Since the dawn of modern chemistry, humic substances are among the most studied among the natural materials. Despite long study, their molecular structure remains elusive. The traditional view is that humic substances are heteropolycondensates, in varying associations with clay. A more recent view is that relatively small molecules also play a role. Humic substances account for 50 – 90% of cation exchange capacity. Similar to clay, char and colloidal humus hold cation nutrients.