Soil Water

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Soil Water

• How much water can soils hold?
• Soil/plant interactions

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How much water can soils hold?

• Determined by soil pore spaces
• Size and distribution
• The type of soil
• Sandy 35-50
• Medium-fine texture 40-60
• Compacted soils 25-30

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Measurements of soil water

• Mass soil water content
• Volumetric soil water content
• The more common measurement
• m3 water per m3 soil

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Some properties of soils

• Deeper soils can generally hold more water than
  shallow soils
• Water moves through unsaturated soils due to
  gravity and matric potential

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Fine textured soil on top, sand on bottom, after
40 minutes
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After 110 minutes, no penetration into sand layer
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After 400 minutes, the matric potential gradient
is finally large enough to cause movement to the
sand
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Measuring soil water

• Measure a soil wet and then dry
• Gypsum blocks
• Measures electrical resistance as a function of
  water content
• Neutron probes
• Measures slowing of neutrons by hydrogen
• Time domain reflectometry (TDR)
• Measures speed of electrical signal

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A Campbell scientific TDR setup
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Saturation versus field capacity

• Saturation is the amount water a soil can hold
  when all pore spaces are full
• Field capacity is the amount of water a soils can
  hold against gravity drainage
• Like the water a sponge holds after it stops
  dripping

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White areas are crop
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Moving water from soils to the atmosphere

• Almost all water moving from soils through plants
  goes through stomata
• From stoma, the Greek word for mouth
• Up to 300 stomata per mm-2 of leaf!

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guard cells
Stomata from a tropical cloud forest tree
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Stomata on a cactus
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Stomata respond to soils and the atmosphere

• Stomata close in response to
• Drying soils
• Increased VPD
• Cold temperatures

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Soil water effects

• As soils dry, stomata tend to open less
• Measured by soil water potential (?)
• A measurement of how hard it is to extract water
  from something (like soils)
• Often in MPa (megapascals)
• Pure water is 0
• More negative, harder to extract

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Wilting point

• The lowest water potential at which plants can
  extract water
• Average wilting point is -1.5 MPa
• Some plants can go down to -10 MPa

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The relationship between soil and leaf water
potentials
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Measure gas flux with a Licor 6400
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A pressure bomb for measuring leaf water potential