Posted: 06/06/2015 at 13:13
If you (temporarily!) stand a pot in a tray of water, the compost will soak up as much water as it needs. That should take about 30 minutes. Then remove the pot from the tray.
The advantage is that the compost will be thoroughly wetted all the way through which may not happen when you water from above as it can run straight down, particularly if the compost has been allowed to dry out too much.
Note that most plants will die if you permanently leave them in pots sitting in a tray of water.