Why measure salinity?
There are several conditions that can give some of the same signs as saline soils: waterlogging, inundation, disease. The only way to know for sure if the problem is salt, is to take measurements. Measuring salinity can provide information on how saline the soil is at different depths, and if done over time, whether the salinity is increasing or not.
Measures of salinity
Salinity is a measure of soluble salts in soil or water. Salt is any molecule comprised of a cation, such as sodium+, potassium+ and calcium2+, and an anion, such as chloride- and sulfate2-. Sodium chloride (NaCl) is the most common salt in groundwater and soils in Western Australia. Most salt in the Western Australian agricultural areas is from rain deposition over many years.
Estimate or measure salinity by:
- the electrical conductivity (EC) of a solution or soil and water mix, in the field or laboratory
- the apparent electrical conductivity of soil using an electromagnetic induction (EM) device
- chemical analysis of total dissolved solids (TDS) of water or soil in a laboratory to identify and measure ion concentrations.
What does soil salinity mean to a plant?
Plants can only grow in soil when the moisture is between the drained upper limit and the crop lower limit: the same amount of salt in the soil root-zone will be less concentrated at the upper limit, and more concentrated at the lower limit.
Annual plants on saline land experience very high salinity as the soil first wets up, then lower salinity as more water fills the pores and leaches down the profile, then higher salinity as the season finishes and soil dries out. Established perennial plants buffer this variation by having access to deeper and possibly less saline water over a much longer period.
When measuring salt levels on a site, the surface few centimetres are the most important for annuals and seed of perennials, and subsoil salinity is more important for established perennials. Soil samples for testing need to reflect this difference.
Mapping salinity from measurements
There are contractors that use vehicle-mounted EM devices to measure points along transects, then analyse the results and map estimated salinity levels onto a farm plan. This type of information is valuable for interpreting yield maps, for choosing and implementing saline land management options, and for monitoring the effectiveness of management.
Units for expressing salinity
The international standard (SI) unit for EC is Siemens. The Department of Primary Industries and Regional Development expresses soil and water salinity in milliSiemens per metre (mS/m).
The Australian standard for EC water salinity is microSiemens per centimetre (µS/cm) or milligrams per litre (mg/L), and for soil salinity, decisiemens per metre (dS/m).
The temperature of a solution also affects its EC. In Australia, the standard temperature for reporting EC is 25°C. If the EC is measured at any other temperature, adjust the reading to EC at 25°C if the EC meter does not do this automatically. More information is available in Wikipedia.
Measuring EC at less than 25°C underestimates salinity; measuring EC above 25°C overestimates salinity. The equation to correct to the 25°C standard is:
EC at 25°C = EC of sample ÷ (1 + (0.02 × (temperature of sample °C – 25))).
TDS can expressed as parts per million (ppm), milligrams per litre (mg/L), and molarity (millimoles per litre, mmol/L).
Conversions between units
Conversion from mS/m to other units
- 100mS/m = 1dS/m (deciSiemens per metre)
- 1mS/m = 6mg/L = 6ppm (this conversion is approximate based on the salts present in much of Western Australia)
- mS/m x 0.0034 ≈ % TDS in 1:5 mix (conversion varies based on salts present)
- 1mS/m ≈ 0.1mmol/dm3 NaCl ≈ 0.1mmol/L NaCl
Seawater is approximately 3.5% salts
Seawater is often used as a comparison:
- 35g per litre (35g per 1000ml seawater)
- 35 000mg/L or 35 000ppm
- ≈ 5500mS/m
- ≈ 550mMol/L
Estimating soil salinity from EC
EC meters are a cheap and easy way for estimating salinity: the higher the conductivity reading, the higher the salt content. Prices range from less than $200 for simple handheld field-use meter, to more than $1000 for more advanced field or laboratory style testers. These meters give results in EC (siemens) or converted to estimated total dissolved solids (in parts per million).
EC using the 1:5 weight-to-volume (EC1:5w/v) method
This system needs you to have accurate scales for light weights and accurate water volume measurement. The procedure is:
- measure 1 part by weight (grams) air-dried soil to 5 parts by volume (mL) distilled water
- agitate the soil mix to get 95% dissolution of salts, then allow the mix to settle:
- 24 hours for low EC soils
- 3 hours for high EC soils
- measure the EC of the solution using an EC meter, plus the temperature of the solution if the EC meter does not automatically correct for temperature
- if required, adjust the EC to that at 25°C
- interpret the salinity class of the EC measure to allow for soil texture differences (Table 1).
|Salinity class||EC1:5 range for sands (mS/m)||EC1:5 range for loams (mS/m)||EC1:5 range for clays (mS/m)||ECe range (mS/m)|
EC using the 1:5 volume-to-volume (EC1:5v/v) method
This system is likely to have significant errors.
