Can nutrient cycles be closed in agricultural systems?
All forms of agriculture involve removal of plant nutrients via harvest of plant material. One major criterion for the sustainability of farming systems is the proportion of nutrients recycled back to the soil. Indeed, there is a widespread belief, as proposed by Steiner (1975), that self-sustaining farms are the real core of sound agricultural production. This is reflected in the view that optimal measures to maintain nutrient levels in agricultural soils involve a high degree of recycling of nutrients with any small losses balanced by soil weathering. Traditional agricultural systems in Europe and elsewhere are held up as an ideal in this context. However, examples of nearly self-sustaining farms, or indeed agricultural systems, are rare (Nolte and Werner 1994; Newman 1997). King (1911) described agricultural practices and, in particular, waste recycling in China, Korea and Japan. He concluded that agriculture was being practiced in a far more sustainable form than found in the USA d
