Calculating the Zakah on the value of the land at the time of sale

Q: I own many plots of land. Some of them have been in my possession for ten years now while other plots have only been a year or two. I do not pay Zakah (obligatory charity) on these plots until I sell them. I calculate the Zakah on the value of the land at the time of sale. Is this method, which I use, right? What should I do if this method is wrong? What about the Zakah of the past years? Please, advise me. May Allah reward you with the best!

A: A person who owns a commercial commodity that equals the Nisab (the minimum amount on which Zakah is due) and a full Hawl (one lunar year calculated from the time a property reaches the minimum amount upon which Zakah is due) passes from the time when he owns it for trade, he should evaluate it by the end of the Hawl. If it equals the Nisab, he must pay 2.5% of its value as due Zakah. Thus, you must pay the Zakah due on the land in your possession on this basis. The Zakah paid every year should be calculated upon the value of the land of this very year.May Allah grant us success. May Allah's Peace and Blessings be upon Prophet Muhammad and upon his family and Companions!