Organic colloids

Humus is the penultimate state of decomposition of organic matter; while it may linger for a thousand years, on the larger scale of the age of the other soil components, it is temporary. It is composed of the very stable lignins (30%) and complex sugars (polyuronides, 30%). Its chemical assay is 60% carbon, 5% nitrogen, some oxygen and the remainder hydrogen, sulfur, and phosphorus. On a dry weight basis, the CEC of humus is many times greater than that of clay. Plant roots also have cation exchange sites.