However, to soil scientists, its something entirely different. Soil humus is a mixture of dark, colloidal organic compounds relatively resistant to decomposition. These compounds result from the decay of organic litter and accumulate in the O and A horizons of soils. Soil humus helps glue mineral particals into aggregates, giving structure to the soil and affecting soil stability. Humin is insoluble but fulvic and humic acids are soluble in dillute NaOH solution.
Humic acids precipitate in acidic solution, but fulvic acids remain soluble.
Figure 1 shows a hypothetical structure of a humic acid with many of the characteristic functional groups. Fulvic acid and humin have similar structures. The COOH and phenolic OH groups are weakly acidic, which give humus its pH buffering ability, pH dependent charge and cation chelating ability. In addition, the toxicity of the phenolic subgroups which make up humus contributes to its resistance to microbial decomposition. The less reactive compounds add onto the polymers through nucleophilic addition to quinones.