2. Soil Water measurement and
Measurement of Water
Submitted by:
Jalpa Panchal
Ph.D. (Agri.)
Agronomy
1st Semester
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ANAND AGRICULTURAL UNIVERSITY
ANAND
Submitted to:
Dr. A. S. Bhanvadia
Research Scientist & Nodal Officer
(Seed) and Head
Regional Research Station
AAU, Anand
Course No. AGRON 605 (2 + 1)
Irrigation Management
3. What is Soil Water Content??
• Soil water content is a measurement of the
amount of water in a known amount of soil;
it can be expressed as % water by weight or
volume of soil, or inches of water per foot of
soil.
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4. Why we measure soil water??
It is essential for proper scheduling of irrigation
and estimating the amount of water needed.
It allows the need for irrigation to be quantified in advance of
a crop showing signs of distress. Knowing the soil moisture
status enables highly efficient irrigation, providing the water as
and when required, and eliminating the wasteful use of water
when irrigation is not needed.
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5. Methods of soil water measurement
1. Feel or Appearance of Soil Method
2. Gravimetric Methods
3. Tensiometric Method
4. Electrical Resistance Method
5. Suction Plate Apparatus Method
6. Pressure Plate and Pressure Membrane Apparatus Methods
7. Neutron Scattering Method
8. Immersion Method
9. Dielectric Method
10. Thermal Conductivity Method
11. Colorimetric Method
12. Chemical Methods
13. Penetrometric Method
14. Air Permeability Method 5
6. 6
• The soil sample is squeezed in the hand and its feel and
appearance are taken into consideration.
• This method is easy to use, and many growers schedule
irrigation in this way.
Advantages:
• most rapid method but gives a very rough estimate.
• Useful to farmers who do not know the various methods of
soil water measurement and their attributes or do not have
equipment to find soil water content.
Feel or Appearance of Soil Method
7. • As its name implies, the feel method involves
estimating soil-water by feeling the soil.
Disadvantages:
• The procedure is fairly labor intensive; every time
one must go into the field, take soil cores, and
make determinations of the soil water content.
• Another disadvantage is at times the crop can
have plenty of water deeper in the soil, but if the
top layer is dry, it may be difficult to get a probe
through to check the deeper levels.
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8. This group includes three methods of measurement
namely,
Thermo-gravimetric
Sorption plug
Spirit burning
• This method provide direct measurement of
soil water content, which is expressed in
percent, based on oven dry soil.
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Gravimetric Methods
9. Thermo-gravimetric (Oven dry method)
• It involves drying a soil sample in hot air oven to drive out the
water.
• The loss in weight of the sample on drying is regarded as the
measure of water present.
Procedure:
• With the gravimetric method, soil moisture is determined by
taking a soil sample from the desired soil depth with the help
of auger.
• Then put into a container, weighing it, drying it in an oven at
105°c to a constant weight and then reweighing the dry
sample to determine how much water was lost.
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10. Water content of soil is expressed either on
weight or volume basis.
W2 – W3
Pw = X 100
W3 – W1
(W2 – W1) – (W3 – W1)
Pv = X 100
dW X Vs
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11. Advantages:
Most accurate method
Relatively cheap method
Does not require many equipments (only requires
oven, a balance, a soil auger or core sampler and
aluminium boxes)
Disadvantages:
Very laborious method it involves laborious process of sampling,
weighing and drying in laboratory.
Time consuming method
Requires several soil samples to avoid variability in
obtaining accurate results
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12. Sorption plug method
Measurement of soil water by determining gravimetrically the water
content of porous plugs placed in soil and in equilibrium with soil water.
It is necessary to calibrate the water contents of porous plugs with those of
soils.
Procedure:
• Porous plugs are placed in soil by making holes to desired depths and at
representative locations in the field.
• An intimate contact of the plug with the soil is necessary
• They absorb water from the soil and the water in sorption plugs gradually
comes to equilibrium with the soil water.
