Determination of Total Readily Soluble Salts in Soil: Test Principles, Methods, and Applications

Determination of the total readily soluble salt content in soil is a fundamental test for evaluating soil salinity, soil aggressiveness toward buried structures, and land suitability for agriculture and construction. This article explains the extraction principle, the gravimetric, electrical conductivity, and ion-summation methods, applicable standards such as GB/T 50123 and LY/T 1251, and typical applications in geotechnical, agricultural, and environmental projects.

Technical Overview

Readily soluble salts in soil are the inorganic salts—mainly chlorides, sulfates, carbonates, and bicarbonates of sodium, potassium, calcium, and magnesium, together with nitrates—that dissolve rapidly when soil is mixed with water under standardized conditions. The total content of these salts, usually expressed in grams per kilogram (g/kg) or as a mass percentage of oven-dry soil, is a key indicator of soil salinity and of the corrosiveness of soil toward buried concrete and steel structures.

The determination is based on preparing a water extract of the soil at a defined soil-to-water ratio (commonly 1:5 by mass), separating the clear extract, and then quantifying the dissolved salts by one of three complementary approaches: measuring the mass of the residue after evaporation (gravimetric method), measuring the electrical conductivity of the extract (conductivity method), or summing the individually determined anions and cations (ion-summation method). The three approaches are mutually verifiable: the ion sum should agree with the evaporation residue within an acceptable tolerance, and both should correlate linearly with electrical conductivity for a given salt composition.

Recognized procedures are given in GB/T 50123 (Standard for Geotechnical Testing Method), LY/T 1251 (analysis of water-soluble salts in forest soils), and related conductivity and water-quality standards such as ISO 11265 and ASTM D1125. Accredited third-party laboratories routinely perform this test as part of geotechnical investigation, agricultural land evaluation, and environmental monitoring programs.

Testing Samples

Disturbed (Bulk) Soil Samples

Representative disturbed samples are collected from boreholes, test pits, trench walls, or stockpiles. A minimum quantity of approximately 200 to 500 g of soil per sample is normally requested. Samples must be sealed in airtight containers or bags immediately after collection, clearly labeled with project name, sampling location, and depth, and protected from moisture loss and temperature extremes, because changes in water content can cause salt migration within the sample and alter the measured distribution.

Air-Dried and Processed Soil

In the laboratory, the soil is air-dried at room temperature, gently crushed with a non-contaminating implement, homogenized, and passed through a 2 mm (or 1 mm) sieve. The water content of the air-dried soil is determined separately (for example in accordance with ASTM D2216) so that all reported salt contents can be converted to an oven-dry mass basis.

Soil Water Extracts

The processed soil is mixed with carbon-dioxide-free deionized water at a 1:5 soil-to-water ratio by mass, shaken for a standardized period (typically 3 to 5 minutes), allowed to stand for equilibration, and then clarified by centrifugation or filtration. The resulting clear extract is the test medium for all subsequent total-salt, conductivity, and ion measurements.

Testing Items

A complete readily soluble salt determination normally includes the following reported parameters:

• Total content of readily soluble salts (evaporation residue dried at 105 to 110 degrees Celsius), in g/kg or percent of oven-dry soil
• Electrical conductivity of the 1:5 soil water extract (EC1:5), in mS/cm at 25 degrees Celsius
• Water-soluble cations: Na+, K+, Ca2+, and Mg2+
• Water-soluble anions: CO3(2-), HCO3-, Cl-, SO4(2-), and, where required, NO3-
• pH of the soil water extract, which assists in interpreting carbonate and bicarbonate speciation

Individual ions are reported in mmol/kg and mg/kg of oven-dry soil, which allows an anion-cation charge balance check; a balance within approximately plus or minus 5 percent is generally regarded as acceptable evidence of data consistency.

Testing Methods

Gravimetric (Evaporation-Residue) Method

The gravimetric method is the classical reference procedure for total readily soluble salts. The steps are as follows:

• Pipette a known volume of clear extract, for example 50 mL, into a pre-cleaned, pre-weighed porcelain evaporating dish.
• Evaporate the aliquot to near dryness on a water bath, add 15 percent hydrogen peroxide dropwise, and heat gently to oxidize residual organic matter until the residue turns white or off-white.
• Dry the dish with residue at 105 to 110 degrees Celsius to constant mass, cool in a desiccator, and weigh accurately to 0.0001 g.
• Carry a reagent blank through the identical procedure and subtract it from the residue mass.

The total soluble salt content is calculated by multiplying the corrected residue mass by the ratio of the total extract volume to the aliquot volume, dividing by the oven-dry mass of soil used, and expressing the result in g/kg. The method directly measures total dissolved solids and is not significantly affected by the specific salt composition, provided organic matter and crystallization water are properly controlled.

