Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

humic acid
     n : 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"