Zakah is due on wealth, and you owe zakah on the land you own: If it is agricultural land, then zakah is due on the harvests and not on the land itself. If, on the other hand, they are bought and sold then the Zakah is calculated on them like wares for sale. In other words, you need to estimate the value of the land and take out zakah on them every year. If any of these pieces are kept for you to build a house for your personal use, then there is no zakah on it.
Zakah is levied at the rate of 2.5% of the total value of the land every year if they are like merchandise or wares of trade.