings around a cylinder) inserted into the soil.
2) An electrical field is given then the soil around the electrodes serves as the
dielectric of the capacitor that completing the oscillating circuit.
3) By varying the frequency soil moisture is then measured
Fig-4: Capacitance probes and FDR [10]

8. V. AMPLITUDE DOMAIN REFLECTOMETRY (ADR)
Principle- When an EM wave traveling along a TL reaches a section, part of the
energy reflected back due to impedance mismatch. This reflected wave
interacts with the original incident wave producing a voltage standing wave.
Impedance of soil depends on the content of moisture that changes wave
amplitude along the length of the TL.
Methodology-
1) Sinusoidal EM wave is generated & applied to a coaxial TL that extends into
the soil through an array of parallel metal rods.
2) It’s outer cover forms an electrical shield across the central rod. Rod assembly
acts as an additional section of the TL having impedance depending on the
dielectric constant of the soil between the rods.
3) Depending on the amplitude change in reflected signal moisture content is
determined
Fig-5: ADR sensor probe [10]

9. VI. PHASE TRANSMISSION
Principle- In travelling sinusoidal wave phase is shifted relatively after traveling
fixed distance with respect to initial phase at the origin.
This phase shift depends on i) length of travel along the TL
ii) frequency and
iii) the velocity of propagation.
As velocity of propagation is related to soil moisture content, so it can be
determined by this phase shift at fixed frequency and length of travel.
Methodology-
1) Two open concentric metal rings are used to design a waveguide probe
2) Phase is then measured at the beginning and ending of the waveguides.
3) At the end soil moisture content can be devaluated easily depending on the
phase shift . [10]
Fig-6: Phase transmission probe and sensor [10]

10. VII. TIME DOMAIN TRANSMISSION (TDT)
Principle- Measurement of the propagation time over a known distance of the
wave can be used to calculate the dielectric characteristics. To determine water
content in a porous dielectric medium, an EM pulse is guided through the
medium by a transmission line.
Methodology-
1) It is similar to TDR, but requires an electrical connection at the beginning as
well as ending of the TL.
2) The probe has a waveguide design as bent rods of metals which are inserted
into the electronic block on both side.
3) Another way is the sensor consists of a long band (~3 ft), with an electronic
block at both ends.
4) Soil moisture is then easily calculated depending on the travel time of EM wave.
Fig-7: Time domain transmission probe [17]

11. VIII. TENSIOMETER METHOD
Principle- Based on the property of the absorption i.e. Suction force of water for
soil.
water comes into equilibrium with the soil solution through a permeable &
saturated porous material if a sealed water-filled tube is placed with the soil.
Methodology-
1) The permeable ceramic cup tip is placed with the soil in the root zone of plant.
2) Water moves from the tube into the soil as the soil is normally not saturated,
So that a partial vacuum(pressure) is made and evaluated by the gauge.
3) Pressure varies with amount of water potential thus gauge indicate the
moisture content in the soil.
Fig-8: Tensiometer example [10]

12. 3. ADVANTAGES & DISADVANTAGES
Parameter (I)
GM
(II)
NM
(III)
TDR
(IV)
FDR
(V)
ADR
(VI)
PT
(VII)
TDT
(VIII)
Tensi.
1) Requirement of Specific
Calibration
N N Y Y Y Y Y N
2) Affected by Soil Salinity & air
gaps
N Y N/Y Y Y Y Y N
3) Measurement at different depths Y Y N Y N N N N
4) Connection with Data Logger N N Y Y Y Y Y N
5) Time efficient N N Y Y Y Y Y N
6) Permanent Installation N Y Y Y Y Y Y N
7) Safety Y N Y Y Y Y Y Y
8) Automation N N Y Y Y Y Y N

13. 4. OPRATING RANGE AND ACCURACY
Technique Operating Range
(ft3 per ft3)
Accuracy
(ft3 per ft3)
Neutron Moderation 0 to 0.6 ± 0.005
TDR 0.05 to saturation ± 0.01
FDR 0 to saturation ± 0.01
ADR 0 to saturation ± 0.01 to 0.05
PT 0.05 to 0.5 ± 0.01
TDT 0 to 0.7 ± 0.05
Tensiometer 0-0.80 bar ±0.01 bar

14. 5. MEASUREMENT VOLUME AND COST
Technique Measurement volume Cost
Neutron Moderation Sphere(radius 6-16
inches)
$10,000-15,000
TDR About 1.2 inches radius
around waveguide
$400-23,000
FDR Sphere(radius 1.6 inches) $100-3,500
ADR Cylinder (radius 1.2
inches)
$500-700
PT Cylinder $200-400
TDT Cylinder (radius 2 inches) $400-1,300
Tensiometer Sphere (Greater than 4
inch radius)
$75-250

15. 6. CONCLUSION
 It is concluded that each technology has some limitations, so it is said that the
ideal method for measurement of soil moisture under field conditions has yet to
be perfected.
 Individual requirements should be indentified sufficiently before adopting a soil
moisture monitoring methodology.
 TDR method can be most superior for soil moisture measurement if it’s cost is
reduce and if limitation over high salinity operation is removed.
FUTURE SCOPE :
Limitation over high salinity operation is removed by use of Polyolefin coated
TDR probes.

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