Methods of measuring soil moisture

METHODS OF MEASURING SOIL
MOISTURE
Presented by: Ali Adnan

Why we measure soil moisture
Information of soil moisture is necessary for
Assessing plant water requirement and Scheduling irrigation
Plant water uptake and consumptive use
Water storage capacity of soil
Rate and quantity of water movement

Methods of measuring soil moisture
Direct methods
Measurement of moisture content in the
soil (wetness)
Indirect methods
Measurement of water potential or stress or tension under which water is held by the
soil.

Direct Methods
1. Gravimetric Method
2. Using Methyl Alcohol
3. Volumetric Method

Direct Methods
1) Gravimetric methods.
Basic measurement of soil moisture is made on soil samples of known
weight or volume.
The soil samples are weighed and they are dried in an oven at 105 oC
for about 24 hours.
Then cooled it on room temperature & weighed.
The difference in weight is amount of moisture in the soil.

Direct Method Conti…
Formula
Moisture content Wet weight –Dry weight
On weight basis = ----------------------------- X 100
Dry weight

2) Using Methyl Alcohol:
Soil sample is mixed with a known volume of methyl alcohol and then
measure the change in specific gravity of solution with a hydrometer.
This is a shot cut procedure but it is no in common use.

3) Volumetric Method
Soil sample is taken with a core sampler or with a tube auger whose
volume is known.
The amount of water present in soil sample is estimated by drying it in
the oven and calculating by formula.

core sampler tube auger

Volumetric Method Conti…
Formula
Moisture content =
Moisture content (%) by weight x Bulk Density (%) by volume

Volumetric Method Conti…
The method is though accurate and simple it is used mainly for
experimental purpose.
 Sampling, transporting & repeated weighing give errors. It is also
laborious and time consuming. The errors of the gravimetric method
can be reduced by increasing the size and number of samples.
 However, the sampling disturbs the experimental plots and hence
many workers prefer indirect methods.

Indirect Methods
Common instruments
1) Gypsum block
2) Tensiometer
3) Neutron probe
4) Pressure plate and pressure membrane apparatus

Indirect Method Conti…
1) Gypsum block
Gypsum blocks or plaster of Paris resistance units are used for
measurement of soil moisture is situ.
These were first invented by Bouycos and Mick in 940.
The blocks are generally rectangular shaped.

Structure:
The blocks are made of various materials like
gypsum
 nylon fiber
 glass
 plaster of Paris or
 combination of these materials

Structure

Principles
It works on principle of conductance of electricity. When two
electrodes A and B are placed parallel to each other in a
medium and then electric current is passed.
 The resistance to the flow of electricity is proportional to the
moisture content in the medium.
R ∞ 1/M
Thus, when the block is wet, conductivity is high and resistance
is low.

Principle Conti..
Generally these read about 400 to 600 ohms resistance at field
capacity and 50,000 at wilting point.
The readings are taken with portable Wheatstone Bridge Bouycos
water Bridge operated by dry cells

Limitations.
While placing the gypsum block in soil, care should be taken that the
blocks must have close contact with undisturbed soil.
After placing, the blocks get wetted with soil moisture due to capillary
movement. Pure gypsum block sets in about 30 minutes.

Limitations.
The gypsum block is sensitive to soil to moisture from 1.0 atm tension
to 20.0 atm.
How ever, the gypsum blocks are not reliable in wet soils.

2) Tensiometer
It is also called irrometer. Tensionmeters provide a direct measure of
tenacity (tension) with which water is held by soil.

Structure:
It consist of
A ceramic or clay cup (7.5 cm)
A cap for closure
A vacuum gauge and
A hollow metallic tube.

Working of Tensiometer.
Working of Tensiometer
At the time of installation, the system is filled with water from
the opening at the top and rubber corked when set up in the
soil.
 Moisture from cup moves out with drying of soil, creating a
vacuum in the tube which is measured with the gauge.
 When desired tension is reached, the soil is irrigated.
Tensiometer works satisfactory up to 0.85 bars of atmosphere.

Merits of tensiometer.
 It is very simple and easy to
read soil moisture.
 It is very useful instrument for
scheduling irrigation to crops
which require frequent irrigations
at low tension.
Limitations:
Sensitivity of a tensiometer is
only up to 0.85 atmospheres
while available soil moisture
range is up to atmosphere.
Hence is useful more on sandy
soils wherein about 80% of
available water is held within
0.85 ranges

3) Neutron meter (neutron scattering method)
Soil moisture can be estimated quickly and continuously
another with neutron moisture meter without disturbing the
soil.
Another advantage is that soil moisture can be estimated
from large volume of soil.
Principle
This method is based use radioactive material for measuring soil
moisture.

working
The probe contains fast neutron source, which may be a mixture of
radium and beryllium or Americium and beryllium. Access tubes are
aluminum tubes of 50 to 100 cm length and are placed in the field
where moisture to be estimated.
Neutron probe is lowered into access tube to the desired depth. fast
neutrons are released from the probes, which scatter into the soil.
When neutrons encounter nuclei of hydrogen atom of water, their speed
is reduced. the scalar or the rate meter counts the number of slow
neutrons, which are directly proportional to water molecules. Moisture
content of soil can be known from the calibration curve with counts of
slow neutrons.

Limitations
It is expensive and moisture content from shallow top layers
cannot be estimated.
The fast neutrons are also slowed down by other source of
hydrogen (present in the organic matter).
Other atoms such as chlorine, boron and iron also slow down the
fast neutrons, thus overestimating the soil moisture content.

4) Pressure membrane and pressure plate
apparatus:
• It is generally used to estimate field capacity, permanent wilting point
and moisture content at different pressures.

5) Gama Ray absorption method:
Measurement of changes in soil water content by change in amount of
gamma radiation absorbed.
The amount of radiation passing through soil depends on soil destiny
which varies chiefly with change in water content.

6) Feel and appearance method:
A practical estimate of moisture content is obtained by the feel and
appearance of soil samples taken from the desired depths.
The soil sample is squeezed in the hand and its feel and appearance
are taken into consideration.

Conclusion
When to irrigate and how much water to be applied

Methods of measuring soil moisture

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