Soils electrical conductivity measurement

Corwin DL, Lesch SM (2005) Apparent soil electrical conductivity measurements in agriculture. Comp Electron Agric 46 11 3... 

The values for electrical conductivity measured on site can provide information on the possibility of utilizing water as irrigation water for agriculture. Thus, in the case of electrical conductivities of up to 250 pS cm at 20 C, no salinization of the soil need be feared. Such water is generally suitable. In the case of electrical conductivities of up to 750 pS cm" at 20 C, the water can as a rule still be tolerated. 

Electrical conductivity (or its mathematical inverse, resistivity) of a soil solution is strongly correlated with total salt content. Therefore, laboratory methods involving solution or saturated paste conductivity are often used to assess soil salinity. Electrical conductivity measurements of bulk soil (designated as ECa for apparent electrical conductivity) were also first used to assess salinity. Resistivity and conductivity measurements are also useful for estimating other soil properties, as reviewed by and. Factors that influence ECa include soil salinity, clay content and cation exchange capacity (CEC), clay mineralogy, soil pore size and distribution, soil moisture content, and temperature. ° For saline soils, most of the variation in ECa can be related to salt concentration. In non-saline soils, conductivity variations are primarily a function of soil texture, moisture content, bulk density, and CEC. The theoretical basis for the relationship between ECa and soil physical properties has been described by a model where ECa was a function of soil water content (both the mobile and immobile fractions), the electrical conductivity of the soil water, soil bulk density, and the electrical conductivity of the soil solid phase.Later, this model was used to predict the expected correlation structure between ECa data and multiple soil properties. ... 

Rhoades JD, Corwin DL, Lesch SM (1999) Geospatial measurements of soil electrical conductivity to assess soil salinity and diffuse salt loading from irrigation, hi Corwin DL (ed) Assessment of non-point source pollution in the vadose zone. American Geophysical Union, Washington, DC, pp 197-215... 

Kitchcm NR, Sudduth KA, Drummond ST (1999) Soil electrical conductivity as a crop productivity measure fw claypan soils. J Prod Agric 12 607-617... 

Electromagnetic (EM) Conductivity Measures the electrical conductivity of materials in microohms over a range of depths determined by the spacing and orientation of the transmitter and receiver coils, and the nature of the earth materials. Delineates areas of soil and groundwater contamination and the depth to bedrock or buried objects. Surveys to depths of SO to 100 ft are possible. Power lines, underground cables, transformers and other electrical sources severely distort the measurements. Low resistivities of surficial materials makes interpretation difficult. The top layers act as a shunt to the introduction of energy info lower layers. Capabilities for defining the variation of resistivity with depth are limited. In cases where the desired result is to map a contaminated plume in a sand layer beneath a surficial clayey soil in an area of cultural interference, or where chemicals have been spilled on the surface, or where clay soils are present it is probably not worth the effort to conduct the survey. 

In addition to obtaining soil samples and transporting them to the laboratory, there has been research into on-the-go soil analysis. This is accomplished using sensors attached to implements in contact with the soil. A soil sample is taken, analyzed, and then returned to the field over a short period of time. In addition to specific sensors, reflection and conductance are other approaches used to obtain information about soil. Electrical measurements of soil are further discussed in Chapter 9. 

One place where soil solution from saturated soils is routinely used is in the determination of salts in soil. Salts can build up in soil in low-rainfall areas. A measure of the salt content is essential for these soils. Typically, a saturated paste of soil is made and the water filtered by vacuum filtration. The filtrate can then be analyzed for electrical conductivity and this measurement, together with calcium, magnesium, and sodium content, can be used to calculate various measures of salt content and potential detrimental effects of salts on crops and water quality. 

In soil analysis, pH, specific ion, oxidation-reduction (redox), electrical conductivity (EC) cells, and oxygen electrodes are commonly used. For each of these measurements, a different specific electrode along with a separate or integral reference electrode is needed. In some cases, with extended use or long exposure to soil or soil-water suspensions, electrodes may become polarized. When this happens, erroneous results will be obtained and depolarization will need to be carried out using the electrode manufacturers directions [3],... 

An example of how kinetic measurements may suggest a mechanism for complex formation between soil compounds is the research of Lopez-Quintela et al. (1984) who studied the reaction between Al ions and citric acid and leaf extract in aqueous solutions. A stopped-flow apparatus was used to study the kinetics and the reaction progress was monitored by electrical conductivity. 

Resistivity meter Used to measure electrical conductivity in soils that may be due to the presence of buried disturbances such as fireplaces, burials, or other structures. 

