exer 4

• Soil moisture is the water that is held in the spaces between soil particles
• Soil moisture


  • Affects soil formation, structure, stability and erosion
  • Affects the upward and downward movement of water and nutrients
  • Helps in development of horizons
  • Supplies water for living plants and organisms
  • Serves as solvent for plant nutrients
  • Influences soil temperature
• Water


  • Excellent solvent property
  • High specific heat
  • High heat of vaporization and boiling point
  • Less density as ice than as liquid
  • Cohesion-Adhesion property
• Soil properties affecting water availability


  • Texture, structure, and the amount and size of pores (micropores and macropores)
• Ways of soil moisture expression


  • Gravimetric moisture content
  • Volumetric moisture content
  • Percentage pore space
  • Soil water depth
• Gravimetric moisture content


  𝑀𝑤% 𝜃𝑚 = 𝑀𝑀𝑠(𝑀𝑡 − 𝑀𝑠) 𝑥 100 = 𝑥 100
• Volumetric moisture content


  %𝜃𝑣 = %𝜃𝑚 𝑥 𝜌𝑏
• Percentage pore space


  %E = (1 - 𝜌𝑏/𝜌𝑠) 𝑥 100
• Soil water depth


  𝐻𝑤 = 𝜃𝑣 𝑥 𝐻𝑡
• Soil moisture tension


  • A measure of the energy state of water
  • Becomes larger when the amount of water stored is smaller
  • When more pressure (energy) is needed for the plant to extract water from the soil, the value is represented by an increasing number
• Classification of soil water


  • Hygroscopic water (adsorbed water)
  • Saturation
  • Field capacity
  • Permanent wilting point
  • Gravitational water
  • Available water
• Available moisture


  • Amount of water that an amount of soil can store and subsequently release readily for use by the plant
  • Directly related with the total amount of pore spaces of medium size (0.2 - 1.0 microns)
  • Pores > 10 microns: drain water readily
  • Pores < 0.2 microns: retain water
• Capillarity


  • Ability of liquid to flow against gravity
  • Due to adhesion and cohesion forces
  • Macropores are wide enough that water in the pores cannot be held against the force of gravity
  • Micropores are small enough to hold water due to cohesion/adhesion forces
• Soil water retention curve is the relationship between water content, θ, and the soil water potential, bar