Humic acid and fulvic acid are both natural components of soil and consist of large and small molecules produced as organic -- that is, living or formerly living -- matter decays and breaks down. Though they're gaining popularity as nutritional supplements, there's no scientific evidence to support their use.
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Humus is composted organic material that is no longer in the process of continuing to break down and decay. It's an important component of the soil system because it allows plants and bacteria to grow in the soil. Humus consists of many different components, some of which are acidic in nature. Both humic acid and fulvic acid are major components of humus. They're not single molecules, however; both are mixtures of many different molecules with similar chemical properties.
Humic Versus Fulvic Structure
Since neither humic nor fulvic acid are single compounds, they don't have defined chemical structures. However, each consists of a group of similar molecules, explain Drs. R. Baigorri and colleagues in a 2009 article published in the "Journal of Agricultural and Food Chemistry." The molecules chemists can extract from humus using strong base include both humic and fulvic acids. The two groups are separated from one another by adding hydrochloric acid to the mixture, which causes humic acids to solidify and leaves fulvic acids behind.
Utility In Soil
Both humic acid and fulvic acid are common soil additives. According to the website Outdoor Gardens, humic acid looks physically different; it is darker in color, where fulvic acid has a more yellow hue. Humic acid molecules are generally larger and more complex than fulvic acid molecules and incorporate more positively charged particles -- these are mostly metals -- that act as minerals in the soil. Humic acid also tends to store water better and absorbs more heat, making it the more useful of the two for soil conditioning purposes.
Many supplement manufacturers claim that fulvic and humic acids have important nutritional or health-promoting properties, and can help with anything from inflammation to hormone balance. However, there is no scientific evidence to support these claims. Further, while both humic and fulvic acid are safe to add to soil that you use to grow vegetables they're not necessarily safe to consume as they haven't been thoroughly tested for human consumption.