Measurement of Soil Salinity and Sodicity
Jim Wang, Tony Provin and Hailin Zhang
Application and Principle
All soils contain soluble salts with major dissolved inorganic ions of Na+, Mg2+, Ca2+, K+,
Cl , SO42-, HCO3-, and CO32-. Soils are considered saline when they contain high levels of soluble
salts, which can have negative impacts on crop growth through the reduction in water availability to
the plant or toxic effects of individual ions such as H2BO3- and Ba2+ under hypersaline conditions. In
arid and semiarid climates, soluble salts accumulate in soil surface due to high potential of
evapotranspiration along with insufficient water to leach soluble salts from the soil. For irrigated
lands, poor irrigation water quality, defined as water with elevated levels of soluble salts, as well as
poor drainage due to high water table and low soil permeability can also result in accumulation of
high salt levels. In addition, salt-affected soils can be caused by salt water spills from oil field
activities as well as high rates of manure and sludge applications. In coastal areas, seawater intrusion
has also increasingly become an important cause of soil salinity. Sodic soils, on the other hand, are
those soils containing high level of Na+ relative to other major cations, which can result in dispersion
of soil particles and poor water permeability. The latter also leads to the disturbed soil-water-plant
interrelationship (Evangelou, 1998).
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Soil salinity is generally characterized by determining total dissolved (TDS) or electrical
conductivity (EC) of soil solution. The two are closely related but EC is often measured. There are
several different approaches to measure soil EC. An appropriate procedure may be selected
depending on the type of information needed in a particular situation. When a rapid in-situ
measurement of the field apparent EC (bulk soil EC) is desired, measurement based on noncontacting terrain EC sensors including electromagnetic (EM) systems may be used (Rhoades, 1990).
Such a procedure has often been used for characterizing salinity of large fields, and it has general
lower accuracy than those otherwise based on EC measurement performed on water extracts of soil
in a conventional laboratory. The laboratory determination of EC is accomplished using a
conductivity cell consisting of metal electrodes, and the measurement is expressed in specific
conductivity unit (deci Siemens per meter, dS m-1), which is the product of the measured
conductance (reciprocal of resistance) and the conductivity cell constant. A conductivity cell constant
is determined by the specific length (L) and cross-sectional area (A) of the two electrodes.
Alternatively, TDS can be determined by a tedious procedure based on residue weight after
evaporation to dryness after filtration (Rhoades, 1996). This chapter will focus on prevalent
laboratory procedures for characterizing soil salinity based on EC measurement.
Most commonly used laboratory procedures for measuring EC or TDS of soil solution is
based on water extracts. The extraction based on the saturated paste has served as a standard
procedure because it mimics the field condition the best for most soils. Although this extract varies
with soil texture, it is often used because it is the lowest extract for most soils for which sufficient
extract can be practically removed from a soil sample for compositional analysis of major
constituents and because it is better related to soil-water contents under most field conditions. For
these reasons, plant tolerance to salinity has been expressed based on EC values of a saturation paste
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extract (USDA, 1954; Mass and Hoffman. 1977). On the other hand, other extracts of fixed soil to
water ratio, especially 1:1 and 1:2 on weight basis, have also been widely used by many laboratories
for reducing the difficulties associated with the preparation of saturated paste extracts.
Soil sodicity is generally characterized by exchangeable sodium percentage (ESP) based on
sodium saturation of cation exchange capacity (CEC). However, it is more often expressed by
sodium absorption ratio (SAR) due to the easy determination of individual ions in soil solutions (or
water extracts) and the close numeric relationship between the two (Evangelou, 1998). SAR is
calculated as Na/(Ca+Mg)1/2 based on concentrations of Na+, Ca2+, and Mg2+ in mmol L-1 in the
extract of a saturated paste or other soil to water ratios as measured by an ICP.
