Water Potential Calculator

Calculate total water potential and understand water flow in plants and soils. Water flows from higher (less negative) to lower (more negative) water potential. Calculate Ψs from concentration, or find total Ψ from all components.

Negative for solutions

Usually small in plants

Negative for adsorption

Height difference

If Ψs unknown

Affects iCRT

WATER POTENTIAL EQUATION: Ψtotal = Ψs + Ψp + Ψm + Ψg Where (all in MPa): - Ψs = Solute (osmotic) potential - typically negative - Ψp = Pressure potential - positive in turgid cells - Ψm = Matrix potential - negative for adsorbed water - Ψg = Gravitational potential = ρgh ≈ 0.01 MPa/m SOLUTE POTENTIAL: Ψs = -iMRT Where: - i = van't Hoff factor (2 for NaCl, 1 for sugar) - M = molarity (mol/L) - R = 0.008314 MPa·L/(mol·K) - T = temperature (K) WATER FLOW: Water flows from high (less negative) to low (more negative) Ψ
Example 1: Calculate Ψs for 0.5 M NaCl at 25°C Ψs = -iMRT = -2 × 0.5 × 0.008314 × 298 = -2.48 MPa Example 2: Plant cell water potential - Ψs = -1.0 MPa (sugars in cell) - Ψp = 0.5 MPa (turgor) - Ψg = 0 (same height) - Ψtotal = -1.0 + 0.5 + 0 + 0 = -0.5 MPa Soil: Ψ = -0.2 MPa Water flows from soil (-0.2) to cell (-0.5)!

What is water potential?

Water potential (Ψ) measures the potential energy of water per unit volume—the tendency of water to move from high to low potential. Measured in megapascals (MPa) or bars. Pure water at standard conditions has Ψ = 0 MPa. Adding solutes makes Ψ negative (lower potential). Water flows from higher (less negative) to lower (more negative) potential.

How does water potential affect plant growth?

Plants need water potential higher in soil than in roots to take up water. Ψsoil around -0.01 to -0.5 MPa is ideal; below -1.5 MPa plants stress. Transpiration creates negative pressure in xylem, pulling water up. When Ψleaf < Ψair, plants lose water faster than they take it in, causing wilting.

What are the components of water potential?

Ψtotal = Ψs + Ψp + Ψm + Ψg: Solute (Ψs) is always negative due to dissolved ions. Pressure (Ψp) is usually positive in plant xylem (turgor). Matrix (Ψm) is negative in soils from hygroscopic water adhesion. Gravitational (Ψg) is ρgh, positive upward.

How does solute concentration affect water potential?

More solutes = more negative Ψs = lower total Ψ = water flows into solution. The equation: Ψs = -iMRT, where i is van't Hoff factor, M is molarity, R is gas constant, T is Kelvin temperature. At 25°C, 1 M NaCl gives Ψs≈-4.8 MPa because NaCl dissociates into 2 ions.

Why is water potential important for cell function?

Cell turgor (Ψp) keeps plants upright. Ψp drops when water potential in environment drops (drought stress). At Ψ = -0.8 to -1.0 MPa, many plants wilt. Osmotic adjustment (accumulating solutes) keeps Ψs more negative, maintaining Ψ gradient for water uptake.

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