How should Zakat be calculated when a person owns two separate agricultural plots watered differently?

Primary text

If a man possesses two adjacent fields, one watered using expense (mu'nah) and the other watered without expense, the yields of both fields must be combined to determine if the threshold (nisab) for Zakat has been met. If the nisab is met through combination, the Zakat due is extracted as follows: One-tenth (ushr) is taken from the yield of the field watered without expense, and one-twentieth (nisf ushr) is taken from the yield of the field watered using expense. This method mirrors combining different types of produce, where the obligation is taken from each according to its due rate.