PLS 366 EXAM #2 STUDY GUIDE
Mass water content (θm) - Answers -The mass of H2O in a set of mass of soil
Usually expressed as kg H2O/kg dry soil
Weigh moist soil, dry at 105 degrees C for 24-48 hours, weigh dry soil
Weigh dry soil
θm = (moist soil (g) - dry soil (g))/dry soil (g)
volumetric water content (θv) - Answers -The volume of H2O in a set of volume of soil
Must know bulk density to calculate
Dw = density water
θv = (Db)(θm)/Dw
= (g soil/cm^3)(g water/g soil)/(1g water/cm^3 water)
= cm^3 water/cm^3 soil
Water molecule - Answers -Polar molecule
Polarity results in many properties: adhesion, cohesion, attraction of water to clay
surfaces, hydration of cations
Hydrogen bonding - Answers -Bond between net negative and net positive water
molecules
In water accounts for high boiling point, expansion on freezing, cohesion and adhesion
Cohesion - Answers -attraction of water for other water molecules
Adhesion - Answers -attraction of water for other substances and surfaces
Surface tension - Answers -consider an air-water interface
results from greater attraction of water molecules for each other within the bulk fluid
(cohesion) than for the air above
water molecules at air-water interface feel a net attraction into the liquid becuase
density of molecules on air side of interface is lower than liquid side
unequal attraction deforms H bonds, which creates membrane-like properties to occur
at the surface
ex) how water bugs can "walk" on water
Capillary rise - Answers -caused by adhesion, cohesion, and surface tension of water
hydrophillic surface: water can move up/absorb by soil
hydrophobic surface: water won't be absorbed, instead it will bead up
as capillary pore radius get smaller, water moves further up (inversely releated to radius
of capillary pore) -> more capillary rise as a result of thinner tubes and more micropores
water can move up or sideways
, better for soil because it fills air space and doesn't disrupt soil (compared to pouring
water ontop)
Capillary Rise Equation - Answers -h = 0.15 (cm^2)/r (cm)
h - height of rise
r - radius of capillary pore (cm)
0.15 - constant including density, gravity, x-sectional area (cm^2)
Soil water potential (SWP) - Answers -(determining which way water will flow in soils)
measure of the difference in energy levels between water in the soil column and a pure
water reference at STP and elevation
SWP defines (in energy terms) what direction soil will move
Water flows from high SWP to a low SWP
high energy state to low energy state
wet to dry soils
large pores to small pores
units are kPa or MPa
kilopascal kPa - Answers -energy unit
kPa or MPa (megapascal)
free pool = 0 kPa
absorbed water = negative kPa values
ranges from 0 to <-800,000 kPa
more negative = more tightly held (how tight water is held in soil)
Soil water potential - Answers -the forces dictating how water moves in soil
difference in energy levels between pure water in reference state and soil water
negative soil water potential would indicate more micropores
matric, osmotic or solute, gravitational, and hydrostatic potential
Ψtotal = Ψm + Ψo + Ψg + Ψh
Matric Potential (Ψm) - Answers -attraction of water for solid surfaces, always negative
because water attracted to soil surfaces has energy lower than pure water
wetter to drier, large pores to small pores
osmotic or solute potential (Ψo) - Answers -atrributed to the presence of solutes in the
soil solution
(lowering from 0)
gravitational potential (Ψg) - Answers -movement of water due to gravity (Ψg = gh)
g - acceleration due to gravity
h - heigh of soil water above reference
hydrostatic potential (Ψh) - Answers -water movement in the saturated zone below
water table
Mass water content (θm) - Answers -The mass of H2O in a set of mass of soil
Usually expressed as kg H2O/kg dry soil
Weigh moist soil, dry at 105 degrees C for 24-48 hours, weigh dry soil
Weigh dry soil
θm = (moist soil (g) - dry soil (g))/dry soil (g)
volumetric water content (θv) - Answers -The volume of H2O in a set of volume of soil
Must know bulk density to calculate
Dw = density water
θv = (Db)(θm)/Dw
= (g soil/cm^3)(g water/g soil)/(1g water/cm^3 water)
= cm^3 water/cm^3 soil
Water molecule - Answers -Polar molecule
Polarity results in many properties: adhesion, cohesion, attraction of water to clay
surfaces, hydration of cations
Hydrogen bonding - Answers -Bond between net negative and net positive water
molecules
In water accounts for high boiling point, expansion on freezing, cohesion and adhesion
Cohesion - Answers -attraction of water for other water molecules
Adhesion - Answers -attraction of water for other substances and surfaces
Surface tension - Answers -consider an air-water interface
results from greater attraction of water molecules for each other within the bulk fluid
(cohesion) than for the air above
water molecules at air-water interface feel a net attraction into the liquid becuase
density of molecules on air side of interface is lower than liquid side
unequal attraction deforms H bonds, which creates membrane-like properties to occur
at the surface
ex) how water bugs can "walk" on water
Capillary rise - Answers -caused by adhesion, cohesion, and surface tension of water
hydrophillic surface: water can move up/absorb by soil
hydrophobic surface: water won't be absorbed, instead it will bead up
as capillary pore radius get smaller, water moves further up (inversely releated to radius
of capillary pore) -> more capillary rise as a result of thinner tubes and more micropores
water can move up or sideways
, better for soil because it fills air space and doesn't disrupt soil (compared to pouring
water ontop)
Capillary Rise Equation - Answers -h = 0.15 (cm^2)/r (cm)
h - height of rise
r - radius of capillary pore (cm)
0.15 - constant including density, gravity, x-sectional area (cm^2)
Soil water potential (SWP) - Answers -(determining which way water will flow in soils)
measure of the difference in energy levels between water in the soil column and a pure
water reference at STP and elevation
SWP defines (in energy terms) what direction soil will move
Water flows from high SWP to a low SWP
high energy state to low energy state
wet to dry soils
large pores to small pores
units are kPa or MPa
kilopascal kPa - Answers -energy unit
kPa or MPa (megapascal)
free pool = 0 kPa
absorbed water = negative kPa values
ranges from 0 to <-800,000 kPa
more negative = more tightly held (how tight water is held in soil)
Soil water potential - Answers -the forces dictating how water moves in soil
difference in energy levels between pure water in reference state and soil water
negative soil water potential would indicate more micropores
matric, osmotic or solute, gravitational, and hydrostatic potential
Ψtotal = Ψm + Ψo + Ψg + Ψh
Matric Potential (Ψm) - Answers -attraction of water for solid surfaces, always negative
because water attracted to soil surfaces has energy lower than pure water
wetter to drier, large pores to small pores
osmotic or solute potential (Ψo) - Answers -atrributed to the presence of solutes in the
soil solution
(lowering from 0)
gravitational potential (Ψg) - Answers -movement of water due to gravity (Ψg = gh)
g - acceleration due to gravity
h - heigh of soil water above reference
hydrostatic potential (Ψh) - Answers -water movement in the saturated zone below
water table
