The most severe challenges to sustainable development occur where many poor people struggle to eke out a living from marginal lands. In some cases, high human populations on fragile lands have led agricultural productivity to deteriorate (García-Barrios and García-Barrios, 1990, Mink, 1993, Zimmerer, 1993), but likewise intensification in some locales has led to sustainable productivity increases (Boserup, 1965, Tiffen, et al., 1994). These mixed results beg closer inquiry, in order to understand how contrary outcomes can come about. For the context of Peru's chilly high plain surrounding Lake Titicaca, this paper examines changes in the stock of natural capital in agricultural soils, how that came about, and what policy tools might contribute to sustaining this key natural capital stock and the agricultural productivity that it enables.