Equipment and Apparatus
1. Weigh balance
2. Plastic containers or glass Erlenmeyer flasks
3. Automatic solution dispenser
4. Reciprocating shaker
5. Vacuum line and suction apparatus
6. Conductivity cell(s)
7. Conductivity meter, preferably with temperature compensation
8. ICP-AES or ion chromatography (IC)/flow injection analysis (FIA) if individual solutes
are determined
Reagents
1. Deionized (D.I.) water
2. Standard KCl solution at 0.010 M: Dissolving 0.7456 g KCl (fw: 74.551, CAS# 7447-407) in 1 liter of D.I. water. The EC for this solution is 0.147 dS/m at 25 oC.
3. Standards for ICP analysis.
Procedure
Extraction
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1:1 or 1:2 Soil to Water Extraction
1. Weigh 20 g (± 0.05 g) of air dried soil ground to pass10 mesh sieve (< 2.0 mm) in a 125mL glass or plastic Erlenmeyer flask.
2. Add 20 mL or 40 mL of D.I. water to the flask to make the soil to water ratio of 1:1 or
1:2 on a weight basis depending the extraction ratio of preference.
3. Place extraction flask(s) on reciprocating mechanical shaker for 1 hr.
4. Filter suspension immediately using highly retentive filter paper and collect the extract in
50 mL plastic containers (refilter if filtrate is cloudy).
Larger soil sample size but with similar soil to water ratio may be used to generate more
extract for other purpose of analysis especially when individual ionic solutes need to be
measured.
1:1 or 1:2 suspensions may also be used for direct measurement of EC without filtration,
especially if the determination of individual solutes such as Na+, Ca2+, and Mg2+ in the
filtrate is not required.
1:2 extraction of soil to water ratio on volume basis has been also used but not common.
Alternative 1:1 or 1:2 Soil to Water Salinity Measurement
1. Weigh or scoop 20 g (± 0.05 g) of soil into a 125 mL beaker or similar container.
2. Add 20 mL or 40 mL of D.I. water to the beaker and stir with glass or Teflon coated rod
until well mixed.
3. Repeat mixing two additional times during the next 30 min.
4. Allow suspension to settle and measure the soil pH and electrical conductivity in the
supernatant.
The alternative method is quicker than the extracted and filtered method but has several
limitations including: 1) tube shaped dipping conductivity electrodes are of limited value
due to time required to clean and potential fouling of the electrode, 2) the presence of
sparing soluble salts and minerals (e.g., gypsum) may not dissolve during the 30 min.
timeframe, thus skewing the soil salinity potential to lower value, and 3) extremely low
buffered soils (CEC<3) maybe over-saturated and thus the result in a low salinity
assessment.
The use of short timeframe salinity tests should be limited to initial determination of soil
salinity potential. This fast assessment, done in conjunction with soil pH is an excellent
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screening tool that laboratories can routinely conduct and its use provides the end user
with greater information on when to conduct more exhaustive soil salinity tests.
Saturated Paste Extraction
1. Weigh 100-250 g moist soil in a 250-mL glass or plastic container. (Alternatively weigh
air dried soil pulverized to pass 10 mesh sieve (<2.0 mm) in a 250-mL glass or plastic
container.)
2. Add D.I. water to the soil while slowly stirring it using a spatula. At saturation, the soil
paste should glisten as it reflects the light. (In addition, it will flow slightly when the
container is tipped, slides freely and clearly off the spatula).
3. After mixing, allow the sample to stand for at least 1 hr and then recheck for saturation.
If the paste stiffens or loses its glisten, add more water and remix. If free-water collects
on the surface after standing, add more soil and remix. Allow the mixed sample to
equilibrate for a total of 18 hrs or overnight.
4. Transfer the saturated paste to a filter funnel with a highly retentive filter (Whatman no. 2
or similar), apply vacuum, and collect filtrate in containers (refilter if the initial filtrate is
cloudy).
Soils high in clay content have considerably higher water holding capacity and therefore
the laboratory may elect to extract a smaller mass of soil.
Vacuum extraction of high clay content soils will require significantly longer extraction
times. Limit vacuum tension to avoid evaporating the filtrate from the suction flasks.
