Farmland soil pollution testing identifies and quantifies contaminants such as heavy metals, pesticide residues, and organic pollutants in agricultural soil to protect crop safety and support compliant land management. This technical guide explains the sampling procedures, testing items, analytical methods, and applicable standards (GB, ISO, ASTM) used by accredited third-party laboratories for agricultural soil quality assessment.
Technical Overview
Farmland soil pollution testing is the systematic process of collecting, preparing, and analyzing agricultural soil samples to determine the presence and concentration of potentially harmful substances, including heavy metals, metalloids, pesticide residues, and persistent organic pollutants. The primary regulatory reference in China is GB 15618-2018, the Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land, which defines risk screening values and risk intervention values for cultivated land categories such as paddy fields, dryland, vegetable plots, and orchards. When measured concentrations fall below the screening values, pollution risk is generally considered low; results between screening and intervention values indicate the need for refined assessment and safe-utilization measures; and exceedance of intervention values calls for strict control, including planting restrictions or remediation. Internationally, comparable frameworks are provided by ISO soil quality standards and ASTM soil testing practices, ensuring that results from accredited third-party laboratories are defensible, comparable, and suitable for regulatory decision-making.
Testing Samples
Surface (Plow Layer) Soil Samples
The plow layer, typically 0–20 cm in depth for dryland and 0–20 cm for paddy fields, is the most frequently tested zone because it directly interacts with crop roots, fertilizers, and agrichemical inputs. Multiple subsamples are usually collected in a grid or diagonal (S-shaped or plum-blossom) pattern and combined into a single composite sample of 1–2 kg to represent a defined sampling unit.
Deep Soil and Profile Samples
Profile samples are collected at depth intervals such as 20–40 cm, 40–60 cm, and 60–100 cm when vertical migration of pollutants is suspected, for example near industrial discharge zones, waste storage areas, or heavily irrigated land. Deep samples help distinguish surface accumulation from geogenic background levels of elements such as arsenic, cadmium, and chromium.
Special-Purpose Samples
Rhizosphere soil from greenhouse vegetable production, orchard soil beneath long-term pesticide application zones, and soil adjacent to irrigation channels can be sampled separately. These samples are often paired with irrigation water testing per GB 5084 or crop tissue analysis to evaluate the complete soil–water–plant pollution pathway.
Testing Items
Heavy Metals and Metalloids
The eight basic inorganic indicators specified in GB 15618-2018 are cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn). Expanded inorganic panels may include manganese (Mn), cobalt (Co), thallium (Tl), antimony (Sb), vanadium (V), beryllium (Be), and selenium (Se) depending on suspected pollution sources.
Organic Pollutants
Typical organic parameters include organochlorine pesticides such as hexachlorocyclohexane (HCH) and dichlorodiphenyltrichloroethane (DDT), organophosphorus pesticides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), total petroleum hydrocarbons (TPH), and plasticizers such as phthalate esters in intensively farmed or peri-urban areas.
Physical and Chemical Properties
Supporting parameters are essential for interpreting pollutant mobility and bioavailability. These include pH, organic matter content, cation exchange capacity (CEC), total nitrogen, available phosphorus, available potassium, soil texture, electrical conductivity, moisture content, and dry matter fraction. Soil pH in particular strongly governs the solubility and plant uptake of cadmium, lead, and other metals.
Testing Methods
Sampling and Sample Preparation
Sampling programs follow GB/T 36197-2018 and the ISO 10381 series, which specify point layout, sampling density, tool decontamination, and documentation requirements. Samples are transported in inert containers, air-dried in clean environments to avoid cross-contamination, gently crushed, and sieved—typically through 2 mm for pH and conductivity tests and through 0.15 mm (100 mesh) for heavy metal and organic analysis, consistent with ISO 11464 pretreatment protocols.
Inorganic Analysis
- Extraction: total heavy metals are commonly extracted with aqua regia according to ISO 11466, while total digestion with hydrofluoric acid-based acid mixtures is used for true total content when required.