People use this in the field to get quick estimates of soil salinity, as it does not require scales. It uses 1 scoop of soil to 5 scoops of water. The soil needs to be well mixed and finely crumbled (not ground) before scooping: this is assumed to achieve a bulk density of about 1 gram per cubic centimetre, which is the same as water's bulk density, and should give about the same result as the EC1:5w/v method.
This method often has large errors relative to the more controlled w/v method, especially for clay soils. It would pay to 'calibrate' any EC1:5 estimated from the v/v technique against the w/v estimated technique.
ECe is the estimated electrical conductivity of the extract from a saturated soil paste, derived from the EC1:5 measure and adjusted for the soil texture class. Estimation of the soil texture factor introduces errors, especially if manual texturing is used, and ECe from this method is only a guide.
ECse is a direct measure of the EC of the saturated extract of a soil sample. This measure is rarely used, as the measure is time consuming and expensive.
ECe and ECse can show salinity classes without adjusting for soil texture.
ECe is most commonly used in research on plant response to salinity levels.
ECse measurement steps:
- Prepare a saturated soil-water paste by adding distilled water to a sample of air-dry soil (200–400g) while stirring.
- Allow the mixture to stand for at least several hours (but often overnight) to permit the soil to fully imbibe the water and the readily soluble salts to fully dissolve, to achieve a uniformly saturated and equilibrated soil-water paste.
- Extract liquid from the saturated paste by suction (using a funnel and filter paper) or centrifuge.
- Measure the EC (and temperature if the EC meter does not automatically correct for temperature) of this extract using standard conductivity meters/cells and thermometers.
- Calculate the EC value of this extract at 25°C – the standard temperature for ECe values – to give ECe.
Converting EC1:5 to ECe
To allow for the soil texture differences, the 1:5 reading is often used to estimate the ECe. Conversion factors from EC1:5 to ECe; and ECe toEC1:5 are in Table 2. Treat the values from these conversions as rough estimates.
Sand particles will not hold as much salt from the soil water as will clay. Therefore, the same level of salt, as measured by the EC1:5 method, will more severely affect plants in lighter textured soils (sands) than heavier textured soils (clays).
|Soil texture||Conversion factor|
Another common soil salinity estimate is apparent electrical conductivity (ECa), using a portable electromagnetic induction (EM) meter, such as the EM38 (Figure 1) or EM31.
The EM measurement strongly correlates with salinity, although other soil factors affect the readings: soil moisture, soil texture (particularly clay content), sodicity and soil temperature. We recommend that the EM readings be calibrated for a soil type against laboratory EC1:5 readings of soil samples to 1.25m deep (for detail, see the EM38 field-operating guide).
Table 3 shows the comparison between ECe, ECa, and EC1:5.
|Salinity class||ECe all soils |
EM38 horizontal mode
|EC1:5 (w/v)loam |
The EM38 (download a guide to using and interpreting the readings) is an expensive unit, but relatively easy to use in the field (Figure 1), and designed to estimate the bulk EC of the rooting zone to about 1.5m. The EM31, a larger machine, estimates bulk EC to about 6m depth.
Measuring salinity from total dissolved solids (TDS)
Analysis of soil or water TDS by an accredited laboratory is the most rigorous method of measuring salinity.
Laboratories can analyse the TDS, which is a measure of the sum of particulate material dissolved in water and represents the total salt content.
Measure or estimate TDS by:
- chemical analysis and summation of all the major anions and cations present in the sample (most accurate measurement of salt content)
- the gravimetric technique where a known volume of sample is evaporated at 180°C to dryness and remaining solid residues are weighed
- converting EC to TDS (an estimate).
TDS is recorded in milligrams of dissolved solid in 1 litre of water (mg/L). Parts per million (ppm) is approximately equal to mg/L when the water density is assumed to be 1kg/L.
Estimating salinisation risk from watertable depth and trend
Depth to watertable and the trend in watertable depth gives an indication of salinity risk. The risk is greatest where groundwater can reach the soil surface by capillary rise and evaporation concentrates salts on the soil surface.
Capillary rise is greater in heavy clay than in sand. The critical depth – where salt toxicity reduces agricultural production by more than 30% – is generally taken to be about 2m.
At the landscape level, salinisation risk is mapped at a vertical height of 2 meters above valley floors. See NRInfo in the layers Soil–Land qualities–Salinity hazard or Salinity–Land Monitor.
The critical depth on a particular site will vary with the concentration and composition of salt in the groundwater, the frequency and amount of rainfall, soil physical properties and the salt tolerance of the crop.
A very shallow layer of coarse sand on the surface provides a mulching effect, which reduces capillary rise and allows rainfall flushing of salts in the topsoil, providing better conditions for seed germination.