Humus extractions

Newer and more complex humus extractions have been developed. These typically involve more steps such as both physical separation on the basis of density and particle size (related to the size of soil inorganic components), and chemical separation based on extractions and washings with hydrofluoric acid (HF), hydrochloric acid (HC1), and sodium hydroxide (NaOH). The products of such separations are then subjected to spectroscopic analysis and interpretation [22,23],... 

Concerning nitrate uptake and assimilation (Albuzio et al., 1986) in barley seedlings incubated with HS from a grassland soil and their fractions—nitrate reductase (NR), glutamate dehydrogenase (GDH),and glutamine synthetase (GS) activities— were analyzed together with the rate of nitrate uptake. The enzymatic activities turned out to be stimulated by the treatment of unfractioned humus extract with increases compared to controls of 65%, 35% and 45% respectively. 

Bog soils, commonly described as peat or muck, are discussed in some detail in Chapters 29 and 30, but need to be mentioned here because most of the chemical studies of humus have been made on humus extracted from these soils rather than from mineral soils. This is in some respects unfortunate because humus from the two sources is not exactly the same, although similar. Bog humus is used in such studies chiefly for practical reasons it is more readily obtained in quantity from peat or muck soils and contains much less clay. In mineral soils much of the 1—5% humus usually present is there in stable combinations with the inorganic portion of the soil and cannot be removed except by very drastic chemical treatments that may markedly decompose or otherwise change the organic matter. 

The extractions should be carried out immediately after filtration since further quantities of humus are deposited if the liquid is allowed to stand. 

Many researchers have attempted to unravel the mystery of the structure of humus. One approach has been to isolate fractions by extracting humus using various extraction procedures. These procedures result in the isolation of three or more fractions humic acid, fulvic acid, and humin. Humic material is isolated from soil by treating it with alkali. The insoluble material remaining after this treatment is called humin. The alkali solution is acidified to a pH of 1.0 and the precipitate is called humic acid, while the soluble... 

Humus, which is extracted using a basic aqueous solution, is an exception to this general rule. Fractionation of humus (see Figure 12.1) is also mostly... 

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)... 

A fractionation procedure has been established and widely applied to studies of humic materials [42-44]. The procedure begins with natural OM (i.e., humus) and uses an aqueous basic solution (e.g., 0.1-0.5 mol/1 NaOH and Na2C03) to solubilize a fraction of the OM. The basic extract is then acidified which causes a precipitate to form, i.e., humic acids (HA). The fraction, which remains in solution, is called fulvic acids (FA). Humin is the name given to the insoluble organic fraction that remains after extraction of humic and fulvic acids. At nearneutral pH (pH 5 - 8), which is characteristic of most natural water, the FA are the most water soluble of these three fractions. HA are somewhat less soluble, with their solubility increasing as the pH increases. Humin is insoluble at all pH values. 

Methanol can also be produced from syngas with hydrogen and carbon monoxide in a 2 1 ratio. Coal-derived methanol typically has low sulfur and other impurities. Syngas from coal can be reformed by reacting with water to produce hydrogen. Ammonium sulfate from coal tar by pyrolysis can be converted to ammonia. The humus substances can be recovered from brown coal by alkali extraction. 

Extraction of Organic Compounds from the Soil. Soil biochemicals that are free or absorbed loosely, but not bound to the humus, were extracted by the following procedures ... 

From land rich in mineral constituents, there may be obtained in one year, or for a series of years, by the addition of ammonia alone, in its salts, or of humus and ammonia, rich crops, without in any way restoring the mineral substances removed in these crops, The duration of this fertility depends on the supply of the availahle mineral constituents existing in the soil. The continued use of these manures produces sooner or later an exhaustion of the soil. If after a time the soil is to recover its original fertility, the mineral substances extracted from it by the crops in a series of years must be again restored. 

These definitions were essentially the same as those put forward by Berzelius. Mulder considered, however, that, besides humus substances, products from the decomposition of organic residues, such as leucine, butyric acid, valeric acid, and formic and ethanoic acids, could exist in soil. These observations are of interest because of the information that has emerged in the past half-century about growth inhibitors and stimulators from low-molecular-weight extracts from SOM and composts. 

Sladky, Z. (1959). The effect of extracted humus substances on growth of tomato plants. Biol. Plant. 1,142-150. 

Various separation methods have been used to isolate, fractionate, and characterize humic materials. Originally it was fractionation, based on solubility differences of humic components in diluted alkalis and acids, which laid the ground work for the first classifications of humic substances (HS) in the 19th century (Mulder, 1861 Sprengel, 1837) and provided for operational definition of HS (Kononova, 1966). And now, alkali extraction is the method of choice for isolating HS from solid humus-containing substrates like soil, peat, coal, and so on (Swift, 1996), while hydrophobic resins (e.g., Amberlite XAD resins) are typically used to extract HS dissolved in natural waters (Aiken, 1985). Initial research on HS began with the used simple separation methods to prove, examine, and define characteristics of components of humic matter (Oden, 1919).Today, however, advances in HS research require ever more sophisticated techniques of separation combined with structural analysis (Orlov, 1990 Stevenson, 1994). 

Organic matter extracted with 50% methanol increases during the EP (Yamanaka 1995a). This may be because of the increased solubility of the humus under the alkaline conditions as previously described. In contrast, water-soluble carbohydrates and phenolics decrease by the treatment. This maybe explained, at least partly, by the damage of plant roots that are known to exude water-soluble carbohydrates to soil (Yamanaka 2002). 

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