Fulvic acids occur as a result of the decay of plants and microorganisms in some soils, sediments, peats, coals, shales, and surface waters. Unlike humic acids, fulvic acids are soluble in water over a large range of pH values (and this is the operational distinction between humic vs fulvic acids). Along with humic acid, fulvic acid may be extracted from solids using a strong base, such as NaOH or KOH. Fulvic acid may actually originate from hydrolysis of large molecules by the strong base during extraction. However, at pH 1, fulvic acid remains in solution but humic acid will precipitate.
Cf., humic acid, humification, humic substances, humin