Praise be to Allah.
The Messenger (blessings and peace of Allah be upon him) explained the nisab for grains and crops, which is five wasqs, and a wasq is sixty saa‘s. So the nisab is three hundred saa‘s measured by the saa‘ of the Prophet (blessings and peace of Allah be upon him), of dates, grapes, and grains such as corn, barley, rice and so on. The saa‘ of the Prophet (blessings and peace of Allah be upon him) is equal to two handfuls scooped up by someone with hands of average size, and each handful is equivalent to a mudd. This is the amount of a saa‘. In terms of weight it is four hundred and eight (480) mithqaal, and a mudd is one hundred and twenty (120) mithqaal, of grains that are of an average size, neither very heavy nor light. [So the nisab is equal to approximately 612 kilograms]
If the grains are irrigated by rainfall and rivers, then (the rate of zakah) is one tenth, or one hundred saa‘s from every thousand saa‘s, and so on. If they are irrigated by mechanical means or using animals (to bring water) such as camels and so on, then one-half of one-tenth is due, i.e., fifty from every thousand. The same ruling also applies to dates. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baz (may Allah have mercy on him)