Humus: soil organic matter

Biochemicals will be present in soil during any analysis and can react with components of interest, either organic or inorganic, including sand, silt, and clay particles. Possible reactions include chelation, decomposition, precipitation, solubilization, or dissolving such as dissolving in soil organic matter (humus). Several of these reactions will take place simultaneously and can lead to nondetection of the component of interest or an analytical result that is much lower than the true value [16]. 

Humus and organic matter can have a dramatic effect on analytical results as shown in the work by Gerke [16]. Metal complexes, particularly aluminum and iron, were found to complex with phosphate. These complexes, which accounted for 50-80% of the phosphate present, were not detectable by standard phosphate analytical procedures. When developing a soil analytical method, it is essential that the method be either applicable to soils of all organic matter contents or that variations of the procedure applicable to soils of differing organic matter be developed [16-19],... 

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

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. 

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

Humin The alkali-insoluble fraction of soil organic matter or humus... 

Bollag JM, LoU Ml (1983) Incorporation of xenobiotics in soil humus. Experentia 39 1221-1225 Bollag JM, Myers CJ, Minard RD (1992) Biological and chemical interactions of pesticides with soil organic matter. Sci Total Environ 123/124 205-217 Bolt GH (1955) Ion adsorption by clays. Soil Sd 79 267-278... 

Soil organic matter is a product of the decay of plants and animals. It usually includes a large number of things in various stages of decomposition. Most plant and animal materials break down rapidly at first, then more slowly as the easily decomposed parts disappear. All of these plant and animal residues are collectively referred to as organic matter. The fraction of organic matter in a relatively advanced state of decay, now decaying only slowly, may be referred to as humus. 

Swaby, R. J., and Ladd, J. N. (1966). Stability and origin of soil humus. In The Use of Isotopes in Soil Organic Matter Studies, Int. Atomic Energy Agency Pergamon Press, Oxford, pp. 153-159. 

Shah, R. K., Choski, M. R., and Joshi, B. C. (1975a). Development studies on soil organic matter Humus. Chem. Era 4,31-34. 

As seen in Table 3.12, the humus content of soils varies within a rather wide concentration range (0.6%-6.6%). However, parameter adjustment is only successful when the protolytic processes of humus are neglected. Consideration of the protonation and deprotonation of aluminol and silanol sites (Chapter 1, Equations 1.54-1.56 Chapter 2, Sections 23-2.5) is sufficient. It is likely caused by the cations of the support electrolyte and the divalent and trivalent (aluminum and ferric) cations dissolved from the soil that react with the acidic functional groups of soil organic matter, limiting the protonation of functional groups (Hargrove and Thomas 1982 Sparks 2003). 

As seen in Chapter 1, Section 1.1.2, soil organic matter is an undefined system, consisting of many components. So, the determination of soil organic content and the study of humic substances are challenging. For practical purposes, the humus content or, more correctly, soil organic matter content, is determined by standard procedures however, the results cannot be considered as absolutely real values, and can only be compared to each other. 

Humus relatively stable dark part of soil organic matter decomposed beyond the stage of visual recognition of the original plant material. 

These reactions may lower soil water pH from 5.6 (its equilibrium value with atmospheric C02 see Box 3.5) to 4-5. This is a simplification since soil organic matter (humus) is not often completely degraded to C02. The partial breakdown products, however, possess carboxyl (COOH) or phenolic functional groups (see Section 2.7.1), which dissociate (Box 4.5) to yield H+ ions that lower the pH even more ... 

Saussure (1804) is usually credited with introducing the term humus (Latin equivalent of soil) to describe the dark-colored organic matter in soil. Somewhat later, Dobereiner (1822) designated the dark-colored component of soil organic matter as Humussdure or humus acid. The origin of the term humic acid to identify the alkali-soluble, acid-insoluble fraction is somewhat obscure but it was in common use by the time of Berzelius (1839). Waksman (1936) pointed out that humus acid and humic acid were often used indiscriminately and seldom was any differentiation made between the two when distinguished, the former referred to the alkali extracted material, whereas the latter referred to the precipitate obtained by acidification. Use of the term humin to describe the alkali-insoluble material had a similar develop-... 

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