Department of Plant Sciences

Long-term experiments started before sustainability became the hot issue it is at present. As a consequence the designs of these experiments were not directed, at least not explicitly, towards solutions of sustainability questions. Nevertheless, long-term experiments may provide, often after re-interpretation, important and sometimes surprising insights in the current research problems. The subject of sustainable agriculture does not only refer to continuing soil productivity, but also to the impacts of farming on the environment. In this paper, however, the discussion is narrowed down to the following questions: do agricultural practices inevitably threaten the sustainability of soil productivity; what is the role of plant nutrients in sustained productivity; what can we learn from the long-term experiments carried out for different purposes than the study of sustainability.

In this paper we try to analyze the outcomes of three long-term trials, carried out in different ecological settings in Kenya, Vietnam, and The Netherlands. For that purpose we apply two simple models, one on formation and decomposition of soil organic matter, and another on the residual effect of fertilizer P, and we make use of some rather recently introduced concepts with regard to soil fertility and required nutrient inputs, balanced plant nutrition, and optimum nutrient management. The general objective of the analysis is to arrive at general insights into the main requirements of sustainable soil productivity. The fact that the studied field trials could exist for a long time does already preclude situations, where processes like erosion, salting-up, acidification and the like, obviously make soil productivity unsustainable.