Soil Water Content and Potential

Soil water content measures how much water a soil holds, while soil water potential measures the energy state of that water, together describing both the quantity and the availability of water in soil.

Definition

Soil water content is the amount of water in a soil expressed as a mass or volume fraction; soil water potential is the potential energy of that water per unit quantity relative to free water, dominated in unsaturated soil by the matric potential arising from capillary and adsorptive forces.

Scope

This topic covers volumetric and gravimetric water content, the components of soil water potential (matric, gravitational, and osmotic), the soil water retention curve, and the agronomic reference points of field capacity, permanent wilting point, and plant-available water. It distinguishes how much water is present from how tightly it is held.

Core questions

• How is soil water content measured and expressed?
• What are the components of soil water potential?
• What does the water retention curve relate, and why does it differ among soils?
• What do field capacity, wilting point, and plant-available water mean?

Key concepts

• Volumetric and gravimetric water content
• Matric, gravitational, and osmotic potential
• Water retention curve
• Field capacity
• Permanent wilting point
• Plant-available water

Key theories

Soil water potential concept

The energy state of soil water is the sum of matric, gravitational, and osmotic potentials; water flows from higher to lower total potential, and plant availability declines as matric potential becomes more negative.

Water retention (characteristic) curve

The relationship between water content and matric potential, governed by pore-size distribution, defines how much water a soil holds at a given energy and underlies field capacity, wilting point, and available water capacity.

Mechanisms

Water is held in soil by capillary forces in pores and by adsorption on particle surfaces, both stronger in fine-pored, fine-textured soils. As a soil dries, water remains only in progressively smaller pores and thinner films, so the matric potential becomes more negative and the remaining water is held more tightly. Field capacity is the content after free drainage, the wilting point the content below which plants cannot extract water, and the difference is the plant-available water.

Clinical relevance

Knowing water content and potential lets growers schedule irrigation, estimate how long a crop can go between waterings, and avoid both drought stress and waterlogging; the same concepts guide drainage design and the interpretation of soil moisture sensors.

History

Buckingham introduced the capillary potential of soil water in 1907, and subsequent work formalized the water retention curve and the field capacity and wilting point concepts, giving agronomy and soil physics a quantitative basis for describing soil moisture.

Key figures

• Daniel Hillel
• Edgar Buckingham
• Lorenzo A. Richards

Seminal works

• hillel1998
• brady2016

Frequently asked questions

What is the difference between field capacity and wilting point?

Field capacity is the water a soil retains after gravity has drained the largest pores, the upper limit of readily stored water, while the permanent wilting point is the drier limit at which the remaining water is held too tightly for plants to extract; the water between them is plant-available.

Can a soil have plenty of water yet still stress plants?

Yes. If water is held at very negative potential, as in dry clay or saline soils, plants may be unable to extract it despite measurable water content, because availability depends on the energy state of the water, not just its amount.