David Ricardo Theory of Rent:

Rent is differential surplus. It’s the excess of the output from better land and the output from the least productive land being used (marginal land).
Assumptions:
Fixed land supply:
Ricardo took it that the amount of land is fixed and could not be altered in either direction.
Land differences:
Lands are different in terms of fertility and location and some are more productive than others.
Perfect competition
There is competition between farmers and landowners for renting land, and between landowners and farmers to rent land.
Law of diminishing returns:
When more labor and capital are used to a tract of land, then ultimately the extra production will fall.
Marginal land:
A “no-rent land” or marginal land was identified by Ricardo, wherein output is just enough to pay for cultivation, and therefore, no rent is given.
How it works:
When population is small, only the best of the land is tilled and little or no rent is paid.
As population and food demand rise, less productive lands are cultivated.
More fertile land owners are able to pay more rents since their land will yield more with the same input than the marginal land.
The differential output between the more productive land and the marginal land will decide the rent.
In effect, Ricardo’s theory purports that rent is not responsible for high food prices, but a result of the variations in the fertility of the land and the growing demand for food due to population increases.