The French legislation for the management of potentially polluted sites requires that " the state of the soils of the site under study" be compared to the state of neighbouring natural soils. That is why diverse institutions try to find local data relating to trace metal concentrations in the soil (often called 'background values'), to use as points of comparison and thereby estimate the contamination level of the studied soil. However, in industrial or urban context, it is very difficult to find "natural soils" located in the vicinity. More often than not, these assessments can at best be carried out with reference to soils considered to be relatively little-contaminated, namely the closest agricultural soils. In France, a large data collection was carried out on a national scale. These data were provided by analyses made in ploughed topsoils, undertaken mainly within the framework of sewage sludge spreading plans. The data collected on the area of the region Centre have been processed and stratified according to "agricultural districts", territories much less heterogeneous from geological and pedological standpoints than administrative subdivisions. The usual measures of position (percentiles) and of dispersion (upper whiskers) have been determined for each agricultural district. These indicators may be used as appropriate reference values.