• Plugs are then removed and weighed in the balance accompanying the unit.
A sorption pug is secured in a metal tube to help its easy removal from the
soil.
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13. It has not been widely accepted for certain
disadvantages….
Disadvantages:
Necessity of weighing the plug accurately in a sensitive balance
Fragile nature of the balance accompanying the unit
Failure of the sorption plug to absorb water effectively to bring that
in perfect equilibrium with the soil water
Costly and time consuming method of preparing holes into soil for
correctly placing the plug in close soil contact and easy removal of
the same for weighing
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14. Spirit burning method
Procedure:
• Burning a small soil sample (25 g) with spirit (5 ml each time)
in a petridish untill complete evaporation of the present water
• The loss of weight of the sample after burning is taken as the
water content
• The soil sample is well mixed with methylated spirit and then
burnt.
• Burning is done three to five times till a constant weight of soil
is obtained.
• The sample is finally weighed and water content is determined
in the same way as with oven dry method.
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15. Disadvantages:
Main limitation is Organic matter present in the soil sample burns
out and that inflates the actual water content of the soil. It is true
particularly with the soil of high organic matter content. So, it is
necessary to calibrate this method with oven drying method.
Besides, Spirit is quite expensive
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Advantages:
Quite rapid method and useful in areas where equipments for
measuring soil water is not easily available
Values of water content obtained are almost reliable
16. An instrument is widely used for scheduling irrigation by measuring the
soil water in units of negative pressure, also known as tension
A tensiometer is a sealed, airtight, water-filled tube (barrel) with a
porous tip on one end and a vacuum gauge or manometer on the other
How does it works??
The instrument must be installed properly so that the porous tip is in
good contact with the soil. Water from the cup moves out to the soil
which is in a drying cycle till the equilibrium is established between the
water in the porous cup and the water in the soil. This water movement
out of tensiometer creates suction in the water system of tensiometer
and the tension is registered in the vaccume gauge in atmosphere.
The tension gradually increases with gradual decrease in soil water
content in the tensiometer.
When irrigation is applied subsequently, water from the soil enters the
cup till an hydraulic equilibrium is reached. This process lowers the
suction in tensiometer and the vaccum gauge shows zero tension.
Soil-water tension is commonly expressed in units of bars or centibars.
One bar is equal to 100 centibars (cb).
Tensiometers work in the range from 0 to 0.85 bar. The suction scale on
the vacuum gauge of most commercial tensiometers reads from 0 to 100
cb.
It is advisable to take gauge readings in the morning hours.
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Tensiometric Method
18. • Soil water tension indicates when to irrigate, but not how
much water to apply.
• The main limitation of tensiometers is that it is useful for
measuring moisture in sandy soils than that of clay soils,
because of higher matric potentials in the former soils. The
actual range of effective measurement is only from 0 to 0.85
bars.
• (In book) it reads soil water in the available range of 35 to 45
per cent in heavy textured soil and 60 to 80 per cent in sandy
soils.
• Requires regular (weekly or daily) maintenance, depending on
range of measurements
• Subject to breakage during installation and cultural practices
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Limitations
19. Electrical Resistance Method
• To measure indirectly soil water content by
electrical conductivity
• Electrical resistance blocks consist of two
electrodes enclosed in a block of porous
material, as shown in Figure.
• The block is often made of gypsum although
fiberglass or nylon is sometimes used.
• Electrical resistance blocks are often referred
to as gypsum blocks and sometimes just
moisture blocks.
• The electrodes are connected to insulated lead
wires that extend upward to the soil surface.
• Resistance blocks work on the principle that
water conducts electricity. (changes in
electrical conductivity with the variation in
soil moisture). 19
20. • When block is properly installed at desired depths of soil, the water suction
of the porous block is in equilibrium with the soil-water suction of the
surrounding soil. As the soil moisture changes, the water content of the
porous block also changes. The electrical resistance between the two
electrodes increases as the water content of the porous block decreases. The
block's resistance can be related to the water content of the soil by a
calibration curve. Free ends of weir leads remain above ground for taking
readings by Wheatstone bridge.
