Humic substances (HS) are colored relatively recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The color of humic substances varies from bright yellow to light or dark brown leading to black. The term comes from humus, which in turn comes from the Latin word humus, meaning "soil, earth".[1] Humic substances represent the major part of organic matter in soil, peat, coal, and sediments, and are important components of dissolved natural organic matter (NOM) in lakes (especially dystrophic lakes), rivers, and sea water. Humic substances account for 50 – 90% of cation exchange capacity in soils.
"Humic substances" is an umbrella term covering humic acid, fulvic acid and humin, which differ in solubility. By definition, humic acid (HA) is soluble in water at neutral and alkaline pH, but insoluble at acidic pH < 2. Fulvic acid (FA) is soluble in water at any pH. Humin is not soluble in water at any pH.
This definition of humic substances is largely operational. It is rooted in the history of soil science and, more precisely, in the tradition of alkaline extraction, which dates back to 1786, when Franz Karl Achard treated peat with a solution of potassium hydroxide and, after subsequent addition of an acid, obtained an amorphous dark precipitate (i.e., humic acid). Aquatic humic substances were isolated for the first time in 1806, from spring water by Jöns Jakob Berzelius.
In terms of chemistry, FA, HA, and humin share more similarities than differences and represent a continuum of humic molecules. All of them are constructed from similar aromatic, polyaromatic, aliphatic, and carbohydrate units and contain the same functional groups (mainly carboxylic, phenolic, and ester groups), albeit in varying proportions.
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, fulvic acid has a higher amount of carboxylic groups and lower average molecular weight than does humic acid. Measured average molecular weights vary with source, however, molecular weight distributions of HA and FA significantly overlap.
Age and origin of the source material determine the chemical structure of humic substances. 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).
Isolation id HS is the result of a alkaline extraction from solid sources of NOM the adsorption of HS on a resin.[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. and surface waters[5] It is the large molecular complexity of the soil humeome[6] that confers to humic matter its bioactivity in, its stabilty in ecosystems ,soil and its role as plant growth promoter (in particular plant roots).[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 directly analyze the soil,[8]
- ^ "Humus". Retrieved 2024-07-20 – via Dictionary.com Random House Dictionary Unabridged.
- ^ Piccolo A. (2016). "In memoriam of Prof. F.J. Stevenson and the question of humic substances". Chemical and Biological Technologies in Agriculture. 3. doi:10.1186/s40538-016-0076-2.
- ^ Drosos M.; et al. (May 15, 2017). "A molecular zoom into soil Humeome by a direct sequential chemical fractionation of soi". The Science of the Total Environment. 586: 807–816. Bibcode:2017ScTEn.586..807D. doi:10.1016/j.scitotenv.2017.02.059. PMID 28214121.
- ^ "Source Materials for International Humic Substances Society Samples". Retrieved 22 July 2020.
- ^ Piccolo A.; et al. (2018). "The Molecular Composition of Humus Carbon: Recalcitrance and Reactivity in Soils". The Molecular Composition of Humus Carbon: Recalcitrance and Reactivity in Soils. In: The Future of Soil Carbon, Wiley and Sons. pp. 87–124. doi:10.1016/B978-0-12-811687-6.00004-3. ISBN 9780128116876.
- ^ Nebbioso A. and Piccolo A. (2011). "Basis of a Humeomics Science: Chemical Fractionation and Molecular Characterization of Humic Biosuprastructures". Biomacromolecules. 12 (4): 1187–1199. doi:10.1021/bm101488e. PMID 21361272. S2CID 45333263.
- ^ Canellas P.L and Olivares F.L. (2014). "TPhysiological responses to humic substances as plant growth promoter". Chemical and Biological Technologies in Agriculture. 1: 3. doi:10.1186/2196-5641-1-3.
- ^ Lehmann, J.; Kleber, M. (2015-12-03). "The contentious nature of soil organic matter". Nature. 528 (7580): 60–8. Bibcode:2015Natur.528...60L. doi:10.1038/nature16069. PMID 26595271.
The development of this extraction method preceded theory, tempting scientists to develop explanations for the synthesis of materials resembling operationally extracted 'humic substances', rather than to develop an understanding of the nature of all organic matter in soil.[...] This lack of evidence means that 'humification' is increasingly questioned, yet the underlying theory persists in the contemporary literature, including current textbooks.[...] The issue has also been approached by redefining 'humic substances' as the portion of soil organic matter that cannot be molecularly characterized or by calling all soil organic matter 'humus'. We argue that this compromise – maintaining terminology but altering its meanings in varying ways – hampers scientific progress beyond the soil sciences. The [need for accurate models] of soil organic matter does not allow a confusing middle path; it requires leaving the traditional view behind to bring about lasting innovation and progress. This is critical as scientific fields outside the soil sciences base their research on the false premise of the existence of 'humic substances'. Thus an issue of terminology becomes a problem of false inference, with far-reaching implications beyond our ability to communicate scientifically accurate soil processes and properties.