Fulvic acids (FAs) are a mixture of weak aliphatic and aromatic organic acids which are soluble in water at all pH conditions (acidic, neutral and alkaline). Their composition and shape is quite variable.

The size of fulvic acids (HFs) are smaller than humic adds (HAs), with molecular weights which range from approximately 1,000 to 10,000. Fulvic acids (FAs) have an oxygen content twice that of humic acids (HAs). They have many carboxyl ( COOH) and hydroxyl ( COH) groups, thus fulvic acids (FAs) are much more chemically reactive. The exchange capacity of fulvic acids (FAs) is more than double that of humic acids (HAs).

This high exchange capacity is due to the total number of carboxyl ( COOH) groups present. The number of carboxyl groups present in fulvic acids (FAs) ranges from 520 to 1120 cmol (H+)/kg. Fulvic acids collected from many different sources and analyzed, show no evidence of methoxy groups ( CH3) groups, they are low in phenols, and are less aromatic compared to humic acids from the same sources.

Because of the relatively small size of fulvic acid (FA) molecules they can readily enter plant roots, stems, and leaves. As they enter these plant parts they carry trace minerals from plant surfaces into plant tissues. Fulvic acids (FAs) are key ingredients of high quality foliar fertilizers. Foliar spray applications containing fulvic acid (FA) mineral chelates, at specific plant growth stages, can be used as a primary production technique for maximizing the plants productive capacity.

Once applied to plant foliage fulvic acids (FAs) transport trace minerals directly to metabolic sites in plantcells. Fulvic acids (FAs) are the most effective carbon containing chelating compounds known. They are plant compatible, thus non toxic, when applied at relatively low concentrations.

Figure 1. Generalized features of the three major humic substances

Molecular weight decreasing    
Cation exchange capacity (c mol/kg)    
and acidity increasing    
100300 5001,000
Carbon content (g/kg) decreasing    
Oxygen content (g/kg) increasing    
Nitrogen content (g/kg) decreasing    
4655 437
Hydrogen content (g/kg) variable    
Fertilizer properties – plant response    
Slow response  Rapid response 

Modified from Dixon, J. B. and S. B. Weed, 1989. Page 95 In “Minerals in Soil Environments”. Soil Science Society of America, Madison, Wisconsin, 1244 pages.


Humates are metal (mineral) salts of humic (HAs) or fulvic acids (FAs) Within any humic substance there are a large number of complex humate molecules. The formation of a humate is based on the ability of the carboxyl ( COOH) and hydroxyl ( OH) groups (on the outside of the polymers) to dissociate (expel) the hydrogen ion.

Once the hydrogen ions are dissociated a negatively charged anion ( COO- or -CO-) results. Two of these anions can bind to positive metal cations, such as Iron (Fe++), copper (Cu++), zinc (Zn++), calcium (Ca++), manganese (Mg++), and magnesium (Mg++). The simplified reaction ( COO- + Fe++ > > COOFe+ + H) proceeds to bind two anions, frequently a COOH and a COH group.

The humate composition of any one humic substance is specific for that substance. Thus there exists a large variability in the molecular composition of different humic substances.