Agricultural Soil Pollution Testing: Standards, Sampling, and Analytical Methods

Agricultural soil pollution testing is an essential process for identifying and quantifying contaminants such as heavy metals, pesticide residues, and persistent organic pollutants in farmland, safeguarding food safety and sustainable land use. By evaluating soil against recognized benchmarks such as GB 15618-2018 and ISO analytical standards, accredited third-party laboratories provide landowners, agribusinesses, and regulators with the reliable data needed to classify land, manage risk, and verify remediation outcomes.

Technical Overview

Agricultural soil pollution testing refers to the systematic sampling, chemical analysis, and risk evaluation of soil in farmland, orchards, tea plantations, vegetable bases, and other land used for food and feed production. Because crops absorb contaminants from soil and transfer them into the food chain, contaminated agricultural land poses direct risks to human health, market access, and ecosystem quality. In most jurisdictions, agricultural soil quality is managed through risk-based standards that define risk screening values and risk control values for key pollutants.

In China, the governing benchmark is GB 15618-2018, the Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land. This standard sets pH-dependent screening values for eight basic inorganic indicators—cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn)—and lists additional controlled items such as hexavalent chromium, methylmercury, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and petroleum hydrocarbons. For example, the risk screening value for cadmium ranges from 0.3 to 0.8 mg/kg depending on soil pH and crop type, while the risk control value is 1.5 mg/kg. Comparable international programs rely on ISO, ASTM, and EN method standards for sampling and analysis.

Testing outcomes support the three-category management logic widely applied in soil pollution prevention programs:

• Priority protection: contaminant concentrations are below risk screening values, and the land can be farmed normally under ongoing monitoring.
• Safe utilization: concentrations fall between screening and control values, requiring detailed risk assessment, adjusted cropping patterns, or soil improvement measures.
• Strict management: concentrations exceed risk control values, restricting certain crops and triggering risk control or remediation measures.

Testing Samples

Representative sampling is the foundation of credible agricultural soil data. Sampling is typically designed on a grid or by land parcel, with each composite sample combining 5 to 20 subsamples collected along an S-shaped or diagonal trajectory. Sample handling follows the general principles of ISO 10381 (soil sampling guidance) and ISO 11464 (pretreatment of samples for physico-chemical analysis).

Dryland and Grain Field Soil

Surface soil from the cultivated layer, typically 0 to 20 cm in depth, is collected using wooden, bamboo, or plastic tools to avoid trace metal contamination. Approximately 1 to 2 kg of soil is retained per composite sample after quartering.

Paddy Field Soil

Flooded or alternately wet-dry paddy soils are sampled from the 0 to 20 cm plow layer, with particular attention to redox-sensitive contaminants such as mercury and arsenic. Standing water is drained before collection, and samples are processed promptly to minimize changes in chemical speciation.

Vegetable Greenhouse and Intensive Cropping Soil

Greenhouse soils often receive high inputs of fertilizers, manure, and pesticides, so sampling frequently covers 0 to 20 cm with denser grids. Elevated cadmium, copper, zinc, nitrate, and pesticide residues are common findings in these systems.

Orchard and Tea Plantation Soil

For deep-rooted perennial crops, sampling may extend to 0 to 40 cm or, in detailed investigations, 0 to 60 cm. Soil pH and heavy metal accumulation from historical pesticide use, such as copper-based fungicides and lead arsenate, are of particular interest.

Soil Profiles and Suspicion-Point Samples

Where point-source pollution is suspected, for example near mining areas, smelters, or irrigation channels, profile samples are taken at 20 cm intervals to depths of one meter or more to characterize the vertical migration of contaminants.

Sample Preservation

• For inorganic analyses: samples are air-dried at room temperature, ground, and passed through 2 mm, 0.25 mm, or 0.149 mm nylon sieves.
• For organic analyses: samples are stored in brown glass jars at approximately 4 °C and analyzed within the holding time specified by the method.
• Containers: clean polyethylene bags or bottles for metals; solvent-rinsed glass containers for organic pollutants.

Testing Items

Basic Inorganic Indicators (Risk Screening Items)

• Cadmium (Cd)
• Mercury (Hg)
• Arsenic (As)
• Lead (Pb)
• Chromium (Cr)
• Copper (Cu)
• Nickel (Ni)
• Zinc (Zn)

Supplementary Inorganic Items

• Hexavalent chromium (Cr(VI))
• Methylmercury in paddy soils
• Total selenium, thallium, antimony, beryllium, cobalt, vanadium, manganese, and fluorine where site conditions warrant

Organic Pollutants

• Organochlorine pesticides: alpha-, beta-, gamma-, and delta-hexachlorocyclohexane (HCH), dichlorodiphenyltrichloroethane (DDT) and its metabolites
• Organophosphorus pesticides: parathion, methyl parathion, chlorpyrifos, and related compounds
• Polycyclic aromatic hydrocarbons (PAHs), including benzo(a)pyrene
• Polychlorinated biphenyls (PCBs)
• Total petroleum hydrocarbons (TPH)
• Phthalate esters and other emerging contaminants as required

Soil Physicochemical Parameters

• pH, essential because screening values in GB 15618 are pH-dependent
• Organic matter and organic carbon
• Cation exchange capacity (CEC)
• Soil texture and particle size distribution
• Total nitrogen, available phosphorus, and available potassium for agronomic interpretation

Testing Methods

Sample Preparation

Field samples are registered, air-dried or freeze-dried as appropriate, disaggregated, homogenized, and quartered following ISO 11464. Metal aliquots are milled with non-metallic equipment and sieved through nylon mesh, while organic aliquots are kept refrigerated and extracted without drying to prevent loss of volatile and semi-volatile compounds.

