Air Filled Porosity (AFP) describes the percentage of air-filled space in a grow mix after excess water has drained away. It tells you how much oxygen is available to plant roots between waterings, not how dry the mix is.
AFP is critical because roots need oxygen to produce energy. When air spaces fill with water for too long, oxygen diffusion slows, root respiration drops, and water and nutrient uptake are impaired, even if moisture is present.
How to interpret AFP values:
- Low AFP means fewer air spaces. The mix stays wetter longer and oxygen returns slowly, increasing the risk of root stress for oxygen-sensitive plants.
- Higher AFP means more air space. The mix re-oxygenates quickly and supports stable root function, especially in low-light indoor conditions where water use is slow.
It’s important to note that AFP is not a watering trigger or a moisture target. A mix with 25–35% AFP is designed to keep roots oxygenated after watering, not to tell you when to water again.
In short, AFP describes how breathable your grow mix is once drained, helping you understand why some plants fail in mixes that stay wet too long even when watering amounts seem reasonable.
See Also
