Humic acid and fulvic acid are two of the three ingredients that make up humic substances in soil's organic matter. Humic acid is only soluble in water at certain pH levels, while fulvic acid is soluble in water at all pH levels. The third ingredient, humins, is alkaline and not soluble at any pH level.
Humic acids are the major extractable elements of the humic substances that, along with non-humic substances, make up soil organic matter. They are black or dark brown in color and the result of secondary synthesis reactions. They are also considered to be of high molecular weight. The level of humic acid in soil affects the fertility of the soil, because it controls the water-holding capacity. The more humic acid, the better the soil retains moisture and the better plants grow.
Fulvic acid remains in the solution after the process of acidification removes humic acid from the soil. The color ranges from light yellow to yellowish brown and it is of lower molecular weight. It is also the byproduct of decaying microorganisms in the soil. Some scientists believe fulvic acid can chelate toxins and render them harmless.