Salinity is a soil condition with a high concentration of soluble salts, high enough to affect plant growth. The USDA Salinity Laboratory defines a soil as saline when its saturated solution has an ECe of 4 dS/m. An ECe of 4 dS/m is equivalent to about 40 mM NaCl. However, the growth of many species is affected by soil with an ECe less than 4 dS/m, as soils are rarely saturated, and the salt concentrates as the soil dries.
Terminology and equations
ECe is the electrical conductivity of the -saturated paste extract’, that is, of the solution extracted from a soil sample after being mixed with sufficient water to produce a paste. This is equivalent to the solution in saturated soil, or in hydroponics. Table 1: Units for measuring salinity, and conversion factors.
Conversion factors relating total dissolved salts or pure NaCl to an electrical conductivity (EC) of 1 dS/m (deci-Siemen per metre), along with equivalent units of various types, old and new. The conversion of EC of 1 dS/m to total dissolved salts (640 mg/L) assumes a composition of salts that is common in groundwater across the world. The exact factor varies from 530 (if the salt is predominantly NaCl) to 900 (if the salts are formed predominantly from divalent ions).
|Measurement and units||Use||Equality to 1 dS/m||Equivalent units|
|Conductivity (dS/m)||soils||1||1 dS/m = 1 mS/cm
= 1 mmho/cm
|Conductivity (μS/cm)||irrigation and river water||1000 μS/cm||1 μS/cm = 1 μmho/cm|
|Total dissolved salts (mg/L)||irrigation and river water||640 mg/L (approx.)||1 mg/L = 1 mg/kg
= 1 ppm
|Molarity of NaCl (mM)||laboratory||10 mM||1 mM = 1 mmol/L|
The electrical conductivity of irrigation or river water is expressed in units 1000 times magnified, as channel or river water would normally have a very low concentration of salts. River water quality is often expressed as μS/cm (1000 x dS/m). Irrigation water quality is often expressed as total soluble salts, an international convention being that 1 dS/m is equivalent to 640 mg/L of mixed salts.
Three treatment approaches can be taken. The most common and convenient one is solution culture, which can be supported by solid material such as fine gravel or high density plastic beads. A recirculating nutrient solution using a modification of the original Hoagland’s solution (Munns and James, 2003) is applied using aeration in pots or subirrigation in tanks. A second method is sand culture, when the sand is irrigated with Hoagland’s nutrient solution (Hoagland and Arnon, 1938). A third method is to use soil as the medium, which is likely to best mimic field conditions, but in small pots the soil needs to flushed periodically with salt-nutrient solution (making is similar to sand culture but with reduced drainage) or rewatered by replacing the water transpired but in this case it creates pockets of low-salinity soil in high-salinity background. Experiments should be conducted over a reasonable time period, and the salt increased in gradual steps to avoid severe osmotic shock. Plant responses in the short-term are primarily osmotic, and only in the longer term (days, weeks or months, depending on the species) does the salt rise to toxic concentrations in leaves and the salt-specific effect is seen (Munns 2002). In all three cases, a decision needs to be made about which salts to use, at which stage of plant develop to start the salinity treatment, and at what concentration.
Ranges of values
Saline soils that support plant growth range have an ECefrom 4 to 12 dS/m, or 40-120 mM NaCl, which means that in reality the EC of the soil solution is usually 2-3 times this, unless after irrigation or rain. Seawater is about 500 mM NaCl, or about 2.5 MPa, and about 42 dS/m. For salinity treatments for experimental purposes, a range of 25-100 mM NaCl is appropriate for sensitive species, for the more tolerant species a range of 50-200 mM, and for halophytes 50-600 mM NaCl (see Figure 1 in Munns and Tester, 2008). In most soils, Na+and Cl–are the dominant ions, but in some regions there are high concentrations of Mg2+and Ca2+, and in others SO42-can also be high. The composition of the treatment solutions should be adjusted accordingly.
Hoagland DR, Arnon DI. 1938. The water-culture method for growing plants without soil. Circular 347, University of California, College of Agriculture, Berkeley.
Munns R. 2002. Comparative physiology of salt and water stress. Plant Cell and Environment 25, 239-250.
Munns, R. 2010. Salinity stress and its impact. In: Blum A, ed. Plant Stress Website. http://www.plantstress.com/Articles/index.asp
Munns R, James RA. 2003. Screening methods for salinity tolerance: a case study with tetraploid wheat. Plant and Soil 253, 201-218.
Munns R, Tester M. 2008. Mechanisms of salinity tolerance. Annual Review of Plant Biology 59, 651-681. USSL. 2010. United States Salinity Laboratory. http://www.ussl.ars.usda.gov