- ICP-MS and ICP-OES: inductively coupled plasma mass spectrometry and optical emission spectrometry enable multi-element determination at trace levels with high throughput.
- AAS: flame and graphite furnace atomic absorption spectrometry, for example GB/T 17141 for lead and cadmium, remains a robust technique for single-element quantification.
- AFS: atomic fluorescence spectrometry per GB/T 22105 is widely applied for mercury and arsenic due to its excellent sensitivity for hydride-forming and volatile elements.
- XRF screening: portable and benchtop X-ray fluorescence provides rapid on-site or in-laboratory screening to guide confirmatory analysis.
Organic Analysis
Organic pollutants are extracted using accelerated solvent extraction, ultrasonic extraction, or Soxhlet extraction, followed by clean-up and determination by gas chromatography with electron capture detection (GC-ECD), gas chromatography–mass spectrometry (GC-MS), or high-performance liquid chromatography (HPLC). GB/T 14550, for example, specifies gas chromatographic determination of HCH and DDT in soil.
Physico-Chemical Analysis
Soil pH is determined potentiometrically in water or calcium chloride suspension per ISO 10390, organic matter by potassium dichromate oxidation, and nutrient parameters by conventional wet-chemical or instrumental procedures aligned with national and international standard methods.
Quality Assurance and Quality Control
Accredited third-party laboratories operate under ISO/IEC 17025 management and technical requirements. Routine QC measures include method blanks, laboratory duplicates, matrix spikes, calibration verification, and analysis of certified reference materials (CRMs), with results reported together with detection limits, measurement uncertainty, and conformity judgments against GB 15618-2018 screening and intervention values.
Applications
- Agricultural land classification: determining whether plots fall under priority protection, safe utilization, or strict management categories.
- Pollution source investigation: tracing contamination from mining, smelting, electroplating, irrigation water, atmospheric deposition, or agrichemical overuse.
- High-standard farmland construction: verifying soil quality before project acceptance and land consolidation.
- Remediation evaluation: confirming that passivation, phytoremediation, soil replacement, or deep-tillage measures have reduced pollutant availability to acceptable levels.
- Safe-utilization planning: selecting low-accumulating crop varieties and adjusting pH, organic amendments, or water regimes based on measured cadmium and arsenic bioavailability.
- Certification and trade support: providing test reports for organic farming programs, green food certification, land transfer due diligence, and agricultural product export documentation.
FAQ
What is the difference between risk screening values and risk intervention values?
Risk screening values in GB 15618-2018 are thresholds below which the risk to agricultural products and human health is generally acceptable. Risk intervention values are higher thresholds above which the soil poses a significant risk and requires strict control measures, such as planting structure adjustment, remediation, or land-use change. Results between the two values trigger refined risk assessment and safe-utilization management.
How deep should farmland soil samples be collected?
The standard sampling depth is the plow layer of 0–20 cm, because this zone reflects tillage inputs and root uptake. Deeper sampling at 20–40 cm and below is recommended when investigating vertical pollutant migration, evaluating remediation by deep plowing, or characterizing suspected point-source contamination.
How often should farmland soil be tested?
General monitoring typically recommends testing every 2–3 years for stable farmland. Annual testing is advisable for plots near industrial zones, land under remediation, areas with known exceedance history, or greenhouse systems with intensive fertilizer and pesticide use.
How long does a complete farmland soil pollution test take?
Standard heavy metal and physico-chemical panels are usually completed within 7–10 working days after sample receipt, while expanded organic pollutant panels involving extraction, clean-up, and GC-MS or HPLC confirmation may require 10–15 working days, depending on sample load and QC requirements.
Can screening with portable XRF replace laboratory analysis?
No. Portable XRF is a valuable rapid screening tool for prioritizing sampling locations, but regulatory conformity assessment requires confirmatory laboratory analysis by validated methods such as ICP-MS, AAS, or AFS on properly prepared samples, because XRF sensitivity for elements like cadmium and mercury at screening-value levels is limited.