"A tithe is due as Zakah, on every plant watered by heaven (rain water), springs, or underground water (i.e. watered without effort).While half a tithe is paid on what is watered by irrigation (i.e. machines are used)." Related by Al-Bukhari. Abu Dawud added in his narration, "When it is watered by underground water (close to the surface and keeping the land wet), a tithe is paid, but half a tithe is paid when the land is watered by wheels or animals."