Capital, Volume Three: The Process of Capitalist Production as a Whole by Karl Marx

In the analysis of differential rent we proceeded from the assumption that the worst soil does not pay any ground-rent; or, to put it more generally, only such land pays ground-rent whose product has an individual price of production below the price of production regulating the market, so that in this manner a surplus-profit arises which is transformed into rent. It is to be noted, to begin with, that the law of differential rent as such is entirely independent of the correctness or incorrectness of this assumption.

Let us call the general price of production, by which the market is regulated, P. Then, P coincides with the individual price of production of the output of the worst soil A; i.e., its price pays for the constant and variable capital consumed in production plus the average profit ( = profit of enterprise plus interest).