The sustainability of agricultural systems as the source of almost all food materials is essential for the continuance of human society. The ability of agriculture to meet the food demands of an increasing population has been questioned since 1800 (Malthus, 1803), on the assumption that technical improvements must ultimately encounter some fundamental biochemical limits. Despite this, the history of World, and European production since the 1940s has shown steady but continuous improvement, with no sign of a slackening in the rate of increase (Open University, 1987).