Soil organic matter extraction

Organic solvents at STP and under supercritical conditions are the most common extractants for soil organic matter. Supercritical C02 and, to a lesser extent, N20 have also been used to extract both native and organic contamination from soil. Humus is extracted using aqueous solutions, but otherwise, water is rarely used to extract organic compounds from soil. A list of common soil organic matter extractants is given in Table 12.2. 

R354 K. M. Haider, M. Spiteller, J. Dec and A. Schaffer, Silylation of Soil Organic Matter Extraction of Humic Compounds and Soil-Bound Residues , Soil Biochem., 2000,10, 139... 

Polysaccharides are present in fulvic acid, humic acid, and humin fractions of soil organic matter. Extractants remove only a small percentage of the total present and hence determinations must be made by means of hydrolysis followed by analysis of the products. These include hexoses, pentoses and uronic acid. There seems to be no doubt that soil polysaccharides are important constituents of soil organic matter. They have a role in... 

NaOH, the suspension is centrifuged, and the aqueous layer is decanted. In the traditional workup of soil organic matter extracts, the alkaline fraction is next... 

The presence of naturally occurring polynuclear aromatic (PAH) compounds is an interesting feature of soil organic matter. These compounds, some of which are carcinogenic, are discussed as air pollutants in Sections 10.8 and 12.4. PAH compounds found in soil include fluoranthene, pyrene, and chrysene. PAH compounds in soil result in part from combustion from both natural sources (grass fires) or pollutant sources. Terpenes also occur in soil organic matter. Extraction of soil with ether and alcohol yields the pigments P-carotene, chlorophyll, and xanthophyll. 

M. Schnitzer, Recent findings on the characterization of humic substances extracted from soils from widely differing climatic zones. Proceedings of Symposium on Soil Organic Matter Studies, Braun.schweig, RFG, 1977, pp. 117-131. 

Soil minerals play a stabilizing role in organic matter. The Al and Fe that complex and stabilize organic matter against microbial decomposition are released from soil minerals during soil formation. The supply rates apparently control the content of soil organic matter to a great extent. This is demonstrated by the relationship between pyrophosphate-extractable C and pyrophosphate-extractable Al plus Fe (Wada 1995). 

This has led to research into identification of labile pools of organic matter (which make up a relatively small proportion of the total pool) that are more sensitive to changes in soil management or environmental conditions than total soil organic matter content. Examples of such pools include C and N held in the microbial biomass and in particulate organic matter and in water-soluble, easily-extractable and potentially mineralizable fractions (Haynes 2005). 

Dilute acid (0.5 M- 2.5 M H2S04) - extractable C or carbohydrate - C have also been used as indicators of soil organic matter status (Angers and Mehuys 1989 Chan and Heenan 1999 Shepherd et al. 2001). The acid hydrolysable fraction generally accounts for about 20-40% of total organic C (Rovira and Vallejo 2002) and 65-85% of the total soil carbohydrate pool (Puget et al. 1999). 

Cao X, Oik D, Chappell M, Cambardella CA, Miller LF, Mao J. Solid-state NMR analysis of soil organic matter fractions from integrated physical-chemical extraction. Soil Sci. Soc. Am. J. 2011 75 1374-1384. 

Figure 12.1. Flow diagram for extraction of soil organic matter and isolation of humus.

Procedure 12.6. Extraction of Soil Organic Matter (Humus)... 

Surfactants can be either found or synthesized to contain both hydrophobic and hydrophilic regions and can be cationic, anionic, or neutral. Thus, surfactants can be chosen to extract a specific type of soil organic matter, whether natural or a contaminant. 

Organic matter extracted from earth materials usually is fractionated on the basis of solubility characteristics. The fractions commonly obtained include humic acid (soluble in alkaline solution, insoluble in acidic solution), fulvic acid (soluble in aqueous media at any pH), hymatomelamic acid (alcohol-soluble part of humic acid), and humin (insoluble in alkaline solutions). This operational fractionation is based in part on the classical definition by Aiken et al. (1985). It should be noticed, however, that this fractionation of soil organic matter does not lead to a pure compound each named fraction consists of a very complicated, heterogeneous mixture of organic substances. Hayes and Malcom (2001) emphasize that biomolecules, which are not part of humic substances, also may precipitate at a pH of 1 or 2 with the humic acids. Furthermore, the more polar compounds may precipitate with fulvic acids. 

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