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

[A rising price of production presupposes that the productivity of the poorest quality land yielding no rent decreases. The assumed regulating price of production cannot rise above £3 per quarter unless the £2 1/2 invested in soil A produce less than 1 qr, or the £5 – less than 2 qrs, or unless an even poorer soil than A has to be taken under cultivation.

For constant, or even increasing, productivity of the second investment of capital this would only be possible if the productivity of the first investment of capital of 1/2 had decreased. This case occurs often enough. For instance, when with superficial ploughing the exhausted top soil yields ever smaller crops, under the old method of cultivation, and then the subsoil, turned up through deeper ploughing, produces better crops than before with more rational cultivation. But, strictly speaking, this special case does not apply here. The decrease in productivity of the first £2 1/2 of invested capital signifies for the superior soils, even when the conditions are assumed to be analogous there, a decrease in differential rent I; yet here we are considering only differential rent II. But since this special case cannot occur without presupposing the existence of differential rent II, and represents in fact the reaction of a modification of differential rent I upon II, we shall give an illustration of it [see Table VII – Ed.].