Soil Science Humic Substances

The relationship between humic substances and melanoidins has been reviewed by Dean et al.,15 who provide an entry to the earlier literature. Both humic substances and melanoidins present enigmas, their structures still being largely undefined. This makes comparing and contrasting them doubly difficult. 

Humic substances seem to be formed primarily by the degradation of lignin, complicated by interactions with other compounds, such as proteins and amino acids, produced from plant litter. Maillard536,537 already recognised that the condensation of sugars with amino acids, peptides, and proteins also plays a role in the formation of humic substances. 

Only 0.05% (about 4X1019 g) of the Earth s carbon is not locked up in sedimentary rocks and only about 9% of that is organic.538 The organic carbon is divided roughly among seawater (45%), soil (40%), and terrestrial plants (15%). Humic substances are traditionally considered to comprise three main fractions humic acids that are soluble in alkali, but insoluble in acid fulvic acids soluble both in alkali and in acid and humin insoluble both in alkali and in acid. 

A modification of the lignin theory of the formation of humic substances is the polyphenol theory, which considers the key interactions to be those between quinones, derived either from polyphenols or lignin, and amino compounds. 

Humic substances are very refractory consequently, 14C-dating suggests long average ages, such as 30 years for Suwannee River fulvic acid, 100-500 years for soil fulvic acid, 700-1600 years for soil humic acid, and 100-2400 years for soil humin. 

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The word humus is Latin for earth, soil, terrain, probably deriving from humi, on the ground, or humilis, which means low, humble. In soil science this term is sometimes used synonymously with soil OM—that is, to denote the organic material in the soil, including humic substances (MacCarthy et al., 1990). In other cases, humus is used to represent only HS (Stevenson, 1982). In order to avoid any misunderstanding in this work, the word humus will be avoided or it will be used as a synonym of humic substances. 

Lichtfouse, E. (2000). Compound-specific isotope analysis, application to archael-ogy, biomedical sciences, biosynthesis, environment, extraterrestrial chemistry, food science, forensic science, humic substances, microbiology, organic geochemistry, soil science and sport. Rapid Commun. Mass Spectrom. 14, 1337-1344. 

Boyd, S.A. and Sommers, L.E., Humic and fulvic acid fractions from sewage sludges and sludge amended soils, in Humic Substances in Soil and Crop Sciences, Clapp, C.E., Malcolm, R.L., and Bloom, P.R., Eds., American Society of Agronomy, Inc. USA, 1990, pp. 203-220. 

Wilson M. A. (1990) Application of nuclear magnetic resonance spectroscopy to organic matter in whole soils. In Humic Substances in Soil and Crop Sciences Selected Readings (eds. P. MacCarthy, C. E. Clapp, R. L. Malcolm, and P. R. Bloom). American Society of Agronomy,... 

Although NMR is probably the most widely used NMR technique in chemical applications, it has only been used sparingly in studies of humic substances. Much of the literature in which this technique is used was published prior to 1987 and is well covered in the review by Wilson.(5) Use of NMR is mostly limited to solutions, and it is thus necessary to dissolve humic substances in aqueous solutions. Of course, one must reduce the amount of H s from solvent molecules by use of deuterated water, but it is often difficult to reduce these background signals to negligible amounts. Wilson(5) described the application of water suppression techniques which have been necessary to obtain H NMR spectra of humic isolates from soil. In the case of solid-state NMR of whole soils and humic substances, the development of a new technique called CRAMPS (combined rotation and multiple pulse spectroscopy) has not received the attention it has in the field of coal science, primarily because of the lack of resolution. Thus, recent applications of either solution or solids NMR has been very limited in humic substance science since Wilson s(5) review. 

Y. Chen, Organic matter reactions involving micronutrients in soils and their effect on plants. Humic Substances in Terrestrial Ecosystems (A. Piccolo, ed.), Elsevier Sciences B. V., Amsterdam, 1996, p. 507. 

Bailey G.W., Akim L.G., Shevchenko S.M. Predicting chemical reactivity of humic substances for minerals and xenobiotics use of computational chemistry, scanning probe microscopy, and virtual reality. In Humic Substances and Chemical Contaminants, C.E. Clapp, M.H.B. Hayes, N. Senesi, P.R. Bloom, P.M. Jardine, eds. Madison, WI Soil Science Society of America, Inc., 2001. 

Chiou CT (1989) In MacCarthy P, Malcolm RL, Clapp E, Bloom P (eds) Humic substances in soil and crop sciences. American Society of Agronomy, Madison, Wisconsin, p 214... 

