As one is forced by the uninterrupted growth of the population to clear and cultivate less fertile tracts of land, so the harvesting of these new units of wheat requires more and more labour: either because the land must be ploughed more deeply, or because a greater surface must be sown, or because more fertilizer is needed; the cost of production is thus much higher for these later harvests than it was for the first ones, which were obtained originally from rich and fertile lands.