Measuring salinity of solutions and soil samples

Protocol

Author

Rana Munns

Overview

Description of units, expected ranges of values and methods relating to measuring the salinity of solutions in nutrient culture and soil samples.

Background

Salinity of tap water, waste water, rivers or soil samples can be measured with an EC (electrical conductivity) meter.

The electrical conductivity of irrigation or river water is usually expressed in μS/cm (1000 x dS/m). The upper limit recommended for human consumption is 830 μS/cm (8 mM NaCl). For irrigation of salt-sensitive species such as grapevines it is about 1200 μS/cm. This particular EC may not stress the plant, but the salt quickly concentrates in the soil as the plant takes up water and leaves the salt behind, and reaches probably four times this concentration around the roots.

Irrigation water quality may also be expressed as total soluble salts. An international convention is that 1 dS/m is equivalent to 640 mg/L of mixed salts (640 ppm). This is based on the composition of irrigation water in California.

For measuring soils in situ, the EM38 meter made by Geonics is used (see more information here). This instrument transmits and receives electromagnetic induction and gives an apparent electrical conductivity (ECa), which is not simply the electrical conductivity of the soil solution but is influenced by the water and the clay content of the soil, so must be calibrated by soil cores as described below.

Materials/Equipment

For soils:

  • Drying oven
  • Soil corer if samples are taken from the field
  • Soil grinder to subsample cores from field

For field use, e.g. river water, bore water, Eutech Instruments make cheap and reliable meters, of which “ECScan High” is suitable for all purposes

For laboratory use, precise values can be obtained with a digital conductivity meter (e.g. CDM 210, Radiometer Analytical SAS, Lyon, France).

Units, terms, definitions

The standard unit of electrical conductivity (EC) is deci-Siemens per metre (dS/m). Table 1 shows the relationship to other units of conductivity. Note that 10 mM NaCl has an EC close to 1 dS/m.

Table 1: Units for measuring salinity, and conversion factors.

Measurement and units Equality to 1 dS/m Equivalent units
Conductivity (dS/m) 1 1 dS/m = 1 mS/cm = 1 mmho/cm
Conductivity (μS/cm) 1000 μS/cm 1 μS/cm = 1 μmho/cm

Procedure

For solutions, the probe of a conductivity meter is immersed in the solution. These should be previously calibrated with NaCl or KCl as per Table 2 below.

For soils, a convenient method for measuring soil salinity is a ‘1:5 extract’. The soil is dried, shaken with 5 g deionised water per 1 g soil, the probe of a hand-held conductivity meter is placed in the suspension, and the EC measured. The EC of the soil that was sampled can be calculated accurately if its water content is known at the time of sampling.

If the soil moisture at the time of sampling is not known, the measurement can be related to the field capacity of the soil using conversion factors. For most clay-loam soils, the EC of the 1:5 extract is multiplied by 10, which gives an approximation of the salinity of the field when it is moist, ie at field capacity. As a rough guide, a sandy soil will have a field capacity of 0.2 g water per g soil, and a clay soil about 0.4 g/g.

Table 2: Electrical conductivity (EC) of pure solutions at 20∘C (dS/m).
The solutions represent those of salts found in soils or in seawater. Data from the Handbook of Physics and Chemistry (CRC Press). Sea water is approximately 500 mM NaCl.

Solution EC (dS/m)
10 mM NaCl 1.0
100 mM NaCl 9.8
500 mM NaCl 42.2
10 mM KCl 1.2
10 mM CaCl2 1.8
10 mM MgCl2 1.6
50 mM MgCl2 8.1

Other resources

  • Handbook of Physics and Chemistry (CRC Press)

Notes and troubleshooting tips

  • The EC of a salt solution depends on the type of salt, the concentration (which is not linearly related to the EC) and the temperature. So the instruments should be calibrated for a given temperature.

Eutech Instruments (for rough measurements)

EM38 meter – Geonics

Radiometer Analytical SAS, Lyon, France (for precise measurements)

Literature references

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