In Wallonia, Belgium, intensive in situ charcoal production that was linked closely to pre‐industrial smelting and steel‐making affected a large part of the forested area in the late eighteenth century. Charcoal kiln relics can be detected under forest as domes of about 10 m in diameter, with the topsoil greatly enriched with charcoal residues. We sampled 19 charcoal kiln sites and the adjacent reference soil by soil horizon on four different soil types (Arenosols, Luvisols, Cambisols and Podzols). Data were analysed with linear mixed models to assess the effect of the charcoal kiln site on soil properties in relation to depth and soil conditions. We also addressed the evolution of soil properties over time by a comparison of the soil characteristics at a currently active kiln site. The charcoal‐rich topsoil has a larger C:N ratio and cation exchange capacity (CEC) per unit of organic carbon than the reference soil. The largest CECs per unit of carbon were observed on soil with coarser textures. On acidic soil, the increase in base saturation in the subsoil reflects the past liming effect of ash produced by wood charring, whereas the topsoil is re‐acidified. The acidity of carbonate‐rich Cambisols, however, is not reduced. Regardless of soil type, the kiln topsoil is greatly depleted in exchangeable K⁺ and available P, which may be attributed to the small affinity of the exchange complex of charcoal for K⁺ and a decrease in P availability with time. Therefore, we recommend further research on the long‐term effects of biochar on the dynamics of plant nutrients.