In general, fulvic acid has higher amount of carboxylic groups and lower average molecular weight than humic acid. Water solubility of humic substances is primarily governed by interplay of two factors: the amount of ionizable functional groups (mainly, carboxylic) and the molecular weight. All of them are constructed from similar aromatic, polyaromatic, aliphatic and carbohydrate units and contain the same functional groups (mainly, carboxylic, phenolic and ester) albeit in varying proportions. In terms of chemistry, fulvic acid, humic acid and humin share more similarities than differences and represent a continuum of humic molecules. Aquatic humic substances were isolated for the first time later, in 1806, from spring water by Jöns Jakob Berzelius. It is rooted in the history of soil science and, more precisely, in the tradition of alkaline extraction, which dates back to 1786, when Franz Karl Achard treated peat with a solution of potassium hydroxide and, after subsequent addition of an acid, obtained an amorphous dark precipitate (i.e., humic acid). This definition of humic substances is largely operational. Hymatomelanic acid is part of humic acid that is soluble in ethanol.

Fulvic acid is soluble in water at any pH. By definition, humic acid is soluble in water at neutral and alkaline pH, but insoluble at acidic pH < 2. 'Humic substances' is an umbrella term covering humic acid, fulvic acid, humin and hymatomelanic acid which differ in solubility. Humic substances represent the major part of organic matter in soil, peat, coal and sediments and are important components of dissolved natural organic matter (NOM) in lakes (especially, dystrophic lakes), rivers and sea water. The colour of humic substances varies from yellow to brown to black. Humic substances (HS) are coloured recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues.

Major component of natural organic matter Humic acid isolated from peat Fulvic acid isolated from peat

What are scids