1. Sources and classification of humic acid
Humic acid is generally considered to be a complex mixture of hydroxybenzene carboxylic acids with non-uniform molecular weight.Lignite, peat and leonardite have high humic acid content and are the main sources of humic acid extraction. The extraction rate of humic acid was highest in leonardite, followed by lignite and peat.
According to the traditional method of soil science, humic acid can be divided into humic acid, fulvic acid and humin. Humic acid is also divided into 3 categories: do not dissolve in alkaline solution, can dissolve in acidic solution called black humic acid; Only dissolved in alkaline solution, can not be dissolved in acidic solution is called brown humic acid; can be dissolved in both alkaline and acidic solutions called Fulvic acid.