How Do You Measure Soil Salinity?

Measure the electrical conductivity (EC) of the soil to ensure that it is being decreased to a safe value (i.e., less than 3 dS/m). There are many different models of EC meters and the cost varies from US$30 to over $100.

Soil salinity varies both spatially and vertically. Therefore, sufficient soil measurements should be made in both directions to have a good idea of the extent of salinity in a specific field. Measurements conducted on standing water in the field may not indicate the level of salinity in the soil. Soil salinity is mostly measured in terms of EC of the soil solution using EC meters, with units of decisiemens per meter (dS/m) or millimhos per centimeter (mmhos/cm); both are equivalent units.

Soil salinity can be assessed in different ways:

Saturation extract: Use this method if laboratory facilities are available. The technique involves measuring the electrical conductivity of the solution extracted from a water-saturated soil paste. Distilled water is added to 200–400 g air-dry soil—while stirring—until the soil is fully saturated. Allow the sample to stand for at least 4 hours. Use a suction filter (pressure plate) or a centrifuge to extract sufficient water. Add 1 drop of 0.1% (NaPO3)6 solution for each 25 mL extract. Measure the ECe of the extract. The measurement is abbreviated as ECe (Electrical Conductivity of the extract). This method gives a more meaningful measure of soil salinity and is independent of soil texture, but it is more time-consuming.
Soil water extract: This method also requires laboratory facilities, but is easier than the saturation extract method above. Prepare a mixture of soil:water (1:5 by weight) using air-dry soil and distilled water. Shake for 1 hour (or, if a shaker is not available, shake the sample by hand for 1 minute at least 4 times at a 30-minute interval). Allow the soil to settle and filter the solution. Add 1 drop of 0.1% (NaPO3)6 solution for each 25 mL extract. Measure the EC of the solution above the settled soil. Measurements are dependent on soil texture. To convert these EC1:5 units to ECe, multiply by 23 (for sandy, loamy, and clayey sand soils), by 14 (for sandy loam, light sandy clay loams), by 10 (for clay loams, silty and fine sandy clay loams), by 9 (for sandy, silty, light clay, and light medium clay), by 8 (for medium clay soils), and by 6 (for heavy clay soils).
Piezometers: This method is quick and can be used directly in rice fields when there is standing water. Plastic or metal tubes perforated or slotted on the sides are inserted in wet soil with the perforated zone aligned with the soil profile where salinity is to be measured. The tube should be long enough so that its top remains a few cm above any water standing in the field. The holes on the side of the tube should be small enough to allow only soil solution to pass through. If excluding soil is a problem, the tube can be covered with a piece of cotton cloth with the bottom sealed. The EC of the solution accumulating in the tube can be measured using an EC meter.

Piezometers can be fabricated using any available tubes and can be inserted in different areas in the field and at different depths along the soil profile. For rice fields, measurements should be made within the top 30 cm of the soil.

Piezomter (Photo courtesy of Solinst, www.solinst.com/Prod/601/601.html)

Reference: Rhoades JD. 1982. Soluble salts. In: Methods of soil analysis. Part 2. Second Edition. ASA. p 167-180.