Water potential
Water potential
Water potential is the energy of water per unit volume, relative to pure water under reference conditions. It tells us which way water will move: from areas of higher potential to areas of lower potential when there’s nothing restricting the flow. Water potential combines several driving forces, including osmosis, gravity, pressure, and how water interacts with surroundings like soil.
Main components of water potential
- Osmotic (solute) potential: dissolved substances lower the water’s energy, making the potential more negative.
- Pressure potential: mechanical pressure adds to the potential; it’s usually positive inside plant cells (helping keep plants turgid) and can be negative in some plant vessels.
- Matrix (capillary) potential: water’s attraction to solid surfaces (like soil particles) creates negative potential, especially in unsaturated soils.
- Gravitational potential: water at higher elevations has higher potential energy, influencing movement uphill or downhill.
- Humidity/other factors: in some contexts, humidity-related effects are considered as part of water potential.
How it works
- Total water potential is the sum of its components. Water tends to move from regions of higher total water potential to lower total water potential until differences are balanced.
In soils and plants (quick examples)
- Saturated soil: about 0 kPa (no tension).
- Field capacity: around -33 kPa.
- Permanent wilting point: around -1500 kPa.
- Dry air: very negative, around -100 MPa in many conditions.
- For plants, water moves from soil (higher Ψ) up through roots and stems to leaves, then to the atmosphere when the gradient favors it.
Measurement in the field
- Tensiometers, gypsum blocks, neutron probes, time-domain reflectometry (TDR), and simple weight measurements can estimate soil water potential.
Why it matters
- Water potential integrates all drivers of water movement, helping explain how water moves through soil, plants, and ecosystems. It predicts uptake by roots, turgor in cells, and water loss to the atmosphere.
This page was last edited on 1 February 2026, at 20:28 (CET).