Keith Goulding, Rothamsted Research, West Common, Harpenden AL5 2JQ, Hertfordshire, England, Elizabeth A. Stockdale, School of Agriculture, Food and Rural Development, Univ of Newcastle upon Tyne, King George VI Building, Newcastle upon Tyne, NE1 7RU, United Kingdom, and Christine A. Watson, Scottish Agricultural College, Crop and Soil Group, SAC, Craibstone Estate, Aberdeen, AB21 9YA, United Kingdom.
Effective nutrient management is essential in organic farm systems. Soluble fertilisers such as ammonium nitrate and superphosphate, which feed the plant directly and are thought to bypass the natural processes of the soil, are not acceptable. Nutrient supplies to crop plants are sustained through recycling, the management of biologically-related processes such as nitrogen fixation by clover and other legumes, and the limited use of off-farm materials. The aim is to achieve as far as possible a closed nutrient cycle on the farm and to minimise adverse environmental impact.Effective waste management is obviously a key to nutrient cycling on organic farms. However complete recycling is limited by the prohibition of the use of sewage sludge because of current concerns over the introduction of potentially toxic elements, organic pollutants and disease transmission. In addition, the current global market, in which food is transported large distances from the farm, results in a significant export of nutrients. These nutrients must be replaced. This paper examines the sustainability of nutrient cycling on organic farms, considering the principles of nutrient cycling in organic agriculture, using data on soil phosphorus and potassium indices and farm nutrient budgets to assess the integrity of nutrient cycling, including some consideration of losses to the environment, and assessing the wider sustainability of organic farming in the context of the limited amount of recycling possible.The data presented suggest some cause for concern over the sustainability of organic systems because of their dependence on feedstuffs and bedding for inputs of phosphorus and potassium, and on the very variable fixation by legumes or imports of manure or compost for nitrogen. Air pollution and net mineralization from soil reserves appear to comprise a large part of the nitrogen supply on some organic farms. Losses of nitrogen from organic systems can also be as large as those from conventional systems. Being dependent on cultivation and the weather, they are even more difficult to limit than those from fertilisers applied to conventional farms. There is some evidence of phosphorus deficiency in soils under organic production. However, with careful management of manure and the effective use of legumes and by using permitted inputs for phosphorus and potassium, nutrients on organic farms can be managed sustainably.Organic and conventional systems both need research to manage the soil-plant system for a more efficient coupling of carbon with other nutrient cycles. This can be viewed in simple terms as the need to build, maintain and better manage soil organic matter, especially the living part of that, the soil microbial biomass. This is plain good practice for many aspects of soil quality as well as nutrient supply, including soil structure and minimising energy use in tillage, and possibly pest and disease control. In particular, research is needed to better understand and manage microbially-mediated processes.