Field capacity is the water content of a soil once gravity drainage has stopped after saturation, usually a day or two after heavy rain in the field. In containers the equivalent is container capacity: the water a pot holds once it stops dripping after a full watering. Because a pot is shallow and has a perched water table at the bottom, container capacity is wetter than field capacity of the same material in the ground.
In practice
Growers treat container capacity as "100% full". In crop steering, substrate water content is read with sensors or pot weight, and the first irrigations of the day bring the medium back up to near field or container capacity. The dryback is then measured from that point. A pot that holds 6 kg of water at container capacity and is down to 4.8 kg has lost 20%.
Taller containers drain better and hold proportionally more air at container capacity.
Don't confuse with
- Saturation: every pore full of water, before drainage; roots can't breathe for long.
- Permanent wilting point: the dry end of the range, where the plant can no longer extract water.
