Water Holding Capacity (WHC)

What WHC is

Water holding capacity describes the amount of water a soil or substrate can retain against gravity after drainage.

It is usually expressed as: - % by volume - % by weight

Why WHC matters

WHC determines: - Irrigation frequency - Root oxygen availability - Nutrient buffering - Drought resilience

Too little WHC leads to rapid drying.
Too much WHC leads to poor aeration.

WHC vs availability

Not all held water is plant-available: - Some water drains freely - Some is held too tightly for roots to extract

The balance between water and air is critical.

Typical behaviour

• Sands: low WHC, high drainage
• Clays: high WHC, slow drainage
• Organic substrates: very high WHC, variable aeration

Agronomic note

Optimal WHC is crop- and system-specific — there is no universal “best” value.