Determination of Soil pH by the Potentiometric Method: Principles, Procedures, and Applications

Soil pH is one of the most fundamental indicators of soil chemical condition, influencing nutrient availability, microbial activity, and contaminant mobility. The potentiometric method is the internationally recognized technique for accurate soil pH determination, employing a glass electrode and reference electrode system in accordance with standards such as ISO 10390, ASTM D4972, and HJ 962. This article provides a comprehensive technical overview of the determination of soil pH (soil acidity and alkalinity) by potentiometry, covering testing principles, sample types, procedures, applications, and frequently asked questions.

Technical Overview

The potentiometric method for soil pH determination measures the hydrogen ion activity of a soil suspension by detecting the electrical potential developed between a glass (indicator) electrode and a reference electrode immersed in the suspension. According to the Nernst equation, the potential of the glass electrode changes linearly with pH (approximately 59.16 mV per pH unit at 25 °C), and a calibrated pH meter converts this potential difference directly into a pH value.

The determination of soil acidity and alkalinity by potentiometry is governed by several established standards, including ISO 10390 (Soil, treated biowaste and sludge — Determination of pH), ASTM D4972 (Standard Test Method for pH of Soils), HJ 962-2018 (Soil — Determination of pH — Potentiometric method), and NY/T 1121.2-2006 (Soil testing — Determination of soil pH — Potentiometric method). These standards harmonize the measurement of soil reaction in water, potassium chloride, or calcium chloride suspensions.

The core instrumentation consists of the following components:

• A pH meter with a resolution of 0.01 pH units and an accuracy of ±0.02 pH units or better, equipped with automatic temperature compensation
• A combined glass electrode (indicator and reference electrodes integrated) or a separate glass electrode paired with a silver/silver chloride or calomel reference electrode
• Certified buffer solutions, typically pH 4.01, 6.87, and 9.18 at 25 °C, traceable to national metrology institutes
• Carbon-dioxide-free distilled or deionized water for suspension preparation, with a conductivity requirement of no more than 0.2 mS/m at 25 °C

Prior to measurement, the pH meter must be calibrated using at least two buffer solutions that bracket the expected soil pH. The electrode slope should fall between 95% and 105% of the theoretical value; electrodes outside this range must be reconditioned or replaced. Key variables affecting accuracy include the soil-to-solution ratio, extraction and equilibration time, carbon dioxide absorption, suspension temperature, and the condition of the electrode junction.

Testing Samples

Agricultural and Farmland Soils

Surface and profile samples from cultivated land, orchards, tea plantations, and paddy fields are the most frequently tested matrices. These samples support fertilization planning, liming or acidification management, and crop suitability assessment. Typical sample requirements are 100–500 g of air-dried soil passed through a 2 mm sieve.

Forest and Grassland Soils

Forest floor litter layers, mineral horizons, and grassland soils are analyzed to evaluate soil acidification trends, base saturation status, and ecological health. Reference procedures for forest soils, such as LY/T 1239, also rely on the potentiometric principle.

Greenhouse Substrates and Growing Media

Peat-based substrates, coconut coir, compost-amended media, and nursery potting mixes are tested to verify that root-zone reaction falls within the optimal range for the target crop, generally pH 5.5–6.5 for most horticultural species.

Contaminated Site and Remediation Soils

Soils from industrial sites, mining areas, landfills, and remediation projects require pH determination because pH governs the mobility and bioavailability of heavy metals and the effectiveness of stabilization treatments such as lime, phosphate, or biochar amendments.

Saline-Alkali and Coastal Soils

High-salt soils from coastal flats, inland saline-alkali regions, and irrigated arid zones are tested to characterize alkalinity and sodicity, supporting gypsum amendment design and reclamation monitoring.

Sediments, Sludges, and Treated Biowaste

River and lake sediments, sewage sludge, and composted organic wastes are measured under ISO 10390, which covers soil, treated biowaste, and sludge matrices using a common potentiometric protocol.

Testing Items

The potentiometric laboratory typically reports the following measurement items:

• Soil pH in water (pHH2O): the routine index of soil acidity and alkalinity, generally determined at a soil-to-water mass-to-volume ratio of 1:2.5 (per HJ 962 and NY/T 1121.2) or 1:5 (per ISO 10390)
• Soil pH in 1 mol/L potassium chloride (pHKCl): reflects exchangeable and reserve acidity; the difference between pHH2O and pHKCl indicates the extent of exchangeable aluminum and hydrogen on soil colloids
• Soil pH in 0.01 mol/L calcium chloride (pHCaCl2): provides a salt-suppressed measurement closer to field conditions, commonly used for acidic and moderately acidic soils
• Lime requirement and acidification indicators: derived parameters supporting amelioration recommendations
• Quality control parameters: calibration slope, buffer verification, duplicate deviation, and temperature records accompanying each result

Testing Methods

Sample Collection and Preparation

Representative samples are collected using standard soil sampling protocols, with subsamples combined and reduced by quartering. Samples are air-dried at room temperature, plant residues, stones, and debris are removed, and the soil is crushed and passed through a 2 mm (10-mesh) sieve. Fresh field-moist samples may also be analyzed when specified, with the moisture content recorded.

