10: Humic Substances

HS compose the largest part of SOM; deep impact on:
buffering capacity
metal-binding ability
stability of aggregates of soil particles
water-holding capacity
sorption of hydrophobic organic compounds, most of which in turn have a large impact on soil fertility

highly heterogeneous mixture (in chemical and physical sense) of organic compounds, many of colloidal and/or polyelectrolytic nature, having a medium to high molecular weight, yellow to black color and a mixed aliphatic/aromatic composition

degredation processes of dead organisms (mainly vegetal, but also including animal) as a result of microbial activity, possibly followed by secondary synthesis/condensation reactions

while their composition is location dependent (due to local vegetation, fauna and climate), HS properties are similar everywhere

fulvic acids (FAs)
humic acids (HAs)
humin

the fraction of HSs that is soluble in water at any pH

the fraction of HSs that is soluble in water at pH > 1

the fraction of HSs that is insoluble in water at any pH

C, H, O, N; sometimes S and other elements are found
O/C atomic ratio higher for FA than for HA (0.7, 0.5 respectively), consistent with their greater solubility

a distribution of sizes and molecular weights

charged with several elemental charges per molecule

HSs display polydispersity and are polyelectrolytic