• Gypsum blocks are sensitive to soil water in the tension range of 1 to 20
atmospheres and are not affected by salt concentrations up to 0.2 percent.
• Life of gypsum block may be five years in well-drained non-saline soils.
• Nylon and fibre glass units have a longer life and work in the soil water
range from saturation to wilting point and in soils with salt concentrations
up to 0.1 percent in the soil water.
• The resistance blocks read low resistance at field capacity and high
resistance at wilting points.
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21. • The water content in the block changes with
corresponding changes in water content in
the soil, and changes within the block are
reflected by changes in resistance between
the electrodes.
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22. Advantages
• Provide an accurate measurement of soil water
• Resistance units can be prepared and installed
in the field easily.
• Resistance measuring meter and the resistance
units are portable and their handling is easy.
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23. Disadvantages
• They are not very sensitive to higher range of
available water and cannot be used in soils that
get waterlogged or are irrigated frequently as
gypsum blocks dissolve slowly in these soils.
• Units are also not durable and are affected by
salts in the soil
• The resistance readings are affected by
temperature variations.
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24. Suction Plate Apparatus Method
• Measure soil water content at low tensions, usually below one
atmosphere.
• It consists of a chamber in which a porous ceramic plate is fitted at
the lower section and a conduit, at the bottom.
• The portion of the chamber below the plate and the conduit is filled
with water.
• A moist soil sample is placed on the porous plate
• When water in the soil sample comes into equilibriuym with water
in the system, the soil water content is determined gravimetrically
• If suction created is more than soil water tension, Water would either
come out of the soil sample
• If soil water tension is higher than the suction created, water get into
the soil sample.
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25. Pressure Plate and Pressure Membrane
Apparatus Methods
• Richards (1949) described the apparatus.
• Pressure membrane and pressure plate apparatus is
generally used to estimate field capacity, permanent
wilting point and moisture content up to two
atmospheric pressure.
• The apparatus consists of an air tight metallic chamber in
which porous ceramic pressure plate is placed. The
pressure plate and soil samples and saturated and are
placed in the metallic chamber. The required pressure of
0.33 or 15 bar is applied through a compressor. The water
from the outlet till equilibrium against applied pressure is
achieved. After that, the soil samples are taken out and
oven- dried for determining the moisture content 25
26. Neutron Scattering Method
• Soil moisture can be estimated quickly and continuously with
neutron moisture meter without disturbing the soil.
• Another advantage is that soil moisture can be estimated from large
volume of soil.
• This meter scans the soil about 15 cm diameters around the neutron
probe in wet soil and 50 cm in dry soil.
• It consists of a probe and a scalar or rate meter. This contains a fast
neutron source which may be a mixture of radium and beryllium or
americium and beryllium. Access tubes are aluminum tubes of 50-
100 cm length and are placed in the field when the moisture has to
be estimated.
• Neutron probe is lowered in to access tube to a desired depth. Fast
neutrons are released from the probe which scatters in to soil. When
the neutrons encounter nuclei of hydrogen atoms of water, their
speed is reduced. The scalar or the rate meter counts of slow
neutrons which are directly proportional to water molecule.
Moisture content of the soil can be known from the calibration curve
with count of slow neutrons
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27. 27
4. Neutron scattering method:
•The most rapid and indirect method for measuring soil water content is probably
that of neutron scattering. In this method number of hydrogen nuclei present per
unit volume of soil is measured.
•Fast moving neutrons emitted from a radioactive source (usually Radium-
Beryllium or Americium-Beryllium) when collide with particles having mass
nearly equal to their own, like hydrogen atom in the soil, release their energy and
are thermalised or slowed down.
•The slowed down neutrons are detected by a detector and recorded on a scaler.
Commonly used detector of slowed down neutron is a tube containing BF3 gas.
•More the neutrons are slowed down, higher will be the water content of the soil.