Suspension Preparation

A weighed portion of soil (commonly 10.0 g) is transferred to a beaker or extraction bottle, and 25 mL of carbon-dioxide-free water is added to achieve a 1:2.5 ratio, or 50 mL for the 1:5 ISO procedure. The suspension is stirred or shaken thoroughly, then allowed to stand for equilibration—for example, stirring for 2 minutes followed by 30 minutes of standing under HJ 962, or shaking for 5 minutes followed by at least 2 hours of standing under ISO 10390—before measurement.

Instrument Calibration

The electrode is rinsed with distilled water and blotted dry, then calibrated with two buffer solutions bracketing the anticipated pH (for example, 4.01 and 6.87 for acidic soils, or 6.87 and 9.18 for alkaline soils). Calibration acceptance requires a slope of 95%–105%, and a third buffer is used periodically to verify accuracy.

Measurement Procedure

The suspension is stirred gently to resuspend the soil, and the electrode is immersed so that the glass bulb contacts the suspension while the reference junction remains immersed. The reading is taken after the displayed value stabilizes, typically within 30–60 seconds, while the suspension temperature is recorded and compensated. The result is reported to two decimal places, together with the extraction medium, soil-to-solution ratio, and temperature.

Quality Control and Assurance

Accredited third-party laboratories apply the following quality measures during the determination of soil pH by the potentiometric method:

• Duplicate determinations with an absolute deviation of no more than 0.1 pH units between parallel samples
• Continuous verification of the electrode slope and response time before and after batch analysis
• Regular electrode maintenance, including junction cleaning and storage in potassium chloride solution, to prevent drift and liquid-junction clogging in clay-rich suspensions
• Use of certified reference materials or control soils to monitor long-term measurement stability
• Documentation of buffer lot numbers, calibration records, and environmental conditions for full traceability

Applications

The determination of soil acidity and alkalinity by the potentiometric method supports a wide range of practical and scientific applications:

• Precision agriculture: formulating fertilizer programs, selecting acid-tolerant or alkali-tolerant varieties, and calculating lime or gypsum application rates
• Environmental monitoring and risk assessment: evaluating soil acidification and salinization, screening contaminated sites, and predicting heavy metal mobility
• Land remediation and ecological restoration: verifying the effectiveness of pH-adjusting amendments and tracking post-remediation recovery
• Construction and geotechnical engineering: classifying soil corrosivity toward concrete and buried steel structures and informing material selection
• Forestry and grassland management: monitoring acid deposition impacts and guiding site preparation for afforestation
• Scientific research: supporting soil surveys, carbon and nutrient cycling studies, and long-term soil quality monitoring networks

FAQ

Why does the soil-to-water ratio affect the measured pH value?

Dilution alters the ion concentration and dissociation equilibrium of the soil solution. A wider soil-to-water ratio generally shifts acidic soils toward higher pH readings and saline or alkaline soils toward lower readings. For this reason, the extraction ratio (1:2.5, 1:2, or 1:5) must always be reported together with the pH value to ensure comparability between datasets.

What is the difference between pHH2O, pHKCl, and pHCaCl2?

pHH2O measures the active acidity of the soil solution, pHKCl displaces exchangeable hydrogen and aluminum from colloids and typically reads 0.5–1.5 units lower, and pHCaCl2 suppresses salt effects to provide a value closer to field moisture conditions. The comparison of these three values is diagnostic of reserve acidity and buffering status.

Should samples be analyzed fresh or air-dried?

Air-dried, 2 mm-sieved soil is the standard preparation for routine analysis because it improves reproducibility and comparability. However, air drying can shift the pH of certain soils, particularly sulfidic or reduced paddy soils, so fresh analysis or specified pretreatment is recommended when biogeochemical processes are of concern.

What accuracy and repeatability can be expected?

With a properly calibrated electrode and controlled suspension preparation, the potentiometric method achieves repeatability within ±0.1 pH units between duplicates and an expanded measurement uncertainty typically in the range of 0.1–0.2 pH units. Regular electrode maintenance and consistent equilibration times are the most critical factors for maintaining this performance.

How should samples be stored before testing?

Air-dried samples stored in sealed, labeled containers at room temperature remain stable for extended periods. Fresh samples should be kept cool (approximately 4 °C) and analyzed promptly to minimize microbial activity and carbon dioxide exchange that can alter pH.

Can field pH measurements replace laboratory potentiometric determination?

Portable pH meters and colorimetric test kits are useful for rapid on-site screening, but regulatory reporting, remediation verification, and scientific studies require laboratory determination under standardized potentiometric procedures with full calibration and quality control documentation provided by accredited third-party laboratories.