Lecture 14 Soil Water (1) Soil Properties Basic Soil Properties Soil Water Storage Soil Water Forces (Potential)

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  • Lecture 14 Soil Water (1)Soil Properties Basic Soil Properties

    Soil Water Storage

    Soil Water Forces (Potential)

  • General Comments Only 0.17% of all fresh water in the world, but very important for plant growth and food production. Short residence time of about 3 months, that effectively quadruples its annual contribution to water cycle.

  • Hydrological Processes in Soil Capillary Rise Infiltration Percolation Runoff

  • Basic Soil Physical Properties Soil system: consists of three phases: solid phase comprising the mineral and organic particles of the porous medium gaseous phase of soil air in the poresliquid phase of soil water A. Wild, 1995. Soils and the Environment: An Introduction. Cambridge University Press

  • Soil Texture Basic Soil Physical PropertiesTexture: size distribution of the mineral particles in the soil. Three basic soil textural types are: clay (small particles), silt (medium particles), sand (large particles) (Figure 2.1 AW) A. Wild, 1995. Soils and the Environment: An Introduction. Cambridge University Press

  • Terms to RememberBasic Soil Physical PropertiesTexture: size distribution of the mineral particles in the soil. Three basic soil textural types are: clay (small particles), silt (medium particles) and sand (large particles) (Figure 2.1 AW)

    Structure: the aggregation of these particles

    Porosity: the percentage of pore space occupied by either air of water in the soil. It is determined mostly by soil texture and by structure to some extent. So it can be separated into textural porosity and structural porosity

    Permeability: connectedness of pores

    Specific surface: particle area per unit volume. Clays have the largest, and sands have the smallest specific area (Table 6.1 WR)

  • Specific Surface AreaBasic Soil Physical Properties

  • Soil Water StorageTerms to RememberSoil water content: percentage of volume occupied by water in the soil, usually in units of cm3/cm3

    Field capacity: the capacity of soil to hold water under gravity

    Wilting point: minimum water content of the soil at which plants can extract water. It depends on texture. Clays have the highest wilting point while sands have the lowest.

    Available water capacity: field capacity less wilting point

    Water table: depth of soil at which the soil is saturated (Figure 3.1 Jones)

  • Water Table in a Hydrological System

  • Soil Water ForcesCapillarity: surface tension at the interface between soil air and soil water

    Adsorption: electrostatic forces in which the polar water molecules are attached to charged faces of the solids. They occur only very close to the surface of solids.

    Osmosis: forces due to solutes in soil water. Saline water, for example, can have large osmosis that prevents plant roots from extracting the water.

    Gravity: vertical forces exerted to water by gravity of the Earth.

    Pressure: forces that increase below water table

  • Soil water potential The potential energy of soil water relative to that of water in a standard state (elevation, purity, atmospheric pressure, temperature) is the total water potential g is gravitational potentialp is pressure potentialo is osmotic potential

  • Water PotentialWhat is the physical meaning of water potential?It is energy per unit volume.It has the same unit as pressure (bar, mb, Pascal)

    Unit of mass: kg (or g)Units of forces: kg m/s2Units of pressure, i.e., force per unit area: (kg m/s2)/m2= kg/(ms2)=Pascal

    Units of energy (equivalent to force times distance): J=kg m/s2 m = kg m2/s2Units of energy per unit volume (pressure): (kg m2/s2)/m3=kg/(ms2)

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