soil water energy concept is all about potential energy,gravitational potential,osmotic potential,pressure potential and total potential energies including units
2. Soil Water Energy
The total energy state of soil water is defined by
its equivalent potential energy, as determined by
the various forces acting on the water per unit
quantity. Different energies are involved including
P.E and K.E.
Kinetic energy is certainly an important factor in
the rapid turbulent flow of water as in river but the
movement of water in soil is so slow that kinetic
energy is usually negligible. Thus in soils kinetic
energy is ignored.
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3. 3
Potential Energy Of Water :
The potential energy of water is the energy the
water obtains as a result of being at some
elevation (position).
P.E is most important in determining the status
and movement of soil water.
Water Potential quantifies the tendency of
water to move from one area to another due to
osmosis, gravity, mechanical pressure, or
matrix effects such as capillary action (which is
caused by surface tension).
4. Forces Affecting Potential Energy
Capillary Forces
Osmotic Force
Gravitation Force
1.Capillary Forces
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5. 5
Adhesion:
Tendency of dissimilar particles or surfaces to
cling to one another.
Mainly adhesion lead to Matric Potential.
Cohesion:
Tendency of similar or identical particles to cling to one
another.
6. 2.Osmotic Force
The attraction of water to ions and other
solutes
result in osmotic forces which lead to osmotic
potential.
Water moving between roots and soil
Through the cell membranes of root hairs,
which are semi permiable according to a water
potential gradient.
7. 3.Gravitation Force
Gravity is the major force acting on soil water
which always pull water downward according
to
concentration gradient and lead to
gravitational
potential.
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8. Soil Water Potential
Total Soil water potential :
It is the sum of all potentials of soil water. All
of
these act simultaneously to influence water
behavior in soil and given as;
Where the ellipsis (….) indicates the possible
contribution of additional potentials not yet
mentioned.
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9. 9
Gravitational Potential :
The force of gravity acts on soil water the
same as it does on any other body, the
attraction being toward the Earth`s center.
Following heavy precipitation or irrigation,
gravity plays an important role in removing
excess water from the upper horizon and
recharging ground water below the soil profile.
Gravitational Potential is always Positive (+).
10. Pressure Potential (Hydrostatic & Matric) :
Besides gravity and solute levels pressure
Potential include;
1)Positive hydrostatic pressure due to the
weight of water in saturated soils and
aquifers, give rise to hydrostatic potential.
2)The negative pressure due to attractive
forces between water and soil solids or the
soil matrix which give rise to the matric
Potential.
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11. A component hydrostatic potential is
operational only for water in saturated
zones below the water table.
Matric Potential influences both the
retention and movement of soil water .It is
most important in supplying water to plant
roots and engineering applications.
It is always negative because water
attracted by the soil matrix has an energy
state lower than that of pure water.
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12. Osmotic Potential :
As water molecules cluster around solute ions
their free movement is reduced(because potential
energy is decreased). Greater the concentration
of solute the more osmotic potential is lowered.
As always water will move from higher solute
concentration (with low osmotic potential) to lower
solute concentration( with high osmotic potential).In
this
case, osmosis is important if a semi permeable
membrane
is present between the high and low osmotic
potential
allowing water through but preventing the movement
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13. 13
Because soil zone are not separated by
membranes
the osmotic pressure has little effect on the
mass
movement of water in soils.
Its major effect is on the uptake of water by
plant
root cells that are isolated from soil solution by
their semi permeable cell membranes.
In soils high in soluble salts osmotic potential
may
be lower in the soil solution than in plant root
cells,
15. Methods of expressing Energy Levels
Energy may be expressed per unit of
mass
(joule/kg)
or per unit of volume (newtons/m2)
but in this case we use Pascal (Pa) or
Kilopascals (kPa).
The SI unit kilopascal (kPa) is equivalent
to
0.01 bars.
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