Lecture 7a Soil Water - Part 1 Water Storage for a Thirsty Planet – more crop per drop and more drink per glass.

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 Soil A horizon - Air Dry

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

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

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) Soil Adhesion Cohesion Gravitational

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

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.

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 sat. wet dry > very dry

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. 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 0 bar Saturated Field Cap Wilt point air dry oven dry AWC

Water moves from areas of high potential (wet soil : -2 or -4) to areas of low potential (dry soil -8) Soil Root