retention forces of water to soil
- capillarity and adsorption
- osmotic pressure
Soil water potential
- 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
gravitational potential of soil water potential
-gravitational attraction relative to point in soil or on soil surface
osmotic potential of soil water potential
- 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
Matric potential
- combined capillarity and adsorption result in suction
- psi = pressure head [m]
vadose zone
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
tensiometer
- 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
TensiMark
- 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
Whats affects the shape of the soil characteristic curve?
- 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
What is air entry tension?
critical suction for air to enter and allow large pores to drain
What is the matric pressure at field capacity? Wilting point?
Field capacity = -340
wilting point = -15,000
Hysteresis for soil moisture explanation
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
