Meta-analysis of nutrient budgets in organic farms across Europe

Nutrient supply to organic farms is a highly discussed topic in Europe, due to the restricted availability of external fertilizer resources and the use of contentious inputs. To optimize the flow of nutrients throughout the organic farming system, it is firstly necessary to obtain valid data on the nutrient status of organic farms. Nutrient budgets are a valid tool to investigate the nutrient demand or surplus of a system. However, there is currently no comprehensive overview of nutrient budgets of European organic farms. We therefore carried out a meta-analysis on 56 individual studies that reported either farm-gate or soil-surface budgets. The analysis showed an imbalance between nutrients, a general surplus of nitrogen (45 kg N ha-1 year-1 [95%-confidence interval (CI): 30, 61]), magnesium (16 kg Mg ha-1 year-1 [-9, 40]) and sulphur (45 kg S ha-1 year-1 [-29, 118]), a balanced phosphorus budget (0 kg P ha-1 year-1 [-2,2]), and a deficit for potassium (-12 kg K ha-1 year-1 [-21, -3]). We observed large differences between farms that could be partly explained by farm type and budgeting method. Arable and mixed farms showed lower nitrogen, phosphor, magnesium and sulphur budgets than dairy/beef farms or even vegetable farms, while all farm types besides dairy/ beef farms showed deficits for K budgets. Further, farm-gate budget studies yielded higher budgets than soil surface budgets. Variations between studied countries could also be detected, but the coverage and comparability is low due to differences in studied farm types and budgeting method.