Salinity measurement and unit conversion
Salinity measurement and unit conversion
Electrical Conductivity
Soil and water salinity is often measured by electrical conductivity (EC). The most commonly used EC units are deciSiemens per metre (dS/m) and millimho per centimetre (mmho/cm), numerically:
1 dS/m = 1 mmho/cm
Simple relationships are used to convert EC to TDS, or vice Versa:
TDS (mg/L or ppm) = EC (dS/m) x 640 (EC from 0.1 to 5 dS/m)
TDS (mg/L or ppm) = EC (dS/m) x 800 (EC > 5 dS/m)
The ratio of TDS to EC of various salt solutions ranges from 550 to 700 ppm per dS/m, depending on the compositions of the solutes in the water. For soil extracts in the EC range from 3 to 30 dS/m, TDS can be estimated using the formula below:The US Salinity Laboratory (1954) also used the following empirical relationship between EC and the total soluble salt concentration (TSS, mmol/L)
TSS (mmol/L) = EC (dS/m) x 10
Sodicity
Changes in the mole fractions of Na+ vs. Ca++ and Mg++ on the cation exchange sites of soils may be assessed by exchangeable sodium percentage (ESP) or by the sodium adsorption ratio (SAR). The ESP of a soil is calculated from:
ESP(%) = Exch. sodium (meq per 100 g soil) / CEC (meq. Per 100 g soil)
Since it is usually difficult to obtain reliable soil exchangeable cation data for calculating ESP, the SAR of soil solution, soil extract, or irrigation water is often used:
SAR = Na+/Sqrt [(Ca++ + Mg++)/2]
where Na+, Ca++, and Mg++ denote the concentrations of respective cations of the water (meq/L).
EC of soil water (ECsw) and EC of saturation paste extract (ECe)
Soil salinity is often measured by electrical conductivity of saturation paste extract (ECe) - The soil sample is saturated with distilled water and mixed to a paste consistency. Nevertheless, the electrical conductivity of the soil water (ECsw) is a better index of soil salinity than is the traditional index ECe because the plant roots actually experience the soil water. However, the EC of soil water depends on the soil to water extraction ratio, i.e., ECsw varies with soil water content and it is not a single number. Thus ECe is used as a criterion for plant salinity tolerance.
In the laboratory, soil EC is often measured by extracts at various soil/water ratios (1:1, 1:2, and 1:5) Empirical equations to convert EC of 1:1, 1:2 or 1:5 soil water extract to ECe are available, but many soil physical properties such as water holding capacity, and chemical properties and processes can influence the conversion from EC of soil water extract to ECe.