Electrical Conductivity Method

The conductivity method is a rapid screening technique. The electrical conductivity of the 1:5 extract is measured at 25 degrees Celsius using a calibrated conductivity meter with automatic temperature compensation, with instrument verification against standard potassium chloride solutions. The total salt content is then estimated from the conductivity using an empirical factor or, preferably, a laboratory regression equation established from local saline soils of known salt composition. The method is fast, inexpensive, and well suited to large-scale salinity mapping, although accuracy decreases when the salt composition deviates markedly from that used to derive the conversion factor.

Ion-Summation Method

The ion-summation method quantifies the individual dissolved ions and adds them to obtain the total, while also providing the compositional information needed for saline soil classification and corrosion assessment:

• CO3(2-) and HCO3-: acid-base titration with standard acid using phenolphthalein and methyl orange indicators, or potentiometric endpoint detection
• Cl-: argentometric (Mohr) titration with standard silver nitrate, or potentiometric titration
• SO4(2-): indirect EDTA titration, barium chloride gravimetry for high contents, turbidimetry, or ion chromatography
• Ca2+ and Mg2+: EDTA complexometric titration at controlled pH with appropriate metallochromic indicators
• Na+ and K+: flame photometry, atomic absorption spectrometry, ICP-OES, or ion chromatography

The sum of all determined ions, converted to g/kg of oven-dry soil, is reported as the total readily soluble salt content and is cross-checked against the gravimetric residue; agreement within roughly 5 to 10 percent validates both determinations.

Quality Control and Data Validation

Reliable results require procedural blanks, duplicate analyses with relative deviation limits defined by the governing standard, periodic verification of balance, oven, and conductivity meter calibration, and a charge-balance and residue-versus-ion-sum consistency review before reporting. Any anomalous divergence between methods triggers re-extraction and re-analysis.

Applications

Total readily soluble salt data are used across multiple sectors:

• Classification and improvement of saline and saline-alkali land, including salt-leaching schedules, drainage design, and crop or halophyte selection
• Geotechnical investigation of saline soils, including evaluation of sulfate-type salt heave and chloride-type collapse behavior affecting foundations and embankments
• Assessment of soil corrosivity toward buried steel pipelines, piles, tanks, and reinforced concrete, in support of durability design and protective coating or backfill selection
• Acceptance of subgrade and fill materials for highways, railways, and airports in saline and coastal regions
• Environmental site assessment, salinization monitoring, and quality control for reclaimed or dredged land
• Agricultural, ecological, and research studies on irrigation water quality, evaporation-driven salt accumulation, and greenhouse or planting-substrate management

FAQ

What is the difference between readily soluble, moderately soluble, and sparingly soluble salts?

Readily soluble salts, such as sodium chloride, sodium sulfate, sodium carbonate, and bicarbonates, dissolve almost completely in the standardized water extraction and dominate short-term salinity effects. Moderately soluble salts, principally gypsum (calcium sulfate dihydrate), dissolve only partially under 1:5 conditions, while sparingly soluble salts such as calcium carbonate are essentially not captured by this test.

Why is a 1:5 soil-to-water extraction ratio commonly used?

The 1:5 ratio provides a reproducible, well-agitated extraction with sufficient extract volume for all analyses, and it forms the basis of the widely used EC1:5 index. Where plant-response interpretation is required, saturation extracts or soil-paste methods may additionally be applied, but the 1:5 extract remains the standard basis for engineering-oriented reporting.

Why is hydrogen peroxide added during the gravimetric determination?

Hydrogen peroxide oxidizes dissolved and colloidal organic matter that would otherwise remain in the evaporation residue and falsely inflate the total salt result. Careful, repeated treatment until the residue is white ensures the weighed mass represents inorganic salts only.

What salt content indicates a saline soil?

Classification thresholds depend on the governing standard, climate, and salt type. As a general guide used in many systems, soils with total salt content below 0.1 percent are non-saline, 0.1 to 0.3 percent is lightly saline, 0.3 to 0.5 percent moderately saline, 0.5 to 1 percent strongly saline, and above 1 percent is typically classified as solonchak-type saline soil. Project-specific acceptance criteria should always be confirmed against the applicable design code.

Can the conductivity method replace the gravimetric method?

Conductivity is excellent for screening, trend monitoring, and mapping large sample sets, but because the conductivity-to-salt relationship depends on salt composition, the gravimetric or ion-summation method should be used whenever results are submitted for regulatory compliance, contractual acceptance, or corrosion classification.

How much sample is needed and how long does testing take?

A minimum of 200 to 500 g of representative soil per sample is typically sufficient for total salt and full ion analysis. Under normal laboratory workflows, results are reported within approximately 5 to 10 working days after sample receipt, subject to sample condition and project scope.