For laboratories conducting numerous saturated paste samples from high clay soils, the
use of syringe vacuum extraction units will allow for unattended operation and will
eliminate the dehydration of filtrates by vacuum pumps.
A filter press using compressed air is more efficient than vacuum in obtaining extracts
from saturated paste or suspensions of other soil to water ratios. The device is available
from Fann Instrument Company
(http://www.fann.com/default.aspx?pageid=199&navid=99&prodid=FPN::JJN5P4CZ).
Analysis of EC with Conductivity Meter
1. A conductivity cell should be selected for EC measurement based on the range of
expected conductivity and cell constant as follows:
Table 1. Recommended cell constants for various EC ranges (Rhoades, 1996).
EC Range
Cell Constant
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dS m-1
m-1
0.001
0.001-0.2
0.01-2
0.01
0.10
0.1-20
1-200
0.5
2. Calibrate the conductivity meter using 0.010 M KCl solution following manufacturer’s
recommendations in the operation and calibration of the meter instrument. This solution
should yield EC of 0.147 dS/m at 25 oC (1 cm cell). A 0.1 M KCl solution should be used
to calibrate the meter when measuring high salinity soils. The EC of 0.1M KCl is 12.9
dS/m at 25oC.
3. Insert the calibrated conductivity cell in the extract and make the measurement.
Analysis of Specific solutes with ICP
1. Calibrate the ICP using multiple element standards following manufacturer’s
recommendations in the operation and calibration of the instrument.
2. Analyzing samples. A portion of extract or extract after EC measurement will be used
for individual solute analysis. Dilution should be made if a sample has concentrations
above the highest standard.
While most inorganic solutes in extracts can be determined by ICP, some ICPs are unable
to analyze Cl- and therefore ion chromatography or flow injection analyzer are used for
chloride and other anion measurements.
Calculations
1. Soil EC — deci Siemens per meter, dS m-1
Report soil salinity as EC with indication of extraction method (e.g. 1:1 extraction or SP)
to the nearest 0.01 dS m-1. The dS m-1 is SI unit for millimho cm-1.
2. Total dissolved solids (TDS)— mg L-1 or parts per million (ppm)
Soil salinity may be expressed as TDS which can be calculated based on EC
measurement as
TDS, in mg L-1 = 640 X EC (in umhos cm-1 or dS m-1)
This relationship is empirical with the conductivity factor (CF) of 640. The CF could
vary in the range of 550-900 depending on soluble components, and the CF is generally
high in chloride-rich solution but low in sulfate-rich solution.
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3. Individual solutes by ICP ---- mg kg-1
Solute in soil, in mg kg-1 = (mg L-1 in extract by ICP- method blank) × N
N = 1 for 1:1 and 2 for 1:2 extraction
This calculation cannot be used for saturated paste extracts, however N may be replaced
by the product of dry soil mass divided by wet soil mass.
4. Soil SAR = Na+/ (Ca2+ + Mg2+)1/2
Na+, Ca2+, and Mg2+ are concentrations of respective ions in mmol L-1 in extract as measured
by ICP.
Some labs convert all analytes to the saturated paste equivalent before making
recommendations (USDA, 1954; Zhang et al. 2005).
5. Conversion of SAR to ESP
For evaluation of soil sodicity, ESP may be obtained by the following:
ESP = 1.475 SAR / (1 + 0.0147 SAR)
This relationship is based on Gapon equation by assuming Gapon constant of Na-(Ca+Mg)
exchange as 0.0147 (Evangelou, 1998) and SAR may be calculated from SP extract.
