Agriculture Reference
In-Depth Information
Maximum Radius of Cylindrical Pores Holding Water
as a Soil Dries from Saturation to Air-dryness
Table 6.1
Soil Water Potential
Pore Radius
(
State of Soil Wetness
(kPa)
m)
Saturation
Water held after free drainage for
48 hours (field capacity)
Approximate water content at which plants
wilt (wilting point)
Soil at equilibrium with a relative vapor
pressure of 0.85 (approaching air-dryness)
0
infinitely large
15
10
1,500
0.1
00
22,000
0.007
motic pressure
(pi) is defined as the hydrostatic pressure necessary to just stop
the inflow of water when the solution is separated from pure water by a semi-
permeable membrane. Osmotic pressure is calculated from the equation
RTC
(6.2)
where
R
and
T
are defined in appendix 7, and
C
is the concentration of solute
particles in solution. When
C
has the units mol/m
3
,
is in pascals. The compo-
nent
s
is significant only in saline soils.
Gravity and Weight.
Gravity exerts a force on water that is directly propor-
tional to the height of the water above or below a chosen reference level. This
gives rise to the component
gravitational potential
g
. If the soil water is at a height
above that chosen for the reference level, usually the soil surface,
g
is positive. In
addition, soil water may experience a pressure greater than atmospheric pressure,
the
pressure potential
p
. In practice, variations in
p
due to changes in air pres-
sure are negligible:
p
becomes significant only when part or all of the soil pro-
file is saturated, as, for example, when groundwater rises into the soil. The upper
surface of the groundwater is called the
water table
, and water below the water
table experiences a hydrostatic pressure (equal to
p
) that is directly proportional
to the depth
h
(see equation B6.1.1).
As stated previously, in saturated soil
m
must be zero, and it becomes neg-
ative as the soil dries (or the water table falls). Thus,
m
and
p
are two sub-
components of a pressure-matric potential continuum for which
• above the water table,
0 (at atmospheric pressure) and
m
is negative;
p
0; and
• at the water table,
m
p
0 and
• below the water table,
p
is positive.
m
Summary
. The total water potential
in soil is the sum of the component
potentials, that is,
(6.3)
m
s
p
g
In nonsaline soils, equation 6.3 simplifies to
g
(6.4)
Soil water potential is measured in the field using a tensiometer or a resistance
block. These measurements are discussed in box 6.3.
m
p
