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"Humic substances" is an umbrella term covering humic acid, fulvic acid and humin, which differ in solubility. By definition, humic acid (HA) is soluble in water at neutral and alkaline pH, but insoluble at acidic pH < 2. Fulvic acid (FA) is soluble in water at any pH. Humin is not soluble in water at any pH.
Potassium humate is used in agriculture as a fertilizer additive to increase the efficiency of fertilizers especially nitrogen- and phosphorus-based fertilizer inputs. Other salts of humic acid are manufactured, mainly sodium humate, which is used in animal health supplements. It also can be used in aquaculture.
Increasing the soil's pH increases the ability of naturally occurring humic substances to improve infiltration in hydrophobic soils. Humic acid is only water-soluble at a pH greater than 6.5, while fulvic acid is soluble at all pH ranges. Both resident acids have a property that enables them to reduce the surface tension of water when in solution.
Soil consists of both mineral (inorganic) and organic components. The organic components can be subdivided into fractions that are soluble, largely humic acids, and insoluble, the humins. Humins make up about 50% of the organic matter in soil. [1]
Humic and fulvic acid are fractions of natural organic matter (NOM). They are defined operationally. The humic acids precipitate from solution at pH < 2. Likewise, the fulvic acids remain in solution. Both are complex mixtures of many organic compounds. Most aquatic NOM is fulvic acid, while a greater fraction of soil NOM (SOM) is humic acid.
Fulvic acid is the genus that contains the matter that has the lowest molecular weight, is soluble in acids and alkalis, and is susceptible to microbial action. Humic acid is the genus that contains the intermediate matter that has medial molecular weight, is soluble in alkalis and insoluble in acids, and has some resistance to microbial action.