Soils 205 Lecture 4
Video tapes Pages
| 6,7 | 176-190 |
Soil Water
Properties-
1) polarity
2) Hydrogen bonding
3) Cohesion and adhesion- result from polarity and hydrogen bonding
Attraction of water molecules to each other Attraction of water to solids
4) Capillarity
-due to cohesion and adhesion
- smaller pores hold water with greater energy
Energy Concepts
All substances, including water, have a tendency to move or change from a state of higher to one of lower energy.
Differences in energy level from one contiguous site to another influences water movement.
Potentials are used to describe the potential energy levels of water in soils.
Factors affecting the energy state of soil water
1) Matric Forces
Matric Potential (Ym)- water in contact with the soil matrix has a lower free energy (Ym < 0)
2) Osmotic forces
Osmotic Potential (Yo)- High salt concentrations
reduce the energy of water (Yo< 0) . Water
will move from an area of low salt concentration to areas of high salt
concentration. Iinfluences
water uptake by plants
more than movement
3) Gravitational forces
Gravitational Potential (Yg)- gravity pulls water downward to areas of lower energy (remember the waterfall); important in wet soils; removes excess water from soils
Total Water Potential
Total soil-water potential = Yt
Yt = Yg + Yo +Ym
Differences in potential cause gradients
Water flows in response to GRADIENT
Water flows from high to low free energy
wet soil ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ® dry soil
H2O movement
H2O free
energy is
higherH2O free
energy is
lower
Units of soil water potential
(a) cm of water
Hanging water column
(b) Bar (approx. 1 atmosphere of pressure)
= 1020 cm of water = 14.7 lb/in2
(c) kilopascals (kPa) - S.I.
bars x 100 = kPa
(d)
cm H2O bars kPa 300 -0.3 -30 1,000 -1 -100 10,000 -10 -1000 15,000 -15 -1500 see table 5.1
Soil Water Content
1. Gravimetric water content (% water on weight basis)
= (grams water in soil/grams dry soil) x 100
2. Soil moisture/energy curves (water-release curves)
