Plant-available water (PAW) is water held between field capacity and wilting point. In which soil condition is PAW generally higher?

• A. Sands
• B. Gravelly soils
• C. Compact clays
• D. Well-structured clays and loams

Answer: (D)

Plant-available water is the water held in the soil between field capacity (water remaining after gravity drainage) and wilting point (water too tightly bound for roots to extract). The amount available depends on the soil’s pore-size distribution and how well the soil is structured. Well-structured clays and loams have a good mix of pore sizes: macro-pores that drain to field capacity and micro-pores that hold moisture more tightly but remain accessible to roots. This balance means there can be substantial water retained in the micro-pores that roots can still extract, while the macro-pores keep things aerated and drainage remains adequate, resulting in a relatively large difference between field capacity and wilting point.

Sands and gravelly soils lack sufficient water storage and have large pores with rapid drainage, so they hold relatively little water between FC and WP. Compact clays, although they can hold a lot of water, trap it more tightly in very small pores and poor structure limits root access, reducing PAW. Therefore, the soil condition with the highest plant-available water is well-structured clays and loams.