SOIL WATERSOIL WATERCHAPTER 7

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 electrons in oxygen-hydrogen covalent bonds

http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html

Polarity of WaterPolarity of Water

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

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 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

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

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

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