'humic acid' definitions:

Definition of 'humic acid'

(from WordNet)
noun
A dark brown humic substance that is soluble in water only at pH values greater than 2; "the half-life of humic acid is measured in centuries"

Definition of 'humic acid'

From: GCIDE
  • Humin \Hu"min\, n. [L. humus the earth, ground.] (Chem.) A bitter, brownish yellow, amorphous substance, extracted from vegetable mold, and also produced by the action of acids on certain sugars and carbohydrates; -- called also {humic acid}, ulmin, gein, ulmic or geic acid, etc. [1913 Webster]