Humic and fulvic acids are types of naturally occurring substances known as humic substances — organic compounds formed during the decomposition of plant and animal matter. Humic acids tend to be larger molecules, less soluble in acidic conditions, and more associated with long-term soil organic matter. Fulvic acids are smaller, more soluble (including under acidic pH), more mobile, more biologically active in some ways. Fulvic acid isolated from peat By Alexignat - Own work