Soil Water3 PhasesSolidLiquidGasBent MoleculeNeutral, but POLARPolarity controls movement and retention in soil2 Stages of Water Movement1) Infiltration2) Subsurface Water FlowFigure 5.8Forms of Water Potential – ΨΨGGravitational Potential (+ value)- determined by the height of water above a reference point- water flows downward under gravityΨMMatric Potential (- value)- determined by the strength of the attraction of water to the soil matrix- most important for unsaturated flow (dry soils)- water flows from zones of wet soil to zones of dry soil (less negative more negative)ΨSSolute (Osmotic) Potential (- value)- determined by the concentration of solutes in the soil water- more negative for high solute concentrations- water flows from zones of low solute to high solute concentrations- most important for root-water interactionΨPPressure Potential (+ or – value)- determined by amount of pressure imposed on soil water- usually zero, but there are exceptionsFlooded SoilsSoils with water tables – ie artesian wellsFigure 5.82 Stages of Water Movement1) Infiltration2) Subsurface Water FlowInfiltration Rate ~ cm/hrin generalInfiltration rate = hydraulic conductivity xHydraulic Conductivity is:a measure of characteristics unique to each soil type which affects water flowit is a function of:1) soil water content2) size distribution and tortuosity of pores∆Ψ∆ΧFigure 5.27Figure 5.252 Stages of Water Movement1) Infiltration2) Subsurface Water FlowSaturated Flow- occurs when pores are water-filled- most rapid in larger pores (e.g. sands)- flow in response to potential gradients:Gravity (ΨG) & Pressure (ΨP) - water drains freely from large pores by gravity and pressure Ψ’sUnsaturated Flow- more common that saturated flow- more important than saturated flow?- controlled by Matric Potential (ΨM)- movement from areas of less negative to more negative potentialCapillary RiseCapillarity is the tendency for water to rise against the gravity Function of Water Adhesion and CohesionFigure 5.6Figure 5.8Figure 5.27Figure 5.65.24Water Movement vs. Water StorageThe soil matrix retains water against the pull of gravity - this is controlled by the matric potential (ΨM)- and is a function of Capillarity – important in wet soilsAdsorption – important when soil is below field capacityFigure 5.2Figure 5.12Figure 5.34Figure 5.35Solute Potential (ΨS)- force that acts across a membrane – ex. root cell membrane- due to hydration of ions- important in saline soilsEffect of is to reduce the free energyHydration – attraction of ions to