Date post: | 29-Jan-2016 |
Category: |
Documents |
Upload: | homer-lawson |
View: | 217 times |
Download: | 0 times |
SOIL WATERSOIL WATER
Functions:plant cells 50-90% waterkeeps turgorseed germination transpiration photosynthesis moves products nutrients available lowers soil strength chemical reactions microbial activity
Water StressWater Stress
Initially, decreased photosynthesis . . .
Continued . . .temporary wilting point
further . . .permanent wilting point
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 electron in oxygen-hydrogen covalent bond
http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html
Water molecule has polarityHydrogen 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
CapillarityCapillarity
Additive force of adhesion and cohesion- can move against force of gravity- small pores conduct capillary water
Soil Water PotentialSoil Water Potential
Work water can doPotential energyTendency of water to flow/move freely in soil
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
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
WATER FILM – WATER POTENTIAL
WATER FILM – WATER POTENTIAL
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
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
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
REFRERENCE POINTS RELATED TO SOIL WATER
REFRERENCE POINTS RELATED TO SOIL WATER
FOUR CATEGORIES OF SOIL MOISTUREFOUR CATEGORIES OF SOIL MOISTURE
Chemically combined . . . unavailable
Hygroscopic . . . unavailable
Gravitational . . . moves downward by gravity
Capillary . . . taken up by plants
WATER RETENTIONWATER RETENTION
Total water-holding capacity and available water-holding capacity are based on soil texture
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
WATER MOVEMENTWATER MOVEMENT
Gravitational flow – moves by gravity• occurs only under saturated conditions• rapid in course soils – large pores• usually percolation through soil profile
SATURATED SOILSSATURATED SOILS
Sandy soil:gravitational water moves rapidly downward
Clay loam:gravitational water retained 2-3 days afterward
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
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
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
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
Effect of Soil Horizons
water flows differently in different textures . . .
stratified layers will slow percolation
Vapor Flow
occurs when water vapor moves from moist to drier soil . . .
- condenses on cooler soil particles- very slow- minimal water moved
Preferential Flow
Saturated soil conditions . . .water enters biopores or other soil channels
Increases infiltration and percolation
May also move pollutants!!!
How Roots Gather WaterHow Roots Gather Water
Governed by Soil Water Potential
Root hairs draw from higher potential regions
Capillary flow moves water
Soil – Plant – Atmosphere continuum
Plants create “unbroken” column of waterDriven by plant transpiration
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
Measuring Soil WaterMeasuring Soil Water
Four methods:
- gravimetric measurements- potentiometers- resistance blocks- neutron probes (mainly research)
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
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 . . .
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
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 . . .
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
Practical Measuring DevicesPractical Measuring Devices
Gravimetric method not very practical management
More useful and practical are . . .
Potentiometers (tensiometers)Resistance Blocks (gypsum blocks)
PotentiometersPotentiometers
- Measure soil moisture potential at given levels- Water exiting tube creates vacuum- Measured by gauge/instrument- Function best at higher potentials
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