Humic substances are divided into three fractions: humic acids, fulvic acids, and the humate fraction. The key difference between them lies in their solubility in water. Humic acids are soluble in alkaline solutions but insoluble in acidic conditions, while fulvic acids are soluble across all pH levels. The humate fraction is insoluble and remains behind during extraction.

Both humic and fulvic acids consist of complex molecules made of long carbon chains (aliphatic) and ring structures (aromatic), with oxygen-containing functional groups (e.g., carboxylic or phenolic groups) that make them soluble in water. The main difference is that fulvic acids have more of these functional groups, allowing them to stay dissolved in water, while humic acids have fewer, causing them to precipitate in acidic conditions.

While humic molecules are often considered larger than fulvic molecules, their molecular sizes (measured in Daltons) overlap, with fulvic acids typically containing more oxygen and humic acids having higher carbon content.

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