Air-filled porosity is the percentage of a medium's total volume occupied by air at container capacity, after free drainage. It sits alongside water-holding capacity: the pores in a pot are shared between water and air, and roots need both. Coarse materials such as perlite, bark, coco chips and clay pebbles raise it; fine peat and fine coco lower it.
In practice
Container media for most crops perform well at roughly 10-30% air-filled porosity after drainage. Below about 10% roots are prone to oxygen shortage and root rot; very high values dry quickly and need frequent irrigation. Pot height matters too: a short, wide pot holds a deeper saturated layer at its base relative to its volume, so the same mix has less air in a shallow pot than a tall one.
This is why adding perlite helps overwatered soil grows and why coco and rockwool can be irrigated many times a day.
Don't confuse with
- Total porosity: all pore space, air plus water.
- Water-holding capacity: the water side of the same split.
Field capacity is the amount of water a medium holds after it has been saturated and allowed to drain freely; in pots it is called container capacity.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Perlite is a light, white, puffed volcanic glass added to growing mixes to increase drainage and air space; it is chemically inert and holds no nutrients.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Clay pebbles (LECA) are fired, expanded clay balls used as an inert hydroponic medium that drains freely, holds a little water and anchors roots.Decay of roots by pathogens such as Pythium, Fusarium, Rhizoctonia or Phytophthora, usually triggered by low oxygen, heat and waterlogging around the roots.Overwatering is watering so often that the medium stays saturated, starving roots of oxygen, which causes drooping, slow growth and root disease.Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.
