and half of it is due when it is irrigated at expense; if it is irrigated equally by both methods, then three quarters of a tenth is due. If the two methods differ in proportion, then the ruling follows whichever is more beneficial; and when the proportion is unknown, a tenth is due. Zakah becomes obligatory when the grain hardens and the fruit begins to ripen. ........

His statement, “and half of it is due when it is irrigated at expense,” means: when irrigation involves expense.

The evidence for this is the Prophet’s saying: “A tenth is due on that which is watered by the sky and springs, or which draws water through its roots; and half of a tenth is due on that which is irrigated by artificial means.” Narrated by al-Bukhari (1).

That which draws water through its roots is that which is nourished by its roots.

The wisdom behind this is the greater expenditure required for what is irrigated at expense, and the lesser expenditure required for what is irrigated without expense. Thus, the Lawgiver has taken this expense and cost into consideration and has reduced the due amount for what is irrigated at expense.

His statement, “and three quarters of it is due when both methods are used,” means that when something is irrigated equally, half the time at expense and half the time without expense, three quarters of a tenth is due.

An example of this is date palms that are irrigated for half the year at expense and for the other half without expense—that is, during the summer they are irrigated at expense, while during the winter they receive water from rainfall. In this case, three quarters of a tenth is due.

His statement, “If the two methods differ in proportion,” means that we are unable to determine precisely whether each method accounts for half, less than half, or more than half.

His statement, “then the ruling follows whichever is more beneficial,” means that whichever method is more beneficial to the date palms, trees, or crops is the one taken into account. Thus, if their growth through irrigation at expense is greater than their growth when they receive water without expense, then half of a tenth is due, because irrigation at expense is more beneficial to them, and so the ruling is based on it.