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Soil moisture tension and related terms

Short answer

Soil moisture tension is the suction force (≈1/3 atm at FC, 15 atm at PWP). Osmotic pressure from salts adds to total stress. FC is the upper limit, PWP is the lower limit; available moisture = FC – PWP.

Explain soil moisture tension, osmotic pressure, field capacity, wilting point and available moisture.

  • Soil moisture tension (suction): the force (negative pressure) with which water is held by the soil; the energy per unit mass that a plant has to apply to take water from soil. It is measured in atm, kPa or pF and increases as the soil dries (about 1/3 atm at FC and 15 atm at PWP).
  • Osmotic pressure: pressure caused by dissolved salts in soil water, which opposes the entry of water into root cells. A saline soil has high osmotic pressure, so total water stress for the plant is the sum of soil moisture tension and osmotic pressure.
  • Field capacity: moisture content retained after free gravity drainage; tension about 1/3 atm; upper limit of available moisture.
  • Permanent wilting point: moisture content at which plants wilt permanently; tension about 15 atm; lower limit of available moisture.
  • Available moisture: the water between FC and PWP that is usable by plants: AM=FC−PWPAM = FC - PWP.

✓ Answer checked by Claude Opus 5.5 (high effort)

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