Soil moisture

Principles of Soil Sciences
(Practical)
Practical 3: Soil Moisture Calculations
Soil and Water science Department
College of Agriculture
University of Duhok
By: Hajar Ameen Ameen
Hajar.ameen@uod.ac

Objectives
 List the objectives of measuring soil water content.
 To identify the forms of soil water.
 To be familiar with definitions of soil moisture content on
oven dry basis and moisture content on wet basis.
 List methods being used for measuring soil moisture content.
 To know how to calculate soil moisture content by
Gravimetric (mass) and volumetric (volume) methods

 Water in the soil (moisture) has a great role in most physico-
chemical reactions and biological activities in the soil.
Why is Measuring Soil Moisture Important?
 Soil forming processes and weathering depend on water
 It is a principal constituent of the growing plant
Physical, chemical and biological properties of the soil can change
as moisture conditions change.

Objectives of measuring soil moisture:
 The prime purpose of soil moisture measurement is to determine
the right amount of water need for irrigation or the right time to
irrigate.
 To convert the soil moisture content from wet – weight basis to
oven dry – weight basis and from weight basis (gravimetric) to
volume basis.
 Soil moisture content is also needed for studies of many
others disciplines, such as hydrology, ecology, meteorology,
engineering and agronomy.
 To standardize the result on dry soil mass

Classification (types) of water in soil
Note:-
1bar = 0.1Mpa = 1000 Kpa

Methods of Soil Moisture Estimation
The methods of measuring soil moisture are divided in to:
A- Direct methods: measure the moisture content in soil by
heating at 105 °C .
B -Indirect methods: Measurement of water potential or tension
under which water is held by the soil. They need to be calibrated
before use.
1- Gravimetric (thermo – gravimetric) methods
2-Volumetric (thermo – volumetric) Method
1. Tensiometer.
2. Gypsum block (Electrical resistance blocks)
3. Neutron scattering probe
4. Pressure plate and pressure membrane apparatus.

Fig. show Soil Water Potential curve for three mineral soils with different
textures.

Time domain reflectrometry (TDR)
Can directly determine the volumetric water content of soil

Measurement of soil moisture by gravimetric method
Gravimetric method is the most widely used method for measuring
soil moisture.
1. Wet- weight basis θ (Wm)
2. Oven dry- weight basis θ (Wd)
 To determine any of these ratios for a particular soil sample, the
water mass must be determined by drying the soil at (105°C) to
constant weight and measuring the soil sample mass after and before
drying.
% Wm =
Weight of water
Weight of moist soil
% Wd =
Weight of water
Weight of oven dry soil
Gravimetric methods of soil moisture measurement
can also be expressed on:
× 100
× 100

1. Obtain an empty container and record the container number.
2. Weigh the empty crucible (M)
3. Add approximately 10 - 30 g of sieved (2 mm) soil and record the weight of
the container plus soil (M1).
4. Place it in a 105 °C oven for 24 hours.
5. Take the container out of the oven and allow it to cool in a dessicator for a
few minutes. Then weigh the container with the dry soil in it and record
this weight (M2).
6. Record all your results in table below:
Procedure:
M M1 M2
Sample
number
Container
number
Container
mass (g)
Container +
moist soil (g)
Container +
oven dry soil (g)
Continue

Calculation:
% soil moisture content (Wd) =
M1 – M2
M2 - M
× 100
% soil moisture content (Wm) =
M1 – M2
M1 – M
× 100
Where:
M = weight of container without soil
M1 = weight of container + moist soil
M2 = weight of container + oven dry soil

 To convert the moisture content from Wd to Wm use the
following equation:
Converting soil moisture content from wet – weight basis to oven dry-
weight basis and vice versa.
Wm =
Wd
100 + Wd
 To convert the moisture content from Wm to Wd use the
following equation:
Wd =
Wm
100 – Wm
× 100
× 100

Problem: Determine the gravimetric moisture content on Wet and
Oven dry weight basis using the following data:
Weight of container = 5g; weight of moist soil + container = 45g;
weight of oven dry soil + container = 40
% (Wd) =
M2 - M
× 100
M1 – M2
=
45 - 40
40 - 5
× 100
= 14.29%
% (Wm) =
M1 – M2
M1 – M
× 100
=
45 - 40
45 - 5
12.5%=
Or
Wm =
Wd
100 + Wd
× 100
Solution:
× 100

Volumetric moisture content (θv)
Soil moisture content can also be expressed on the basis of volume using
the following equation:
θv =
Vw (volume of water)
Vt (Total volume of soil)
× 100
Where:
Vt = (Vs + Vv)
Vv = (Va + Vw)
Vs= volume of soil particles
Vv = volume of pore spaces in soil
Va = volume of pore space equipped by air
Vw = volume of pore space equipped by water

Problem: Determine the volumetric moisture content of soil using the
following data:
Mass of water in soil = 20g; volume of soil (Vt) = 500 cm3
θv =
Vw
Vt
× 100
Vw =
Vw =
Mass of water
Density of water
20g
1g/cm3
= 20 cm3
θv
20
500
=
× 100
= 4 %
Solution:

Volumetric soil moisture content can also be determined using
the following equation:
θv = Wd
Bulk density of soil (Db)
Density of water
×

Problem: A soil sample has 20 % gravimetric moisture content
(Wd) and bulk density (Db) of 1.25 g/cm3. calculate the
volumetric moisture content (θv).
θv = Wd
Bulk density of soil (Db)
Density of water
×
Solution:
θv = 20 ×
1.25 g/cm3
1 g/cm3
θv = 25%

Water content expressed as depth (dw)
1m
1m
1m
0.5m
1m3 soil
0.5 m3 water
θv = 50%
dw = 500 mm/m
θv = 0.5m3/ 1m3 × 100
dw =
θv × Dsoil
100
dw =
100
50 × 1m
= 0.5 m
Depth of water in soil

1. Texture: Finer the texture (sand), more is the pore space and
also surface area, greater is the retention of water.
2. Structure: Well-aggregated porous structure favors better
porosity, which in turn enhance water retention.
3. Organic matter: Higher the organic matter more is the water
retention in the soil.
4. Density of soil (compaction): Higher the density of soil, lower is
the moisture content.
5. Climate factors: such temperature, wind relative humidity. For
example, cooler the temperature, higher is the moisture
retention.
Factors Affecting Soil Water

Soil moisture

More Related Content

Soil moisture