SOIL WATER CHAPTER 7. SOIL WATER Functions: plant cells 50-90% waterkeeps turgor seed...
Transcript of SOIL WATER CHAPTER 7. SOIL WATER Functions: plant cells 50-90% waterkeeps turgor seed...
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SOIL WATERSOIL WATERCHAPTER 7
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SOIL WATERSOIL WATER
Functions:plant cells 50-90% waterkeeps turgorseed germination transpiration photosynthesis moves products nutrients available lowers soil strength chemical reactions microbial activity
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Water StressWater Stress
Initially, decreased photosynthesis . . .
Continued . . .temporary wilting point
further . . .permanent wilting point
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Forces on Soil WaterForces on Soil Water
Gravitational – pull of gravity downwardAdhesion – attraction of water to soilCohesion – attraction of water to water
adhesion and cohesion result from shape of water molecule and sharing of electrons in oxygen-hydrogen covalent bonds
http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html
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Polarity of WaterPolarity of Water
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Polarity of WaterPolarity of Water
Effects of Water Molecule Polarity:Hydrogen of one molecule attracted to oxygen of
another molecule in a hydrogen bond accounts for cohesion
Hydrogen bond between hydrogen of water and oxygen of silica (SiO2) accounts for adhesion
Adhesion water is very tightly held!!!Cohesion water can move and is available for use
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CapillarityCapillarity
Additive force of adhesion and cohesion- can move against force of gravity- small pores conduct capillary water
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Soil Water PotentialSoil Water Potential
Work water can doPotential energyTendency of water to flow/move freely in soilhttp://www.fhsu.edu/biology/ranpers/ert/wp_tut.htm
Water will always try to move from a state of high energy to a low-energy state
The lower the soil water potential the more tightly water is adsorbed to soil particles
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Water POTENTIALWater POTENTIAL
Refers to the ability of water to move in soil
More water in soil = More water potentialAt saturation, potential is near 0 (zero)As soil dries, values become more
negativeWater is held more tightly by soil!!
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WATER FILM – WATER POTENTIAL
WATER FILM – WATER POTENTIAL
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Three Forces of Water PotentialThree Forces of Water Potential
Gravitational – potential energy due to gravity positive
Matric – most common force; effect of soil on waternegative
Osmotic – special case of salty soilsnegative
Total water potential is sum of three forces
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Units of PotentialUnits of Potential
Official unit is the Pascal (Pa), kilopascal (kPa), or Megapascal (MPa)
- common usage of older unit bar
- equivalent to 0.1 MPa or 100 kPaSoil water potential is usually negative because of
negative matric potential
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TYPES OF SOIL WATERTYPES OF SOIL WATER
Gravitational – at saturation, will drain from larger pores within 24 to 48 hours in well-drained soils
Available – can be absorbed by plants; held between gravitational water and wilting point
Cohesion – held between gravitational and adhesion (hygroscopic) water
Hygroscopic – held tightly by soil particles; air dry
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REFRERENCE POINTS RELATED TO SOIL WATER
REFRERENCE POINTS RELATED TO SOIL WATER
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FOUR CATEGORIES OF SOIL MOISTUREFOUR CATEGORIES OF SOIL MOISTURE
Chemically combined . . . unavailable
Hygroscopic . . . unavailable
Gravitational . . . moves downward by gravity
Capillary . . . taken up by plants
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WATER RETENTIONWATER RETENTION
Total water-holding capacity and available water-holding capacity are based on soil texture
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WATER RETENTIONWATER RETENTION
Medium-textured soils have the highest available water-holding capacity e.g. Silt Loam
Organic matter influences water-holding capacity
Increases amount of available water
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WATER MOVEMENTWATER MOVEMENT
Gravitational flow – moves by gravity• occurs only under saturated conditions• rapid in course soils – large pores• usually percolation through soil profile
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SATURATED SOILSSATURATED SOILS
Sandy soil:gravitational water moves rapidly downward
Clay loam:gravitational water retained 2-3 days afterward
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Once soils lose gravitational water (drain) movement is by . . .
Capillarity – movement due to attraction between water molecules and soil particles
Rapid in sandy soils but limited in distanceSlow in clay soils but may move great
distances
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WATER MOVEMENTWATER MOVEMENT
Unsaturated flow – lateral movement; capillary flow• depends on unbroken films of water spreading
through connected capillary pores• moves from moist to dry soil• can move in any direction
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WETTING FRONTWETTING FRONT
A distinct “line” where water is moving in soil –Wet behind, Dry ahead
• Soils must be nearly saturated in order for the front to advance; Why?
• Dry soil cannot “pull” the water deeper• All the soil must be wet in order for the front to
advance
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CAPILLARY RISECAPILLARY RISE
Upward movement of water from higher to lower potentials
• Explains evaporation of water from soil to atmosphere• Continuation of capillary rise when entire soil column dries• Boundary in soil serves to protect from further losses• Unsaturated flow only moves over short distances• Saturated soil near the surface encourages capillary riseResponsible for accumulation of salts at surface of soils
in dry climates and in potted plants
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Effect of Soil Horizons
water flows differently in different textures . . .
stratified layers will slow percolation
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Vapor Flow
occurs when water vapor moves from moist to drier soil . . .
- condenses on cooler soil particles- very slow- minimal water moved
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Preferential Flow
Saturated soil conditions . . .water enters biopores or other soil channels
Increases infiltration and percolation
May also move pollutants!!!
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How Roots Gather WaterHow Roots Gather Water
Governed by Soil Water Potential
Root hairs draw from higher potential regions
Capillary flow moves water
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Soil – Plant – Atmosphere continuum
Plants create “unbroken” column of waterDriven by plant transpiration
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Patterns of Water RemovalPatterns of Water Removal
Plants will use water near the surface first
Oxygen is highest . . . Respiration drives uptake
As surface dries, plant roots grow deeper . . .absorption shifts downward
If surface is rewetted, absorption shifts upward
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Measuring Soil WaterMeasuring Soil Water
Four methods:
- gravimetric measurements- potentiometers- resistance blocks- neutron probes (mainly research)
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GravimetricGravimetric- measures soil water content by weight
water content = moist wt – dry wt dry wt
Example: soil sample at field capacity 162 grams dry sample 135 grams
water content = 162g – 135g = .20 135g
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Volume BasisVolume Basis
More useful – utilizes gravimetric water content
volumetric water content =
gravimetric water content x soil bulk density water density
From previous gravimetric example . . .
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If bulk density of soil is 1.4 grams per cubic cm, and we know density of water is 1.0 g/cc
Volumetric water content =
.20 x 1.4g/cc = .28 1.0 g/cc
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Soil Depth BasisSoil Depth Basis
Measures “inches of water” per foot of soil- Uses volumetric water content- Simple calculation . . .
Inches water per foot = 12 inches x volumetric water contentContinue from previous example . . .
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Inches water per foot soil =
12 inches x .28 = 3.36
Or simply stated . . . Each foot of soil depth contains 3.36 inches of water assuming constant soil conditions
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Practical Measuring DevicesPractical Measuring Devices
Gravimetric method not very practical management
More useful and practical are . . .
Potentiometers (tensiometers)Resistance Blocks (gypsum blocks)
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PotentiometersPotentiometers
- Measure soil moisture potential at given levels- Water exiting tube creates vacuum- Measured by gauge/instrument- Function best at higher potentials
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Resistance BlocksResistance Blocks
- Measure resistance of electrical flow between two electrodes embedded in block buried in soil
- moist soil with ions of salts in solution carry electrical flow
- resistance blocks designed to buffer salt effects (gypsum accomplishes this)
- works well between field capacity and WP