Humic Acid and Fulvic acid definition

Humic substance is of polymeric nature,with molecular weights determined by small angle X-raw scattering ranging from 500 to 2000 Dalton for both aquatic and soil fulvic acid,and 1000 to more than 12000 Da aquatic humic acid .According to this polymeric model,HS have been operationally classified into three fractions,based on their relative solubilities which were thought to be a function of molecular weight.These three fractions include:

Humin,which is insoluble in water on whole pH range;

Humic acids,which are insoluble in water but soluble at alkaline media;

Fulvic acid,which are soluble at all pH values.

The composition of humin,Humic acid and fulvic acid is essentially accounted for by carbon,hydrogen and oxygen,with less amount of nitrogen and sulfur. Respresetative elemental compositions of soil humic acid and fulvic acid by following table

Humic acids   Fulvic acids

Carbon          53.8-58.7%       40.7%-50.6%

Hydrogen     3.2-6.2%            3.8-7.0%

Oxygen         32.8%-38.3%    39.7%-49.8%

Nitrogen      0.8%-4.3%         0.9%-3.3%

Sulfur           0.1%-1.5%          0.1-3.6%