How do humic substances from soil affect the color of cave deposits?

Humic substances act as a primary coloring agent for cave deposits, such as flowstone and other speleothems. These substances are organic products resulting from the decay of plants in overlying soils.1

When rainwater infiltrates the soil, it picks up these organic compounds and carries them into the cave via seepage water. Once inside, these substances tint the otherwise pure white calcite deposits, giving them various shades of tan, orange, and brown. This process is similar to how humic substances contribute to the brown color of certain soils and the tea-like appearance of some swamp and lake waters. While iron oxides can also pigment these deposits, humic substances are considered the more common coloring agent.1