1. Sources and classification of humic acid (Some following parts are called HA.)
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 HA content and are the main sources of HA extraction. The extraction rate of HA was highest in leonardite, followed by lignite and peat.
According to the traditional method of soil science, humic acid can be divided into HA, fulvic acid and humin. HA is also divided into 3 categories: do not dissolve in alkaline solution, can dissolve in acidic solution called black HA; Only dissolved in alkaline solution, can not be dissolved in acidic solution is called brown HA; can be dissolved in both alkaline and acidic solutions called Fulvic acid.