The rent may increase, because the price of the product of the land rises, in which case the rate of differential rent always rises, whether the rent on the worst cultivated soil be large, small or non-existent. By rate we mean the ratio of that portion of surplus-value converted into rent to the invested capital which produces the agricultural product. This differs from the ratio of surplus-product to total product, for the total product does not comprise the entire invested capital, namely, the fixed capital, which continues to exist alongside the product. On the other hand, it covers the fact that on soils yielding differential rent an increasing portion of the product is transformed into an excess of surplus-product. The increase in price of agricultural product of the worst soil first creates rent and thereby the price of land.
The rent, however, may also increase without a rise in price of the agricultural product. This price may remain constant, or even decrease.
If the price remains constant, the rent can grow only (apart from monopoly prices) because, on the one hand, given the same amount of capital invested in the old lands, new lands of better quality are cultivated, which merely suffice, however, to cover the increased demand, so that the regulating market-price remains unchanged. In this case, the price of the old lands does not rise, but the price of the newly cultivated lands rises above that of the old ones.
Or, on the other hand, the rent rises because the mass of capital exploiting the land increases, assuming that the relative productivity and market-price remain the same. Although the rent thus remains the same compared with the invested capital, still its mass, for instance, may be doubled, because the capital itself has doubled. Since no fall in price has occurred, the second investment of capital yields a surplus-profit just as well as the first, and it likewise is transformed into rent after the expiration of the lease. The mass of rent rises here, because the mass of capital producing a rent increases.