They remains in solution after removal of humic acid by acidification.
Fulvic acids are light yellow to yellow-brown in color.
Humin - the fraction of humic substances that is not soluble in water at any pH value and in alkali. Humins are black in color. The postulated relationships are depicted in figure , in which it can be seen that carbon and oxygen contents, acidity and degree of polymerization all change systematically with increasing molecular weight. Electron microscope observations revealed the humic acids of different soils to have polymeric structure, appearing in form of rings, chains, and clusters. The sizes of their macromolecules can range from 60 - 500 A, what is mainly decided of by the occuring humification process, which also exerts an influence on their spatial sructure.