Hassett IJ, Banwart WL (1989) The sorption of nonpolar organics by soils and sediments In Sawhney BL, Brown K. (eds) Reactions and movement of organic chemicals in soils Soil Science Society of America, Madison, Wl, pp 31 5p Hayes MHB, Malcom RL (2001) Considerations of compositions and aspects of the structure of humic substances. In Clapp CE, Hayes MHB, Senesi N, Bloom PR Jardine PM, Humic substances and chemical contaminants. Soil Science Society of America, Madison, Wl, pp 1-39 Herbillon AJ, Erankart R, Vielvoye L (1981) An occurrence of interstratified kaoUnite-smectite minerals in a red-black soil top sequence. Clay Miner 16 195-201 Horne RA (1969) Marine chemistry. Wiley, New York... 

Schofield RK, Samson HR (1954) Flocculation of kaolinite due to the attraction of opposite charged crystal faces. Discuss Faraday Soc 18 135-145 Schofield RK, Samson HR (1953) The defiocculation of kaolinite suspensions and the accompanying change-over from positive to negative chloride adsorption. Clay Miner BuU 2 45-51 Schulten HR (2001) Models of humic structures association of humic acids and organic matter in soils and water. In Qapp CE et al. Humic substances and chemical contaminants. Soil Science Society of America, Madison, Wl, pp 73-88... 

Assaf-Anid, N., Hayes, K. F. Vogel, T. M. (1994). Reductive dechlorination of carbon tetrachloride by cobalamin(II) in the presence of dithiothreitol mechanistic study, effect of redox potential and pH. Environmental Science Technology, 28, 246-52. Ballard, T. M. (1971). Role of humic carrier substance in DDT movement through forest soil. Soil Science Society of America Proceedings, 35, 145-7. 

Wang,T. S. C., Huang, P. M., Chou, C-H. Chen, J-H. (1986). The role of soil minerals in the abiotic polymerization of phenolic compounds and formation of humic substances. In Interactions of Soil Minerals with Natural Organics and Microbes, ed. P. M. Huang M. Schnitzer, pp. 251-81. Madison, WI Soil Science Society of America. 

Senesi, N. 1992. Binding mechanisms of pesticides to soil humic substances. Science of the Total Environment 123/124 63-76. 

Tipping, E., and M. A. Hurley. 1988. A model of solid—solution interactions in acidic organic soils, based on the complexation properties of humic substances. Journal of Soil Science 39 505-519. 

Tipping, E., and C. Woof. 1990. Humic substances in acid organic soils Modeling their release to the soil solution in terms of humic charge. Journal of Soil Science 41 573-586. 

Chen, Y., and Aviad, T. (1990). Effects of humic substances on plant growth. In Elumic Substances in Soil and Crop Science, MacCarthy P, ed., ASA, SSA, Madison, WI, pp. 161-186. 

Humic substances account for various definitions according to different authors, and also its definition changes in literature through the years. These substances are conventionally defined as a series of relatively high-molecular-weight, yellow-to black-coloured substances, formed by secondary synthesis reactions (Stevenson, 1982 Soil Science Society of America, 1996) or as a category of naturally occurring,... 

Figure 8.3. Fluorescence micrographs of Pinus sylvestris roots at 1mm (top) and 2 mm (bottom) behind the root tip after a treatment with LMS (low-molecular-size humic substances). Note the much smaller rate differentiation of the roots treated with LMS than the roots treated with HMS (high-molecular-size humic substances). Reprinted from Nardi, S., Pizzeghello, D., Remiero, F., and Rascio, N. (2000). Chemical and biochemical properties of humic substances isolated from forest soils and plant growth. Soil Sci. Soc. Am. J. 64,639-645, with permission from the Soil Science Society of America.

Clapp, C. E., Chen, Y., Hayes, M. H. B., and Cheng, H. H. (2001). Plant growth promoting activity of humic substances. In Understanding and Managing Organic Matter in Soils, Sediments, and Waters, Swift, R. S., and Sparks, K. M., eds., International Humic Science Society, St. Paul, MN, pp. 243-255. 

Hayes, M.H.B. (1991). Concepts of the origins, composition, and structures of humic substances. In W.S. Wilson, ed., Advances in Soil Organic Matter Research The Impact on Agriculture and the Environment. Special Publication No. 90. Cambridge, England The Royal Society of Chemistry, Thomas Graham House, Science Park, pp. 3-22. 

Schnitzer, M. 1986. Binding of humic substances by soil mineral colloids. In M. Huang andM. Schnitzer, Ed. Interactions of Soil Minerals with Natural Organics and Microbes. Special Publication No. 17. Soil Science Society of America Madison, WI, pp. 77-101. 

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