Lecture 7a Soil Water Part 1

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Lecture 7aSoil Water - Part 1

• Water Storage for a Thirsty Planet
• more crop per drop!

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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
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Soil Water Adhesion Water- water attracted to
solid surfaces

• 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

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Soil Water Cohesion water water attracted
to other water molecules

• held by hydrogen bonding (when - of water
  molecules are close together)
• liquid state in water film
• major source of water for plants
• greater energy than adhesion water

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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)

Adhesion Cohesion Gravitational
Soil
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Soil Water Soil Moisture

• 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
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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------- gt very
  dry

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Water tension () or Potential (-)
0 bar -.33 bar

• 1 bar
• -15 bar

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Soil Water Classification- a way to
quantitatively describe the water in the soil.

• 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
-10000
Saturated Field Cap Wilt point air dry
oven dry
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Water moves from areas of high potential (wet
soil -2 or -4) to areas of low potential (dry
soil -8)
-.4
-3
-7
-8
-2
Root
Soil
Soil
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The End Part 1