Lal R., Shukla M.K. Principles of Soil Physics
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TABLE 11.4 The Value of 1 Bar of Expressing
Soil-Water Potential on a Volume Basis
Unit Equivalent quantity
One bar 100 centibars (cb, 1 cb=1 J/kg)
1000 millibars (mb)
1020 cm of H
2
O
401.57 inches of H
2
O
75.01 cm of Hg
0.9869 atmosphere
100 J/kg
14.5 Ibs inch
2
(PSI)
10
6
ergs/g
10
6
dynes/cm
2
10
2
kPa
soil-moisture potential expressed in cm of water. The pF value corresponding to soil
matric potential of − 1, −10, −100, and −1000cm of water is 0, 1, 2, and 3 respectively
Principles of soil physics 314
(refer to Appendix 11.1).
Example 11.8
Study the column setup shown below. Compute components of soil-moisture potential for
three possible reference levels.
Solution
There are three solutions based on the choice of reference level.
1. Reference level at the top of soil column-AA.
Depth (cm) Φ
z
(cm) Φ
m
(cm) Φ
p
(cm) Φ
t
(cm)
10 −10 −200 0 −210
20 −20 −160 0 −180
30 −30 −110 0 −140
40 −40 −80 0 −120
50 −50 0 0 −50
60 −60 0 10 −50
70 −70 0 20 −50
80 −80 0 30 −50
Soils moisture potential 315
Principles of soil physics 316
2. Reference level at the watertable-BB.
Depth (cm) Φ
z
(cm) Φ
m
(cm) Φ
p
(cm) Φ
t
(cm)
10 40 −200 0 −160
20 30 −160 0 −130
30 20 −110 0 −90
40 10 −80 0 −70
50 0 0 0 0
60 −10 0 10 0
70 −20 0 20 0
80 −30 0 30 0
3. Reference level at the bottom of soil column-CC.
Depth (cm) Φ
z
(cm) Φ
m
(cm) Φ
p
(cm) Φ
t
(cm)
10 80 −200 0 −120
20 70 −160 0 −90
30 60 −110 0 −50
40 50 −80 0 −30
50 40 0 0 40
60 30 0 10 40
70 20 0 20 40
80 10 0 30 40
Soils moisture potential 317
11.7 SOIL MOISTURE CHARACTERISTICS
The fundamental relationship between soil’s moisture content (θ) and soil-matric
potential (Φ
m
) is called “soil moisture characteristics,” “soil moisture characteristic
curve,” or “pF curve.” This unique relationship depends on soil structure as determined
by total porosity and the pore size distribution. Thus, change in structure and pore size
distribution leads to changes in soil moisture characteristics. The unique relationship, at
the time of obtaining the undisturbed core sample, may be mathematically expressed as in
Eqs. (11.15) and (11.16), and graphically depicted as in Fig. 11.5.
θ=f(Φ
m
)
(11.15)
Φ
m
=f(θ)
(11.16)
An example of hypothetical data on soil moisture characteristic for two soils of
contrasting texture is shown in Table 11.5. As expected, soil wetness increases with
decrease in soil matric potential from a very high negative value for an extremely dry
condition to a near zero suction for a saturated soil when all pores are full of water. As
the suction increases from saturation to a low value of 10 or 20 cm of water, soil wetness
remains the same in a heavy-textured soil. The suction at which soil wetness begins to
decrease is called the “air entry point.”
There are numerous factors that affect soil moisture characteristics. In addition to
particle size distribution, soil organic matter content plays an important role, especially at
low suctions (or field capacity). Soil wetness at field soil moisture capacity (pF 2.5)
increases linearly with an increase in soil organic matter content (Fig. 11.6). All other
factors remaining the same, soil moisture retention at a specific suction also depends on
the ambient temperature. Soil moisture retention decreases with increase in ambient
temperature (Fig. 11.7).
Principles of soil physics 318
FIGURE 11.5 A schematic showing
the soil moisture characteristic curve.
TABLE 11.5 Soil Moisture Characteristic for Two
Soils of Contrasting Texture
Soil matric potential
Volumetric wetness (θ)
Soil
wetness
cm of H
2
O pF
Heavy-textured Light-textured
Saturated 1 0 0.60 0.40
10 1 Free Water 0.60 0.38
Wet 50 1.7 0.55 0.35
100 2
0.50 0.25
330 2.5 Field Moisture Capacity 0.45 0.18
1000 3 0.40 0.15
Moist 10,000 4
0.35 0.12
15,000 4.2 Permanent Wilting Point 0.20 0.07
30,000 4.47
0.15 0.02
Dry 100,000 5 Residual water 0.10 0.005
1,000,000 6 0.06 0
10,000,000 7 Bonded water 0.05 0
It is important, therefore, that soil moisture characteristics are determined in a laboratory
with constant temperature. Soil structure is the most important factor affecting pF curve.
