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Soil Water (Classification of soil water (Available water capacity…

- - - - Texture
        
        Coarse texture - low AWC as large pores drain easily, few storage pores
        
        Silt loam best - has a range of pore sizes
        
        Clay can have high wilting point reducing AWC
      - Structure
        
        Inter and intra-aggregate pores important
      - Organic matter
        
        Improves structure
        
        Improves water holding capacity
        
        Adsorbs water (but may not be available water)
      - Salt
        
        Affects solute potential (saline soils, fertiliser granules etc.)
- - - - Free water above a reference point has a positive potential
      - Free water below a reference point has a negative potential
    - - Develops due to hydraulic head (below water table) due to overlying weight of water
      - Is always positive or 0
      - Only exists in saturated soil
    - - Water is held strongly by the soil surfaces and within pore spaces
      - Due to adhesion and capillarity in soil matrix - water held under suction
      - Work needs to be done to obtain water from the soil matrix so matric potential is always negative
      - Plants must overcome matric potential to obtain water
      - As soil dries, it becomes harder for plants to extract water as it is held in smaller pores at low potential - matric potential becomes more negative - higher suctions required
      - In unsaturated soils, matric potential may strongly influence the total potential
      - The availability of water to plants is often determined by how tightly water is held in soil and this is determined by matric potential
      - Measurement
        
        Tensiometer (most common method)
        
        Measures how tightly held water is
        
        Porous cup connected via water-filled (saturated) tube to a vacuum gauge
        
        If soil is drier than saturation, water is 'sucked' from porous cup into soil and registers a reading on gauge
        
        In saturated soil, potential on inside and outside of tube are equal - 0 suction
        
        Very useful for irrigation scheduling
        
        Tensiometers restricted to use in 0 to -80 kPa range
        
        Matric potential of -50 -80 kPa often used as trigger of soil dryness for irrigation
      - Suction (h) (cm) x Diameter of min draining pore size = 0.3
    - - Solutes attract water by hydration which reduces the potential energy of the water molecules
      - Osmotic potential of pure water is zero
      - Solute potential is always negative since work is required to obtain the water
      - Not generally a large part of the total potential
  - - - Silt loam has a large range of pore sizes
      - Sandy soils dewater quickly due to large pores - small amount of energy required
      - Structure also important, as pores exist between aggregates
      - Clay has many small pores - dewaters only at large suctions - large amounts of energy required
- - - - Particle size
      - Clay mineralogy
      - Organic matter