The zone of influence is generally 15-20 cm around the detector.
28. Advantages
• Quick measurement of soil water in situ over a wide range of
soil water giving values on volume basis.
• Not affected by slight salinity in the soil
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Disadvantages
• Instrument is expensive and delicate
• Its handling is dangerous because of high energy emissions.
• Gives inaccurate results for soil layers near the surface, as the
radiation is lost to the air.
• Variation in soil density and presence of stone, pl;ant roots,
lithium, boron, cadmium and high organic matter lead to
erroneous results.
29. Immersion Method
• Prihar and Sandhu (1967 and 1968) suggested the immersion method
• to measure the soil water content on volume basis by a gauge known
as soil moisture gauge.
• Gauge consists of a volumetric flask with a graduated tubular stopper.
• Quite rapid method
Procedure:
• Take 20 g moist soil into the flask and 100 ml water pored in
instalments driving out the air from the soil
• Increase in total volume is proportional to the amount of soil water
present in the soil sample
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Ww = Dw(DsVi – Wsm)
Ds – Dw
Where, Ww = weight of water
Ds = particle density
Vi = volume increase of water
Wsm= weight of moist soil sample
Dw = density of water
30. Time Domain Reflectometer
• This instrument is used to measure soil water on
volumetric basis up to a depth of 60 cm.
• It can be portable or permanently installed in situ
for periodic soil water measurement.
• Water content can be determined quickly both in the
laboratory as well as in the field.
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31. How does it works?
• Two parallel rods or stiff wires are
inserted into the soil to the depth at
which the average water content is
desired.
• The rods are connected to an
instrument that sends an
electromagnetic pulse (or wave) of
energy along the rods.
• The rate at which the wave of energy is
conducted into the soil and reflected
back to the soil surface is directly
related to the average water content of
the soil.
• One instrument can be used for
hundreds of pairs of rods. This device,
just becoming commercially available,
is easy to use and reliable.
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32. Thermal Conductivity Method
• An electric heating element with temperature measuring
device is inserted into the soil to the desired depth.
• A constant energy is supplied and the rise in temperature
is noted at intervals.
• The electrical energy consumed and the temperature
gradient obtained are used as a measure of soil water.
• Soils, when dry, act as a heat insulating medium and
consequently a considerable rise in temperature occurs on
application of heat which is not dissipated easily
• When soils are wet and rise of temperature is slow, heat
gets quickly dissipated from the source.
• This temperature is then calibrated to find out the soil
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33. • Method is not popular because of laborious
calibration with the instrument and variations
of temperature due to differences in soil at
different depths.
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34. Colorimetric Method
• Adding the soil sample to a 0.34 per cent solution of
anhydrous cobalt chloride and comparing the colour
change with standards which are previously prepared.
• Colorimetric hydrometer containing cobalt chloride
saturated silica gel is used for determination of soil
water.
• The hydrometer is inserted into a freshly dug up holi in
soil and it shows a change in colour according to the
amount of water present in soil.
• Colour developed is compared with a colour chart
showing the water content.
• Not widely accepted
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35. Chemical Methods
• Calcium carbide produces acetylene gas
when it reacts with water and the amount of
acetylene produced is directly proportional to
the amount of soil water in soil.
• Sometimes, concentrated sulphuric Acid is
used to dry up the water present in the soil.
The loss of weight of the soil gives the
measure of the soil water.
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36. Penetrometric Method
• Penetrometer is used to measure the force
required to push a rod into the soil.
• The force is measured in dyes/cm
• The resistance offered by the soil is taken as a
measure of the soil water content.
Limitaions
• Forces varies with soil texture, soil density,
organic matter content and several other factors
• Soils with hard pan this method is not workable
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37. Air Permeability Method
• Passing air through soil mass and than
measuring the outflow of air
Limitations
• Gives rough measurement
• Flow of air is influenced by many factors such
as soil texture, organic matter content, soil
structure presence of stones, roots and cracks
etc.
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