Analytical Performance
Laboratory measurements of EC for soil and water salinity are generally accurate and
reproducible to 0.005 dS m-1. Common errors in EC measurement are often due to inadequate
sample circulation and electrode fouling. The four electrode EC cell may be used to overcome
the problems of electrode fouling associated with the conventional two electrode conductivity
cell. Exposure of an extract sample to air may cause changes in EC due to the dissolution of
atmospheric gases such as CO2. The latter is significant when the extract sample contains little
dissolved solids. When EC measurement is not immediately performed after obtaining an extract,
1 drop of 0.1% (NaPO3)6 solution may be added per 25 mL of the extract to prevent the
precipitation of CaCO3. For most EC measurements for assessing soil and water salinity,
common pipet-type and dip- type conductivity cells will serve the purpose although flow-through
conductivity cells may be used for EC measurements expected below 0.01 dS m-1 to minimze
exposure of measurements to atmosphere. Soil EC may be determined by direct measurement in
soil suspensions when individual solutes are not required. However, these measurements were
found to be generally lower than those made in extracts of similar soil to water ratios (Hogg and
Henry, 1984).
For calculation of SAR, an ICP may be used to determine individual ions of an extract.
Modern ICP can reliably detect metal elements in water extracts of soils to 0.1 mg L-1 for major
elements and 0.01 mg L-1 for trace elements, which are satisfactory for making soil and water
salinity evaluations.
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Interpretation
The methods outlined here are convenient ways of assessing soil salinity by measuring
EC and sodicity by calculating SAR based on ICP measurements of individual solid
concentrations of K+, Na+, Ca2+, and Mg2+. While the 1:1 and 1:2 extracts of soil to water ratio on
weight basis for determining soil salinity are commonly done in routine laboratories, the
interpretations of soil salinity and sodicity as related to crop response are often based on the
measurements made in saturated pastes. Therefore, the conversion of EC or SAR in different
extracts of soil to water ratios to that of SP extract may be necessary. On the other hand, although
EC and individual ions of 1:1 or 1:2 soil/water ratio extracts are generally correlated with SP extracts,
the correlations appear to be different for different soils (Hogg and Henry, 1984; Zhang et al., 2005).
Caution needs to be taken for the interpretation of soil salinity and sodicity of a specific extraction
of soil to water ratio under local soil-climate-crop conditions. A general guideline of
interpretation based on saturated paste extraction is illustrated in Figure 1.
Figure 1. EC(dS/m) vs. ESP(%)
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EC (dS/m)
8
12
0
E 15
S
P
% 30
Saline Soil
Normal
Soil
Increase salt hazard
Sodic Soil
Saline-Sodic Soil
General limit for most plants
Suggested value for salt sensitive plants
Effects of Storage
1. Air-dried soils may be stored several months without affecting the EC and SAR
measurement.
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Safety and disposal
1. The chemicals used in this procedure pose no safety risk and therefore can be stored and
disposed of according to routine laboratory procedures.
References
Evangelou, V.P. 1998. Environmental soil and water chemistry. John Wiley & Sons, Inc., New York.
Hogg, T.J. and J.L. Henry. 1984. Comparison of 1;1 and 1:2 suspensions and extracts with the
saturation extract in estimating salinity in Saskatchewan soils. Can. J. Soil Sci. 64:699-704.
Mass, E.V. and G.J. Hoffman. 1977. Crop tolerance –current assessment. J. Irrig. Drain. Div., Am.
Soc. Civ. Eng. 103(IR2):115-133.
Rhoades, J.D. 1990. Determining soil salinity from measurement of electrical conductivity.
Commun. Soil Sci. Plant Anal. 21:861-901.
Rhoades, J.D. 1996. Salinity: Electrical conductivity and total dissolved solids. P. 417-435. In D.L.
Sparks (ed.) Methods of soil analysis: Chemical methods, Part 3. ASA and SSSA, Madison, WI.
USDA, 1954. Diagnosis and improvement of saline and alkali soils. U.S.Dept. of Agriculture
Handbook no. 60. USDA, Washington, DC.
Zhang, H., LJ.L. Schroder; J.J. Pittman, J.J. Wang, and M.E. Payton. 2005. Soil salinity using
saturated paste and 1;1 soil and water extracts. Soil Sci. Soc. Am. J. 69:1146-1151.
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