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

The assumption here implies that the market-price is regulated as before by the capital invested in the worst soil A.

I. If the additional capital invested in any one of the rent-bearing soils – B, C, D – produces only as much as the same capital upon soil A, i.e., if it yields only the average profit at the regulating price of production, but no surplus-profit, then the effect upon the rent is nil. Everything remains as before. It is the same as though an arbitrary number of acres of A quality, i.e., of the worst soil, has been added to the cultivated area.