The researchers studied how the maturity of wood waste compost affects the structure of humic acids. They separated humic substances from non-humic substances to calculate a humification index. As the compost matured, the humification index increased. The researchers found that the more mature the compost, the more humic acids were present, indicating a higher level of decomposition. This suggests that the maturity of compost can be determined by analyzing the structural features of humic acids.