Soils moisture potential 319
The soil moisture characteristic curve of a well-structured soil has a strong inflection
point as indicative of change in pore size distribution. Such inflection points are not well
defined in a weekly
FIGURE 11.6 The pF curve of soils of
similar texture but with high and low
organic matter content.
Principles of soil physics 320
FIGURE 11.7 Soil moisture retention
decreases with an increase in room
temperature.
FIGURE 11.8 Effect of soil structure
on pF curves.
structured or a structureless soil (single-grained sand) (Fig. 11.8). Change in soil structure
due to change in land use or soil management (plow till to no till or vice versa) are
evidenced by plotting the differential “soil moisture characteristic curve.” A plot of the
slope of the soil moisture characteristic curve (dΦ
m
/dθ) versus is indicative of the
change in soil structure over time (Fig. 11.9). Change in soil’s moisture content per unit
change in
is also called “specific water capacity” [Eq. (11.17)].
C
θ
=dθ/dΦ
n
(11.17)
With the exception of determining soil moisture retention at pF 4.2 (permanent wilting
point) which is usually determined on a ground and sieved sample, soil moisture retention
curves are measured on undisturbed soil either by using a core or a clod. There are also
numerous methods of measuring moisture characteristics at different suctions (Table
11.6). These methods can be divided into four groups, depending on the suction range.
1. Low suction (0–60cm of water):
Sintered glass funnel or Haines funnel technique
Tension table
Sand box technique
Plastic (porous) membranes
Soils moisture potential 321
2. Medium suction (100–1000 cm of water):
Pressure plate extractors
Pressure membrane (cellophane)
FIGURE 11.9 A plot of differential
soil moisture characteristic over time
indicates progressive decline in soil
structure or degradation of soil
physical quality.
TABLE 11.6 Methods of Determining Soil
Moisture Characteristics
Matric potential (kPa) Method use
0–10 Sand box
0–20 Haines Funnel containing porous/sintered glass plate
0–70 Suction plate
10–50 Sand/Kaolin combination
0–60 Tension table (glass or plexi glass)
1–1000 Consolidation (by applying direct load on porous disks)
10–1500 Porous plate extractors
10–3000 Centrifuge
30–1500 Osmosis using glycol and other solutions
Principles of soil physics 322
100–2000 Psychrometer s
10–10,000 Pressure membrane
3000–1,000,000 Vapor pressure equilibrium using sorption balance
1000–10,000,000 Filter paper
3. High suction (1000–20,000 cm of water):
Pressure plate extractors
Pressure membrane extractors
4. Very high suction (20,000–10,000 cm of water):
Vapor pressure equilibrium using vacuum desiccators
The mathematical function to compute matric potential using the vapor pressure is shown
in Eq. (11.18).
Φ
m
=[RT ln(p/p
0
)]/M
(11.18)
where R is gas constant (8.3143/K/mole), T is absolute temperature (°C+273.16) in K, M
is mass in kg/mole of water (0.018015 kg), and In is the natural logarithm. At 20°C, Φ
m
is
−21,988 J/kg for relative vapor pressure of 0.85 and − 1500 J/Kg for relative vapor
pressure of 0.989 (note the p/p
0
of 0.989 is equivalent to the permanent wilting point). A
commonly used empirical formula to compute pF from the equilibrium value of relative
humidity (in %) is Eq. (11.19).
pF=6.5+log
10
(2-log R.H.)
(11.19)
Using Eq. (11.19), pF for relative humidity of 98.9% is 4.2, or the permanent wilting
point. High sensitivity of pF to changes in relative humidity is shown by the calculations
in Tables 11.7. Some relevant models of estimating soil moisture characteristic curves are
discussed in Chapter 13. Humidity values for different chemicals are shown in Appendix
O.
TABLE 11.7 High Sensitivity of pF to Even
Minute Changes in Relative Humidity of Soil Air
Relative humidity (%) PF=6.5+log1
0
(2−log1
0
R.H.)
0.001 Undefined
1 6.8
10 6.5
20 6.35
30 6.22
40 6.10
60 5.84
Soils moisture potential 323