Indirect soil measurements

Direct measurements 1 Mediated direct measurements 1 Indirect soil measurements 1 Destructive soil analytical methods J Soil solution Soil solids... 

Because radon is a gas, its occurrence in soil is most appropriately referred to as its occurrence in "soil-gas," which is in the gas or water-filled space between individual particles of soil. Factors that affect radon soil-gas levels include radium content and distribution, soil porosity, moisture, and density. However, soil as a source of radon is seldom characterized by radon levels in soil-gas, but is usually characterized directly by emanation measurements or indirectly by measurements of members of the uranium-238 series (National Research Council 1981). Radon content is not a direct function of the radium concentration of the soil, but radium concentration is an important indicator of the potential for radon production in soils and bedrock. However, Michel (1987) states that average radium content cannot be used to estimate radon soil-gas levels, primarily due to differences in soil porosity. 

Indirect methods of estimating sorption have been used when actual measurement of sorption isotherm is impossible (44). For instance, sorption coefficients have been estimated from soil organic carbon and a specific surface of soil, and from semiempidcal equations using pesticide properties. 

There is a direct and an indirect method of measuring specific soil resistivity. The direct method is carried out in the laboratoiy on a soil sample using a soil box as shown in Fig. 3-16. The resistivity of a soil specimen of cross-section, S, and length, I, is measured and the specific resistivity determined ... 

The most commonly applied indirect method of measuring soil resistivity using the four-electrode arrangement of Fig. 3-14 is described further in Section 24.3.1. The measured quantities are the injected current, /, between the electrodes A and B, and the voltage, t/, between the electrodes C and D. The specific soil resistivity follows from Eq. (24-41). For the usual measuring arrangement with equally spaced electrodes a = b,ii follows from Eq. (24-41) ... 

The content of the material in a carbon reservoir is a measure of that reservoir s direct or indirect exchange rate with the atmosphere, although variations in solar also create variations in atmospheric content activity (Stuiver and Quay, 1980, 1981). Geologically important reservoirs (i.e., carbonate rocks and fossil carbon) contain no radiocarbon because the turnover times of these reservoirs are much longer than the isotope s half-life. The distribution of is used in studies of ocean circulation, soil sciences, and studies of the terrestrial biosphere. 

The Rothamsted Carbon Model (RothC) uses a five pool structure, decomposable plant material (DPM), resistant plant materials (RPM), microbial biomass, humified organic matter, and inert organic matter to assess carbon turnover (Coleman and Jenkinson 1996 Guo et al. 2007). The first four pools decompose by first-order kinetics. The decay rate constants are modified by temperature, soil moisture, and indirectly by clay content. RothC does not include a plant growth sub-module, and therefore NHC inputs must be known, estimated, or calculated by inverse modeling. Skjemstad et al. (2004) tested an approach for populating the different pools based on measured values. 

Direct and indirect measurements are important in understanding the complex chemistry of soil and the soil environment. For a full understanding of its chemistry, all measurements of all types must be synthesized into a whole picture of that soil. The various specific types of measurements will be discussed in greater detail in subsequent chapters. In this chapter, only a general outline of the basic concepts and tools used to analyze different soil components will be covered. A flowchart of methods of analyzing soil is given in Figure 8.1. 

Indirect measurement of soil and its constituents is necessary in a majority of cases. Extracting solutions have been devised for inorganic and organic constituents of soil such that they can be removed from the soil matrix, in some cases purified and concentrated, and subsequently measured (see Chapters 11 and 12). 

Soil solids are either inorganic or organic, and each requires different analyses. Direct measurements on whole soil are not as common as indirect measurements. Direct measurements are limited to X-ray, IR, and NMR analysis. X-ray analysis is commonly used to investigate mineral structure, while IR is used to... 

Explain the difference between direct, mediated direct, and indirect measurement of soil components. 

Explain how indirect measurement of soil chemistry is different from direct measurement. 

The identities of the solid phases that form remain a mystery. Direct identification is difficult because Fe(II) and Mn(II) solid phases are readily oxidized by O2 and it is therefore necessary to maintain scrupulously anoxic conditions to ensure that the material examined actually represents that in anoxic soil. An alternative is to make indirect assessments through measurements of pe, pH and [Fe +] in solution, but these too are difficult (see section on measurement of redox potential in soil). 

Indirect exposure of humans via the environment may occur by consumption of food (fish, crops, meat, and nulk) and drinking water, inhalation of ambient air, and ingestion of soil. For existing substance, measured levels in various environmental compartments may be available however, for new substances, usually no relevant measured data are available and concentrations of a substance in the environment must be estimated. 

Adding limestone to soils to correct soil acidity and to supplement available calcium will, of course, indirectly affect human and calcium nutrition, but this is a difficult quantity in measure. 

The emissions from all soil sites other than the . altemiflora marsh followed a pattern that was consistent with release of microbial metabolites from the soil surface. H2S and DMS were the predominant species found, with emission rates 10-100 times greater than those of CS2 and DMDS. Peak emissions of all compounds were measured in the early afternoon, corresponding to the maximum soil temperature. No effects were found due to the extent of vegetation coverage. Die difference between these sites and the . altemiflora sites provides indirect evidence for the role of DMSP cleavage in the release of DMS from the tissues of the , altemiflora. Elevated concentrations of DMSP have been reported to occur in the leaves of . altemiflora. when compared to other American salt-marsh species (6). 

Reconstruction of faunal records in deep-sea sediments and oxygen isotope measurements have been yielding useful information of the ocean s paleotemperature. However, the lack of these tools in the case of continents has hampered the estimation of paleotemperature in continents. The latter estimate has mainly been made on somehow indirect approach such as pollen data, periglacial feature, and soil carbonate, all of which suffer from considerable uncertainty stemming from the assumption that must be made to convert the observation to temperature. The advantage of the noble gas thermometer, as compared to the other paleotemperature methods, is that it is based on the relatively simple physical principle that directly relates noble gas concentration to the ambient temperature (see Stute Schlosser, 1993, for a recent review of the noble gas paleotemperature method). However, in actual practice, we need some cautions. 

Measurement of soil activity there are a number of laboratory methods which are suitable for measuring the biological activity of the soil. In principle, a distinction is made between direct and indirect methods for the determination of soil activity. The biomass in the soil, for example, can be estimated by counting the individual organisms in the soil, or the measurement of respiration after the addition of a nutrient in excess can provide an indication of active biomass. Moreover, in determinations of activity, a distinction is made between actual and potential activity. Actual activity values are values measured at the time that the sample was taken. Determinations of potential activity, on the other hand, show the level of performance that microorganisms are capable of under optimum experimental conditions, after the addition of a nutrient substrate and prolonged incubation. 

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