An idea from On the Principles of Political Economy and Taxation

Rent starts when the best land runs out. Say three fields, farmed with the same work, grow 100, 90 and 80 measures of wheat. Once the 90 field is needed, the best field can charge 10 in rent. Once the 80 field is needed, it charges 20.

From On the Principles of Political Economy and Taxation by David Ricardo