In this book, ‘sustainable’ is taken to mean that a practice can be continued in­definitely. As explained in Chaps. 2 and 3, this severely limits the extent to which geochemically scarce resources which have been formed in slow geological pro­cesses, such as fossil fuels and phosphate ore, can be converted into wastes. Sus­tainability also requires that renewable resources such as fertile soil, soil organic matter, groundwater and nutrients are maintained and retain their quality. Regarding cropping, this in turn leads to preferences for conservation tillage, much improved nutrient recycling and improved water efficiency. In forestry, this leads to a prefer­ence for long rotations and nutrient recycling. Sustainability also limits achievable biomass production. In Chap. 3, biomass production from currently abandoned, in­cluding fallow, agricultural land was estimated to have a sustainable yearly yield of approximately 23-28 EJ, about one order of magnitude below the yield suggested by de Vries et al. (2007).