Humic acids are chemically diverse acids that result from the decay of plants and microorganisms in some soils, sediments, peats, coals, shales, and surface waters. These acids are insoluble in water at low pH, can be precipitated at a pH = 1, and extracted from solids using a strong base, such as NaOH or KOH.
Cf., fulvic acid, humic substances, humification, humin