Heavy Metal and Metalloid Analysis

For element determination, soils are digested with aqua regia in accordance with ISO 11466, or by microwave-assisted digestion such as ISO 12914, with stronger acid mixtures used when total content including silicate-bound fractions is required. Instrumental determination is performed by:

• Inductively coupled plasma mass spectrometry (ICP-MS) for ultra-trace multi-element determination with a wide linear range.
• Inductively coupled plasma optical emission spectrometry (ICP-OES), consistent with ISO 22036, for multi-element analysis at higher concentrations.
• Graphite furnace atomic absorption spectrometry (GFAAS) for cadmium and lead, consistent with GB/T 17141.
• Flame atomic absorption spectrometry (FAAS) for copper, zinc, nickel, and chromium, consistent with ISO 11047.
• Atomic fluorescence spectrometry (AFS) for mercury and arsenic, consistent with the GB/T 22105 series methods.
• Portable or benchtop X-ray fluorescence (XRF) spectrometry for rapid in-situ screening and survey-level delineation of contaminated areas.

Mercury may additionally be determined by cold vapor atomic absorption, and hexavalent chromium by alkaline digestion followed by colorimetric determination.

Organic Pollutant Analysis

Organic contaminants are extracted by accelerated solvent extraction, ultrasonic extraction, or Soxhlet extraction, purified through silica gel or florisil clean-up columns, and determined by:

• Gas chromatography with electron capture detection (GC-ECD) for organochlorine pesticides, consistent with ISO 10382 and GB/T 14550.
• Gas chromatography-mass spectrometry (GC-MS) for PAHs, PCBs, organophosphorus pesticides, and confirmation of screening results, consistent with ISO 13877 for PAHs.
• High performance liquid chromatography (HPLC) or LC-MS/MS for thermally labile or polar pesticides.
• Infrared spectrometry or gravimetry for total petroleum hydrocarbons, depending on the carbon range of interest.

Physicochemical Parameter Analysis

• pH is measured in a soil-water suspension with a glass electrode according to ISO 10390.
• Organic carbon is determined by dichromate oxidation or dry combustion according to ISO 10694.
• Cation exchange capacity is measured by compulsive exchange or ammonium acetate methods consistent with ISO 13536.

Quality Assurance and Quality Control

All analyses are performed by accredited third-party laboratories operating under an ISO/IEC 17025 quality management system. Standard QA/QC measures include:

• Field blanks, transport blanks, and 10 to 20 percent field duplicate samples to assess sampling precision.
• Method blanks, laboratory duplicates, and calibration verification with every analytical batch.
• Certified reference materials and matrix spike recoveries, typically accepted within 70 to 130 percent, to verify accuracy.
• Reporting of results on a dry-weight basis (mg/kg dw) with stated detection limits, measurement uncertainty, and comparison against applicable screening and control values.

Applications

• National and regional soil pollution status surveys and detailed investigations of agricultural land.
• Classification of farmland into priority protection, safe utilization, and strict management categories.
• Safety evaluation of agricultural product producing areas and support for green food and organic certification programs.
• Adjustment of cropping structure, for example substituting non-food crops on land where cadmium exceeds control values.
• Verification of remediation and passivation (immobilization) projects, including pre- and post-treatment comparisons.
• Assessment of irrigation water, fertilizer, manure, compost, and sludge application practices that may accumulate contaminants.
• Environmental impact assessment, land transfer due diligence, pollution dispute resolution, and regulatory enforcement.

FAQ

Which standard defines acceptable contaminant levels in agricultural soil?

In China, GB 15618-2018 establishes risk screening values and risk control values for agricultural land. Screening values vary with soil pH and land use type (paddy field versus other agricultural land), while control values define the threshold above which strict risk management or remediation is required. International projects typically combine ISO or ASTM analytical methods with locally applicable soil quality guidelines.

How does agricultural land assessment differ from construction land assessment?

Agricultural land is evaluated primarily on the pathway of contaminant transfer from soil to crops and livestock and then to humans, whereas construction land standards such as GB 36600-2018 focus on direct ingestion, inhalation, and dermal contact. As a result, the screening values, indicator lists, and pH-dependence differ significantly between the two frameworks.

At what depth are agricultural soil samples collected?

The standard sampling depth is the cultivated layer of 0 to 20 cm for dryland, paddy fields, and vegetable bases. Orchard and tea plantation soils may be sampled at 0 to 40 cm, and vertical profile sampling in 20 cm increments is used when investigating contaminant migration to deeper layers.

How much soil is required for a test, and how are composite samples formed?

Each composite sample generally requires 1 to 2 kg of soil. It is formed by combining and homogenizing 5 to 20 subsamples collected across a parcel, then quartering down to the required laboratory mass. Using non-metallic sampling tools prevents cross-contamination with trace metals.

What happens if a result exceeds the risk screening value?

An exceedance does not automatically prohibit farming. It triggers further steps: detailed risk assessment considering crop type, consumption patterns, and soil properties; combined testing of crops and, where relevant, irrigation water; and implementation of safe utilization measures such as soil conditioning, variety substitution, or cropping adjustments. Concentrations above risk control values require strict management and potential remediation.

How long does agricultural soil pollution testing take?

Typical turnaround is 7 to 15 working days after laboratory receipt for routine heavy metal and pH packages. Extended panels covering pesticides, PAHs, PCBs, or speciation analysis, as well as projects requiring additional sampling rounds, may require more time.

Are the laboratory reports legally or regulatorily recognized?

Reports issued by accredited third-party laboratories operating under ISO/IEC 17025, with data validated through certified reference materials and full QA/QC documentation, are recognized for regulatory submission, remediation acceptance, certification audits, and dispute resolution.