A number of commentaries over the last several years have reminded us of the importance of agriculture as the foundation of human civilization. In this article, David R. Montgomery, professor of earth and space sciences at the University of Washington, examines the conflicting trends of long-term population growth and the likely end of improvements in per-acre agricultural yield. Even as continued population growth removes ever more land from agricultural production, we are probably reaching the limits of our ability to improve agricultural productivity. Interesting and ultimately hopeful perspective based in the notion of "agroecology". The author's book Dirt: The Erosion of Civilizations is forthcoming from the University of California Press.