"It is He who produces gardens, with trellises and without, and dates, and tilth with produce of all kinds, and olives and pomegranates, similar (in kind) and different (in variety): eat of their fruit in their season, but render the dues that are proper on the day that the harvest is gathered. But waste not by excess: for Allah loves not the wasters." [Al-Quran 6:141]
Further, Salim bin 'Abdullah narrated from his father that the Prophet said, "On a land irrigated by rain water or by natural water channels or if the land is wet due to a near by water channel Ushr (i.e. one-tenth) is compulsory (as Zakat); and on the land irrigated by the well, half of an Ushr (i.e. one-twentieth) is compulsory (as Zakat on the yield of the land)." (Bukhari, Vol.2, Book 24, No. 560)
The amount of fruits and grains that require Zakat is five Wasqs, each of which is 60 sa', and a Sa' is almost 3 kg. Hence, the minimum Nisab that requires Zakat on fruits and grains is 300 Sa' (or approximately 900 kg). If the crop is watered without much effort, such as by spring water or by rain, it requires one-tenth (10%) as Zakat payment. This means that half of Wasq (90kg) is due for every five Wasqs. If such produce is watered with effort, such as from wells or irrigation channels, it requires half of a tenth (5%) as Zakat payment. This means for every five Wasqs, there is a quarter of a Wasq (45kg) due for Zakat.