The Prophet ﷺ said, "On crops watered by the sky, by springs, or that draw water on their own, the tithe is due. On crops watered by irrigation, half the tithe is due."
عن النبي ﷺ أنه قال: فيما سقت السماء والعيون أو كان عثريا العشر وما سقي بالنضح نصف العشر.
Bukhari narrated it in the Book of Zakat (1483) from Saeed bin Abu Maryam, who said, Abdullah bin Wahb narrated to us, he said, Yunus bin Yazid informed me, from Zuhri, from Salim bin Abdullah, from his father, may Allah be pleased with him, and he related the hadith.In some of its routes of transmission, from the hadith of Ibn Wahb himself, it reads, "For what is watered by the sky, and the rivers, and the springs, or what is bal, a tenth is due, and for what is watered by sawani or by nadh, half a tenth is due." Abu Dawud (1596) and others narrated it.Regarding his saying, "or was athari," the athari crop is that which drinks through its roots without being irrigated. Its derivation is from al-aathur, which is the channel through which water flows, because the one walking stumbles in it. This also includes what drinks from rivers without any labor, or what drinks through its roots, as when it is planted in land where the water lies close to the surface, so the roots of the tree reach it and it has no need of watering. See Fath al-Bari (3/349).Regarding his saying, "bal," Abu Dawud reported from Waki that al-bal is the crop that grows on its own from rainwater. He also reported from Abu Iyas Asadi, who said it is that which is watered by rainwater. And he reported from Nadr bin Shumayl that al-bal is rainwater. End of quote.The gist of it is that it is what drinks through its roots, without effort being spent in watering it. As for nadh, it is watering by means of a rope and bucket.From what has been explained above, it is understood that the Prophet ﷺ set the charity due on what required little effort and yielded abundant benefit at the doubled rate, as a generosity toward the poor, and set what required much effort due at half that rate, as leniency toward the owners of wealth.As for al-sawani, it is the plural of al-saniyah, which is the camel used for drawing water, that is, for irrigation.