The Prophet (blessings and peace of Allah be upon him) connected the ruling to irrigation, and did not pay attention after that to how the crop is harvested or what takes place before that such as levelling the ground. This is something else that does not have anything to do with zakaah. The Messenger (blessings and peace of Allah be upon him) connected the ruling to something else. And the prescriptions of the Messenger of Allah (blessings and peace of Allah be upon him) are for all of his ummah, the first and the last of them. They are not laws that were only for the people of his own time; rather they were for the people of his own time and for all those who come after them until the Day of Resurrection. Allah, may He be glorified and exalted, knew what would happen in the future of newly-invented machines and their need for fuel when harvesting, planting and so on.
The things mentioned by the questioner having to do with land that is irrigated with rain water do not have any effect on the rate of zakaah; what must be paid is one-tenth, because of the report that the Prophet (blessings and peace of Allah be upon him) said: “On that which is irrigated by rain, springs and the like, one tenth (is due), and on that which is irrigated by artificial means, half of one tenth (is due).” Narrated by al-Bukhaari in al-Saheeh; there are also corroborating reports. This indicates that the Messenger (blessings and peace of Allah be upon him) did not pay attention to anything done after irrigation or planting, or anything done before that.
The ruling is based on the method of irrigation: that which is watered by springs, rivers and rains is subject to complete zakaah of one-tenth: one out of every ten, one hundred out of every thousand, and so on. That which is watered by machines or by using camels or cattle to bring the water and so on, or by sprinklers, is subject to zakaah of half of one tenth, because of the effort and expense incurred to irrigate it. And Allah is the source of strength.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him),