To measure soil conductivity, the soil is mixed with water until it has a pastelike consistency. A pair of electrodes, commonly made of platinum metal, is inserted into this saturated paste. The electrodes are characterized by their surface area, A cm (usually about 1 cm ), and their separation distance of L cm (usually about 1 cm), as illustrated in Figure 8.3. An alternating current is then applied to the electrode pair, and the conductance of the paste is measured in units of reciprocal ohms (mho), or Siemens (1 Siemen = 1 mho). But this conductance depends on A and L and is therefore a characteristic of the measuring device as well as the solution. It is more useful to define a specific conductance, termed the electrical conductivity or EC, by the equation ... 

Figure 8.3. Two-electrode cell used to measure electrical conductivity of solutions and saturated soil pastes.

The conductivity of a soil is precisely the specific conductivity at 25°C of a water extract obtained at a definite soil water ratio. The electrical conductivity is measure on an electrical conductivity bridge and is normally reported in mmhos cm k A fairly quantitative estimate of the soluble salt content of solutions extracted from the soils can be made from their electrical conductance. Soil extracts obtained using high water to soil ratios are as less accurate measure of the solute content of the soil since more salts may be removed than are ever present in the soil, at field moisture contents. Usually soil water ratio of 1 2.5 or 1.5 is used for routine measurement. Thus the soil water ratio employed must be specified with the analysis. 

Textural analysis was conducted using temperature-controlled water baths and the Bouyoucos hydrometer method ( 8) with 2 modifications. First, soil samples were agitated for 20 seconds with plastic plungers instead of mixing the soil by inverting the container, and second, the clay suspension was not washed and sieved to determine fractional sizes. Organic matter percentages were determined with a dichromate reduction method (9). Electrical conductivity (ec) and pH were measured from the saturation extract of... 

The saltpan in the basin of Fig. 11.1 exemplifies a more common occurrence of soil salinity. Soils in low-lying areas, even in arid regions, may have high water tables. Water from groundwater tables within a few meters of the surface can move by capillarity to the soil surface, where it evaporates and leaves behind its salts. Figure 11.2 shows an example of salt distribution above a water table 90 cm below the soil surface. The soil salinity concentration is expressed as electrical conductivity, the common method of measurement. 

More recently salinity has been measur ed in terms of tire electrical conductivity (EC) of a solution. In addition to overcoming some of the ambiguities of TDS measurements, the EC measurement is quicker and sufficiently accurate for most purposes. To determine the EC, the solution is placed between two electrodes of constant geometry, including constant distance of separation. When an electrical potential is imposed, the electrical current varies directly with the total concentration of dissolved salts. The current is inversely proportional to the solution s resistance and can be measured with a resistance bridge. Conductance is the reciprocal of resistance and has units of reciprocal ohms or siemens (formerly mhos). The EC of the saturation extract of the soil measures the salinity of the soil. 

Salinity defines the concentration of ions dissolved in soil water, and is measured directly by electrical conductivity (EC). Soil water is held under tension or suction and as such is not available for analysis. To compensate for this, the standard procedure is to measure soil EC on a water extract. Soil scientists use what is called a saturated paste soil water extract (for more details see Radojevic and Bashkin, 1999). 

The salinity may be determined directly as a total content of water-soluble salts, or as a sum of contents of particular ions. The measurement of the specific electric resistance of the soil is a very rough, but simple and rapid method. It is used for soil surveys on a wide scale. For achieving more precise results, it is possible to use a method standardized by measuring the resistance in saturated soil paste (the soil paste is prepared by mixing a soil sample with distilled water to obtain a moderately liquefied paste) and the results are corrected with respect to a temperature of 16°C. Tables are used to convert the measured electrical resistance values to the soil concentration. For more precise data, it is possible to separate the solution from the saturated soil paste and to measure the specific electric conductivity. The solution separated from the soil paste is called the saturated soil extract. 

Conductance measurements are useful practically to monitor ionic distribution profiles in electrokinetic laboratory test cells and to characterize the electrical properties of soil samples. 

The electrical conductivity of a sediment depends on the soil type, porosity, water content, and pore water composition. Devices to measure conductivity have been combined with a standard electrical cone to give measurements of cone tip resistance, sleeve friction, and friction ratio (ratio of these two). 

A general measures of salinity indicative of aggressiveness of ground and related to electrical conductivity or soil resistivity Undisturbed specimens required in sterilized containers... 

The chemical characterization of soils and soil components include a number of methods, ranging from simple measurements such as pH or electrical conductivity to elemental analysis after total dissolution by digestion with HNO3/HCI/HF (Sparks 1996). Table 7.8 summarizes the most important analysis methods for elements of interest in soils. Many of these methods have been covered in previous sections. Some of them (e.g., chromatography, colorimetry) are common, well-known analytical techniques (Skoog et al. 2004 Harris 2010). 

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