soil water


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SOIL WATER. Functions: plant cells 50-90% waterkeeps turgor seed germinationtranspiration photosynthesis moves products nutrients available lowers soil strength chemical reactions microbial activity. Water Stress. Initially, decreased photosynthesis . . . Continued . . . - PowerPoint PPT Presentation


  • SOIL WATERFunctions:plant cells 50-90% waterkeeps turgorseed germinationtranspiration photosynthesis moves products nutrients available lowers soil strength chemical reactions microbial activity

  • Water StressInitially, decreased photosynthesis . . .

    Continued . . .temporary wilting pointfurther . . .permanent wilting point

  • Forces on Soil WaterGravitational 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

  • Water molecule has polarityHydrogen of one molecule attracted to oxygen of another molecule in a hydrogen bond accounts for cohesionHydrogen bond between hydrogen of water and oxygen of silica (SiO2) accounts for adhesionAdhesion water is very tightly held!!!Cohesion water can move and is available for use

  • Capillarity

    Additive force of adhesion and cohesion- can move against force of gravity- small pores conduct capillary water

  • Soil Water PotentialWork 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 stateThe lower the soil water potential the more tightly water is adsorbed to soil particles


    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


  • Three Forces of Water PotentialGravitational potential energy due to gravity positiveMatric most common force; effect of soil on waternegativeOsmotic special case of salty soilsnegativeTotal water potential is sum of three forces

  • Units of PotentialOfficial 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 WATERGravitational at saturation, will drain from larger pores within 24 to 48 hours in well-drained soilsAvailable can be absorbed by plants; held between gravitational water and wilting pointCohesion held between gravitational and adhesion (hygroscopic) waterHygroscopic held tightly by soil particles; air dry



    Chemically combined . . . unavailable

    Hygroscopic . . . unavailable

    Gravitational . . . moves downward by gravity

    Capillary . . . taken up by plants

  • WATER RETENTIONTotal 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 LoamOrganic matter influences water-holding capacityIncreases amount of available water


    Gravitational flow moves by gravityoccurs only under saturated conditionsrapid in course soils large poresusually percolation through soil profile


    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


    Unsaturated flow lateral movement; capillary flowdepends on unbroken films of water spreading through connected capillary poresmoves from moist to dry soilcan move in any direction

  • WETTING FRONTA distinct line where water is moving in soil Wet behind, Dry aheadSoils must be nearly saturated in order for the front to advance; Why?Dry soil cannot pull the water deeperAll the soil must be wet in order for the front to advance

  • CAPILLARY RISEUpward movement of water from higher to lower potentialsExplains evaporation of water from soil to atmosphereContinuation of capillary rise when entire soil column driesBoundary in soil serves to protect from further lossesUnsaturated flow only moves over short distancesSaturated 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 channelsIncreases infiltration and percolation

    May also move pollutants!!!

  • How Roots Gather WaterGoverned 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 RemovalPlants will use water near the surface first

    Oxygen is highest . . . Respiration drives uptake

    As surface dries, plant roots grow deeper . . .absorption shifts downwardIf surface is rewetted, absorption shifts upward

  • Measuring Soil WaterFour methods:

    - gravimetric measurements- potentiometers- resistance blocks- neutron probes (mainly research)

  • Gravimetricmeasures soil water content by weightwater content = moist wt dry wt dry wtExample: soil sample at field capacity 162 grams dry sample 135 gramswater content = 162g 135g = .20 135g

  • Volume BasisMore 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 BasisMeasures 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 DevicesGravimetric method not very practical management

    More useful and practical are . . .

    Potentiometers (tensiometers)Resistance Blocks (gypsum blocks)

  • Potentiometers

    Measure soil moisture potential at given levelsWater exiting tube creates vacuumMeasured by gauge/instrumentFunction best at higher potentials

  • Resistance BlocksMeasure 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