The aim of sustainable heavy-metal management in agro-ecosystems is to ensure that the soil continues to fulfil its function in agricultural production, in environmental processes such as the cycling of elements, and as a habitat of numerous organisms. Assessment of sustainability has to be carried out in time, since metal accumulation in soil is largely irreversible and may cause problems if certain concentration levels are exceeded. In this study, we provide a concept to assess the sustainability of current metal cycles in agro-ecosystems based on dynamic heavy-metal balances for the plough layer. After presenting some general aspects of dynamic metal balances, we introduce 'sustainability indices'. These characteristic numbers can be used as indicators for potentially adverse effects of current agricultural practices, since they account for (ecotoxicologically founded) soil quality standards and for quality standards for produce, groundwater and surface water. They can also be used to assess the effects of different management options that aim to prevent quality standards from being exceeded as they provide insight in the dynamics governing input-output relationships. This is illustrated with a case study on heavy-metal flows in different arable farming systems.