It is difficult to produce systems for judging sustainability, despite general enthusiasm for the concept. Here we evaluate the 'capitals' formulation for sustainability, which attempts to bring together the social and the environmental dimensions of the issue, and which has gained wide currency. We concentrate our attention on the 'natural capital' element in this framework, which has apparently been seen as its least problematical component. We use data on soil erosion from a Sahelian agricultural community in Niger. Despite apparently high rates of erosion, we find it difficult to decide whether the system is sustainable (using the capitals or any other framework). It is even dubious whether sustainability is an urgent concern. We caution against imposing yet another poorly formulated set of concepts on this and similar systems.