Lecture Panel 9

Question 1

retention forces of water to soil

Answer 1

• capillarity and adsorption

- osmotic pressure

Question 2

Soil water potential

Answer 2

• work per unit quantity necessary to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water at a specified location and at atmospheric pressure to a given soil location
• water moves from location of higher potential energy to lower
• soil water potential = gravitational potential + matric potential + osmotic potential

Question 3

gravitational potential of soil water potential

Answer 3

-gravitational attraction relative to point in soil or on soil surface

Question 4

osmotic potential of soil water potential

Answer 4

• solutes affect thermodynamic properties of water and lower potential energy
• important for interactions bewteen plant roots and soil
• typically not considered if soil solution is dilute

Question 5

Matric potential

Answer 5

• combined capillarity and adsorption result in suction

- psi = pressure head [m]

Question 6

vadose zone

Answer 6

part of Earth between the land surface and the top of the phreatic zone (the position at which the groundwater is at atmospheric pressure)
-pressures here are negative relative to atmospheric pressure

Question 7

tensiometer

Answer 7

• hollow tube closed by cup of porous ceramic material and sealed at the other end
• filled with water, sealed, and inserted into soil
• pressure is initially at atmospheric, therefore there is pressure induced flow into the cup
• gauge reads the tension in the tube and in a spherical region surrounding the cup
• typically installed as a group at different depths to provide info about the vertical tension gradient

Question 8

TensiMark

Answer 8

• matric potential instrument
• water equilibrates with ceramic cup at tip
• matric potential is calculated from thermal storage properties of the soil
• allows for continuous monitoring

Question 9

Whats affects the shape of the soil characteristic curve?

Answer 9

• texture of soil
• clays have greater retention at any suction and a more gradual curve
• sands have large pores and drain quickly once these are emptied, apart from very strongly adsorbed water

Question 10

What is air entry tension?

Answer 10

critical suction for air to enter and allow large pores to drain

Question 11

What is the matric pressure at field capacity? Wilting point?

Answer 11

Field capacity = -340

wilting point = -15,000

Question 12

Hysteresis for soil moisture explanation

Answer 12

soil moisture characteristic curve is different for wetting and drying - when wetting, we will have a lower moisture corresponding to our pressure than when drying.
-because - wetting is easier due to the relative size of effective radius of the pores - drying depends on narrow radii connecting channels whereas wetting relies on the larger pores