Humic substance
Humic substances (HS) are colored relatively recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues.
In terms of chemistry, FA, HA, and humin share more similarities than differences and represent a continuum of humic molecules.
Water solubility of humic substances is primarily governed by interplay of two factors: the amount of ionizable functional groups and (mainly carboxylic) and molecular weight (MW).
In general, humic substances derived from soil and peat (which takes hundreds to thousands of years to form) have higher molecular weight, higher amounts of O and N, more carbohydrate units, and fewer polyaromatic units than humic substances derived from coal and leonardite (which takes millions of years to form).
[2][3][4] A newer view of humic substances is that they are not mostly high-molecular-weight macropolymers but rather represent a heterogeneous mixture of relatively small molecular components of the soil organic matter auto-assembled in supramolecular associations and are composed of a variety of compounds of biological origin and synthesized by abiotic and biotic reactions in soil.
[7] The academic definition of humic substances is under debate and some researchers argue against the traditional concepts of humification and seek to forgo alkali extract method and to analyze the soil directly.
Their structure, elemental composition and content of functional groups of a given sample depend on the water or soil source and on the specific procedures and conditions of extraction.
Humic substances are extracted from soil and other solid sources using 0.1 M NaOH, under a nitrogen atmosphere, to prevent abiotic oxidation of some of the components of HS.
Despite long study, their molecular structure remains debatable The traditional view has been that humic substances are hetero- poly-condensates, in varying associations with clay.
The number of individually distinct components in HS, as measured by mass spectroscopy is in the thousands The average composition of HA and FA can be represented by model structures.
[24] The formation of (chelate) complexes is an important aspect of the biological role of humic acids in regulating bioavailability of metal ions.
[25] CriticismDecomposition products of dead plant materials form intimate associations with minerals, making it difficult to isolate and characterize soil organic constituents.
[8] However, modern chemical analysis methods applied to unprocessed mineral soil have not directly observed large humic molecules.
This suggests that the extraction and fractionation techniques used to isolate humic substances alter the original chemical composition of the organic matter.
[26] Despite these concerns, the 'humification' theory persists in the field and in even textbooks, and attempts to redefine 'humic substances' in soil have resulted in a proliferation of conflicting definitions.
For instance, humic and fulvic acids can react with the chemicals used in the chlorination process to form disinfection byproducts such as dihaloacetonitriles, which are toxic to humans.
[27][28] Accurate methods of establishing humic acid concentrations are therefore essential in maintaining water supplies, especially from upland peaty catchments in temperate climates.
As a lot of different bio-organic molecules in very diverse physical associations are mixed together in natural environments, it is cumbersome to measure their exact concentrations in the humic superstructure.
Extraction procedures are bound to alter some of the chemical linkages present in the soil humic substances (mainly ester bonds in biopolyesters such as cutins and suberins).
[31] A 1998 study by scientists at the North Carolina State University College of Agriculture and Life Sciences showed that addition of humate to soil significantly increased root mass in creeping bentgrass turf.
[40] The heavy-metal binding abilities of humic acids have been exploited to develop remediation technologies for removing lead from waste water.
