Lecture 7a
Soil Water - Part 1
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Water Storage for a
Thirsty Planet –
more crop per drop!
Describe in your own words what happens to the water in the
diagram below as it is added to a dry soil. How does the water
enter and move through the soil?
Water
A horizon - Air Dry
Soil
Soil Water
Adhesion Water- water attracted to solid surfaces
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held by strong electrical
forces - low energy
little movement- held tight
by soil
exists as a film
unavailable to plants
removed from soil by drying
in an oven
Soil Water
Cohesion water – water attracted to other water molecules
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held by hydrogen
bonding (–when + & - of
water molecules are close
together)=………
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liquid state in water film
major source of water
for plants
greater energy than
adhesion water
Gravitational Water
exists in macro -pores
 has greatest energy (true liquid)
 moves freely due to gravitational
forces. (2 hands, 1 hand, 3 fingers, 1 finger, touch)
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Adhesion
Soil
Cohesion
Gravitational
Soil Water = Soil Moisture
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Water has energy - substances tend to
change from a state of high energy to
states of low energy (water runs down
hill)
Soil water potential - water is held in soil by
“tension” or attraction of water molecules to
solid surfaces and to other water molecules
Water Potential
Tension = - pressure
 Soil water potential = amount of work
that must be done per unit quantity of
water in order to transport a quantity
of water from a pool of pure water to
the soil water.
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Soil water potential
Total soil water potential = Matric potential
+ gravitational potential + Osmotic (salts)
 As the soil dries the matric potential
decreases or a larger negative number
 00
-5 -8 -10
-15
-55
100
 sat.
wet - -------- dry------- > very dry
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Water tension (+) or Potential (-)
0 bar
-.33 bar
-1 bar
--15 bar
Soil Water Classification-
a way to
quantitatively describe the water in the soil.
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0 to -0.3 bar = Gravitational (not available)
-0.3 = Field Capacity
-15 bar = wilt point
Between -0.3 & -15 is plant available water (AWC)
-15 to -100 bar = stages of air dry
-10,000 bar = oven dry
AWC
0 bar -0.33
-15
-100
Saturated Field Cap Wilt point
-10000
air dry
oven dry
Water moves from areas of high potential
(wet soil : -2 or -4) to areas of low
potential (dry soil -8)
-.4
-3
-7
-8
Root
-2
Soil
Soil