Air-Filled Porosity (AFP)

What AFP is

Air-filled porosity describes the percentage of pore space occupied by air after drainage.

It represents the amount of oxygen available to roots and soil microorganisms.

AFP is typically expressed as: - % by volume

Why AFP matters

Roots require oxygen for respiration.
Insufficient AFP leads to: - Reduced root growth - Poor nutrient uptake - Accumulation of toxic metabolites - Increased disease pressure

Plants usually experience stress before visible wilting occurs.

Typical guidance values

• Mineral soils: >10–15% after drainage
• Container substrates: 20–30% is often targeted
• Highly sensitive crops may require higher AFP

AFP vs WHC

• High WHC does not guarantee good aeration
• AFP and WHC must be balanced
• Excessive water retention reduces AFP

Agronomic note

Plants respond more strongly to oxygen limitation than to moderate water stress.