The most common substances we collectively refer to as “humus”

Some of the most common substances we collectively refer to as “humus” include:

Fulvic acid: a yellow to yellow-brown humic substance that is soluble in water under all pH conditions and is of low molecular weight.
Humic acid: a dark-brown humic substance that is soluble in water only at higher soil pH values and is of greater molecular weight than fulvic acid. Humic acid may remain for centuries in undisturbed soil.
Humin: a black humic substance that is not soluble in water at any pH, has a high molecular weight, and is never found in base-extracted liquid humic acid products.