Water Content

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Water Content Water Potential
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Water
Soil
Potential
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Water Content vs Water Potential

• Water content
• amount of water in soil
• calculated by mass or volume (g/g or cm3/ cm3)
• produces a percent content
• tells you how much water is present
• Water Potential
• energy of water in soil
• measured in energy/mass soil
• units are J/kg or kPa (these are equivalent)
• tells you which direction water will move and how
  easy or difficult it will be for it to do so

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Gravimetric (Mass) Water Content (W)

• units g/g or kg/kg
• W (mass water / mass dry soil) x 100
• Example 40 g water in 75 g soil
• (40g / 75g) x 100 53
• To get weight values
• weigh soil sample
• place in 105C oven for 24 hours
• reweigh sample to get dry soil mass
• subtract dry weight from wet weight to water mass

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Volumetric Water Content (?)

• units cm3/cm3 or m3/m3
• ? (volume water / volume soil) x 100
• Example 0.3 m3 water in 1 m3 soil
• ? (0.3 m3 / 1 m3) x 100 30 water
• can also solve if know bulk density and
  gravimetric water content
• BD mass dry soil/unit bulk volume
• ? (BD x W) x 100
• Example BD 1.4 g/cm3 and W 20
• ? (1.4 x 0.2) x 100 28

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Water Potential (?)

• measures the energy of water
• units J/kg or kPa (these are equivalent)
• () water potential tells you how much energy
  will be released when the water moves
• e.g. water coming down over a falls or rapids
• (-) water potential tells you how much energy
  will have to be exerted to move the water
• e.g. water down in soil being taken up by plant
  roots
• (0) water potential is free water
• e.g. water in a pool
• serves as your reference level
• water always flows from high to low potential

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Water Potential (?)

• ?Total ?g ?m ?p ?o
• ?g gravitational potential
• only matters in soil science once a soil is
  saturated
• ?m matric potential
• THE water potential of soil science
• ?p pressure potential
• negligible in soil science
• ?o osmotic potential (also called solute
  potential, ?s)
• only matters in soil science when the soil is
  salty

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Matric Potential (?m)

• for our purposes, ?T ?m
• generated from the attraction of polar water
  molecules to negatively charged colloid surfaces
  (adhesion) and each other (cohesion)
• also referred to as suction or tension
• ?m always has a negative value

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Negative ?

• free water has a water potential of zero
• in soil, ? is always negative (unless the soil is
  saturated, then ? 1)
• this means it requires energy for a plant to take
  up water
• the more negative the ? value, the more suction
  it has to the colloids, the harder it is for a
  plant to take it up
• dont forget
• a low ? is a very big negative number (e.g -2000)
• a high ? is a very small negative number (e.g.
  -10)

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Plant Available Water

• once saturated, gravity (?g) will drain a soils
  total ? from 1 to between -10 and -30 kPa (field
  capacity)
• plant available water is between -10 and -1500
  kPa
• -1500 kPa is the permanent wilting point
• below this point, most plants can no longer draw
  up water
• if more water is not added to the soil soon, the
  plant will permanently wilt and die
• In irrigation systems especially, being able to
  measure water potential could be helpful!

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Tools for Measuring ?
Tensiometer - measures suction of soil on water
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Tools for Measuring ?
Thermocouple Sample Chamber Psychrometers -
measure air humidity in equilibrium with sample
and infers ? with math equation
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Water Content and Water Potential

• both important, but ? more so
• ? will determine whether or not a plant is
  capable of taking up water, regardless of the
  water content
• i.e. it doesnt matter what the water content of
  a soil is if the water potential is too low for a
  plant to be able to take up water
• different soil textures will reach the same ? at
  different water contents

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Texture, Water Content and Water Potential
Why?
Higher surface area and negative charge of clays
means more water is held onto tightera little
clay goes a long way!