Abstract The transformation of poplar bark during humification was studied. Data are reported on the pH, ash, organic carbon and total nitrogen contents of products at different stages of humifications (0, 3, 6, 12, 30 and 84 months). The results show that the van Bemmelin factor (organic C = (organic carbon) ×f) had an average value of 2·01. The amount and quality of humic substances were also investigated and the results obtained show that composting leads to a mature product after not less than 12 months' decomposition. Non-humified materials were extracted by Na 4P 2O 7 solution (ionic strength, 0·5 m; pH, 9·0) from fresh and slightly humified bark. The experiments are discussed in order to establish the limiting values of some analytical characteristics of mature bark manures.