Determination of soil organic matter

Paul EA, Clark FE (1989) Soil microbiology and biochemistry. Academic, New York Paul EA, Follet RF, Leavitt SW, Halvorson A, Peterson GA, Lyon DJ (1997) Radiocarbon dating for determination of soil organic matter pool sizes and dynamics. Soil Sci Soc Am J 61 1058-1067... 

Specific directions for determination of soil organic matter are given in the bibliographic references. 

Soil and soil suspensions are colored and hard to see through. Thus, it is hard to directly titrate them using colored indicators. There are typically only two cases where direct titrations of soil are carried out. The first is to determine the amount of amendment needed to bring the soil to a desired pH. The second is in the determination of soil organic matter where organic matter is oxidized with chromate and the unreacted chromate is titrated (actually called a back titration) to determine, by subtraction, the amount of dichromate reduced and thus the amount of organic matter present. 

Storer DA. A simple high sample volume ashing procedure for determination of soil organic matter. Soil Sci. Plant Anal. 1984 15 759-772. 

Method 5.7a. Determination of soil organic matter by loss on ignition... 

Ahmad R, Kookana R, Alston A, Skjestad J (2001) The nature of soil organic matter affects sorption of pesticide. 1. Relationships with carbon chemistry as determined by C-13 CPMAS NMR spectroscopy. Environ Sci Technol... 

Skjemstad JO, Le Feuvr RP, Prebble RE (1990) Turnover of soil organic matter under pasture as determined by 13C natural abundance. Aust J Soil Res 28 267-276... 

For these and other reasons, direct oxidation of soil organic matter using high temperature and atmospheric air, although commonly used, is not the best procedure for determining soil organic matter content. 

There is one case where total destruction of soil organic matter and its constituents is standard practice and that is in the determination of nitrogen... 

Other methods for determining the amount of soil organic matter are available [4], However, they are not as commonly used as is chromate oxidation, which is commonly called the Watley-Black method. Usually, these other methods are both more time-consuming and less accurate than is the dichromate oxidation method. The dichromate oxidation of organic matter is the standard by which all other methods of determining soil organic matter must be compared [4,5],... 

The determination of the methoxy group (OCH ) content of soils is important in studies concerned with the degree of humification of soil organic matter. Methods used to determine methoxy groups in soils have generally been based on volumetric modifications of the classical Zeisel method, which is a complicated and tedious procedure requiring specialised apparatus. 

TABLE 14.3. A Summary of the Abundance of Compound Classes Determined by Py-FIMS in Major Soil Units of the World and in Samples Showing the Influence of Primary Organic Matter Input Either from Vegetation or with Farmyard Manure (FYM) on the Composition of Soil Organic Matter... 

Determination of Humification Degree of Soil Organic Matter and Humic Substances in Different Ecosystems,... 

Ahmad, R., RN. Nelson, R.S. Kookana (2006). The molecular composition of soil organic matter as determined by 13C NMR and elemental analyses and correlation with pesticide sorption. Eur. J. Soil Sci., 57 883-893. 

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. 

Soil organic matter can be measured with or without their separation from the mineral phases. In the case of there being no separation, the usual procedure is the oxidation of soil organic matter, for example, by potassium dichromate in acidic medium. In this way, the carbon content is determined, from which soil organic matter content is calculated using the average elementary composition of humic substances (Kononova 1972). 

Investigations on the influence of the carbohydrates on soil properties, and attempts to determine the composition of soil organic matter, have prompted many workers to determine, quantitatively, the proportion of carbohydrates in soils. 

Pentoses have also been determined in the hydrolyzates of soils (IN sulfuric acid, 1 hour, 120°), after the removal of the uronic acids with anion exchanger, by Dial s orcinol method. They constituted 3 to 5 % of soil organic matter. 

The quantitative determinations, although not always satisfactory, have shown the carbohydrates to constitute about 10% of soil organic matter. The major immediate problem is development of better methods for the determination of the total carbohydrates and their individual monosac-... 

Soils. The structural composition of soil organic matter (SOM) has been determined in 27 soils with different vegetation from several ecological zones of Australia and Pakistan using solid-state CPMAS NMR. Binding of protons, Cu(II) and Pb(II) to a soil fulvic acid, humic acid, and two different size fractions of the humic acid has been investigated solid-state C NMR combined with other techniques. ... 

Soil Organic Matter Organic compounds may exist in soil either in solution or the vapor phase and can be absorbed through the roots in either state, absorption from solution would be the most likely process. From the discussion of the sorption process in soil (see Sorption, Chapter 3) the concentration of the compound in aqueous solution, Caq, would be a function of the concentration in the soil (Csoii). and the soil distribution ratio, K, which in turn is dependent primarily on SOM content. One would predict that uptake would be inversely related to the level of SOM. Observations of the uptake of diel-drin by carrots raised in different soils provide an opportunity to evaluate this relation and demonstrate the dependence on the concentration of the compound in soil solution. If the uptake efficiency is defined by the ratio of the concentration of dieldrin in carrots to that in the soil it is clear that higher levels of soil organic matter reduce uptake by carrots (Table 5.6). The ATom for dieldrin is 6980 mL g from which values were calculated for dieldrin in each soil. Since = Cjoii/Caq the concentration of dieldrin in soil solution can be determined. If uptake is defined as Ccarrot/Caq consistent value... 

The soil fauna play such an important role in humus formation that any reasonably complete discussion of soil organic matter and its effect on plant growth must deal with them. In the limited space available the author will refer briefly to the chief types of animals involved, mention some of the main factors that determine their abundance, and consider the role that they play in soil formation and crop production. 

The carbon content of soil organic matter was early determined by the combustion method to be about 58%, and from this value it follows that the organic matter content of a soil can be obtained by multiplying its carbon content by the factor 1.724. This procedure has been much used, even though it is not accurate, and has been much criticized. Broadbent (1953), for example, presents data for 63 surface soils showing that the factor varied between 1.6 and 3.3 with the bulk of the values falling in the range of... 

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