An irrigation system for irrigating a plant having a root comprises: a valve to begin irrigation from a water source at a first time-point； a moisture sensor to sense moisture at a second, subsequent, time-point, said moisture sensor being located underground at an insertion depth between ground surface and a maximum depth of the root； a water drainage speed calculator to calculate a water drainage speed based on the first time-point, the second time-point, and the insertion depth of the moisture sensor； and a target drainage time calculator to calculate a remaining time for irrigation, based on the calculated water drainage speed and further based on a difference between the insertion depth and the maximum depth of the root.
