Agricultural land soil environment assessment is the systematic process of sampling, laboratory testing, and risk evaluation of farmland soil against national risk control standards such as GB 15618-2018, with the aim of protecting food safety, arable land quality, and human health. This technical guide explains the regulatory framework, sampling strategy, testing items, analytical methods, quality assurance requirements, and practical applications of farmland soil environmental evaluation for regulators, agribusinesses, and landowners.
Technical Overview
Agricultural land soil environment assessment refers to the comprehensive investigation and evaluation of the environmental quality of cultivated soils, including paddy fields, dry farmland, vegetable bases, orchards, and tea plantations. The assessment determines whether contaminant concentrations in soil pose a risk to agricultural product safety, arable land productivity, or ecological and human health, and provides the technical basis for land use classification, safe utilization planning, and soil remediation decisions.
In China, the assessment framework is established by the Soil Pollution Prevention and Control Action Plan and the Soil Pollution Prevention and Control Law, with GB 15618-2018 (Soil Environmental Quality — Risk Control Standard for Soil Contamination of Agricultural Land) serving as the core evaluation criterion. This standard defines two tiers of benchmark values: risk screening values and risk control values. Screening values are pH-dependent and are set for four acidity bands (pH ≤ 5.5, 5.5 < pH ≤ 6.5, 6.5 < pH ≤ 7.5, pH > 7.5), with separate thresholds for paddy fields and other agricultural land for certain elements such as arsenic and chromium. Risk control values apply to cadmium, mercury, arsenic, lead, and chromium, which are directly linked to food safety.
Based on assessment results, agricultural land is classified into three management categories: Category I (uncontaminated or mildly contaminated, subject to priority protection), Category II (contaminated but usable under safe utilization measures such as variety adjustment, agronomic control, and passivation amendment), and Category III (heavily contaminated, subject to strict control including restricted planting of edible crops and alternative land use). Sampling and monitoring activities follow HJ/T 166-2004 (Technical Specification for Soil Environmental Monitoring) and NY/T 395-2012 (Technical Rules for Soil Environmental Quality Monitoring of Farmland), complemented by international practices from ISO soil quality standards.
Testing Samples
Surface Cultivated-Layer Samples
Topsoil samples are collected from the plow layer, typically 0–20 cm in depth, which represents the root zone and the primary receptor of fertilizers, pesticides, irrigation water, and atmospheric deposition. For each sampling unit, 5 to 20 subsamples are collected in an S-shaped, plum-blossom, or checkerboard pattern, combined into a composite sample of approximately 1–2 kg after debris removal and quartering. Sampling points must avoid field edges, fertilizer bands, manure heaps, and irrigation outlets to ensure representativeness.
Profile and Subsurface Samples
Soil profile samples are collected at depth intervals such as 0–20 cm, 20–40 cm, and 40–60 cm (or by genetic horizon) to characterize vertical contaminant migration, establish geochemical background levels, and support source identification. Profile sampling is particularly important near mining areas, smelters, and historical industrial sites where deep contamination may occur.
Specialized Land Use Samples
Dedicated sampling schemes apply to orchards and tea plantations (commonly 0–40 cm, corresponding to deeper root distribution), protected greenhouse soils with intensive input histories, sewage-irrigated districts, and fields adjacent to point emission sources. Grid-based systematic sampling with densified spacing is applied in detailed investigations around suspected contamination sources.
Ancillary Environmental Samples
To support source apportionment and pathway analysis, the assessment may include paired samples of irrigation water (evaluated against farmland irrigation water quality standards), ditch and canal sediments, atmospheric deposition dust, fertilizers, manure, and compost inputs, as well as crop and edible-part samples for combined soil–crop evaluation.
Testing Items
The testing program is tiered according to the evaluation objective and land use sensitivity:
- Basic (mandatory) inorganic items under GB 15618-2018: cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), zinc (Zn), together with pH as the reference parameter.
- Other listed pollutants: total hexachlorocyclohexane (HCH) and total DDT organochlorine pesticides, which are included in the screening framework of GB 15618-2018.
- Extended organic suites for source investigations: polycyclic aromatic hydrocarbons, polychlorinated biphenyls, petroleum hydrocarbons, phthalate esters, and antibiotic residues.
- Physico-chemical indicators: soil organic matter, cation exchange capacity, particle size distribution, electrical conductivity, and moisture content.
- Fertility and nutrient indicators: total and available nitrogen, phosphorus, and potassium, exchangeable calcium and magnesium, and micronutrients (Fe, Mn, B, Mo).
- Advanced characterization: chemical fractionation (sequential extraction) of heavy metals to distinguish total, reducible, oxidizable, and acid-soluble (bioavailable) pools, supporting refined risk judgment and remediation design.
Testing Methods
Sample Preparation
Field samples are transported under chain-of-custody procedures, air-dried in the shade at controlled temperature, gently crushed, and sieved through 2 mm and 0.149 mm (100-mesh) nylon sieves as required by the target analytes. Prepared aliquots are stored in acid-cleaned or amber glass containers to prevent cross-contamination and photodegradation.
Inorganic Analysis
Heavy metals are determined after acid digestion using aqua regia extraction (ISO 11466, HJ 803-2016) or total digestion with hydrochloric–nitric–hydrofluoric–perchloric acid, including microwave-assisted procedures (HJ 832-2017). Instrumental platforms include inductively coupled plasma mass spectrometry (ICP-MS, HJ 803-2016, ISO 17294-2), inductively coupled plasma optical emission spectrometry (ICP-OES, ISO 11885), graphite furnace atomic absorption spectrometry (GFAAS, GB/T 17141), flame atomic absorption spectrometry (FAAS, GB/T 17137, GB/T 17138, GB/T 17139), and atomic fluorescence spectrometry (AFS, GB/T 22105, HJ 680) for mercury and arsenic. Soil pH is measured potentiometrically in a water-to-soil suspension (HJ 962-2018, ISO 10390, ASTM D4972).
Organic Analysis
Organochlorine pesticides are extracted by accelerated solvent extraction or Soxhlet extraction, cleaned up on florisil or silica-alumina columns, and quantified by gas chromatography with electron capture detection (GC-ECD, GB/T 14550) or gas chromatography–mass spectrometry (GC-MS, HJ 835). Polycyclic aromatic hydrocarbons are determined by high-performance liquid chromatography with fluorescence detection (HJ 784) or GC-MS, with quantification against isotope-labeled internal standards.
Quality Assurance and Quality Control
All analyses are performed by accredited third-party laboratories operating under ISO/IEC 17025 quality systems. QA/QC measures include field and laboratory duplicate samples at no less than 10%, certified reference materials (GBW series) with each analytical batch, method blanks, matrix spikes, calibration verification standards, and documented holding times. Data undergo multi-level review before being released for risk evaluation, with measurement uncertainty reported for critical parameters.
Evaluation Approach
Analytical results are compared with the pH-band-specific risk screening values of GB 15618-2018 using the single-factor exceedance method; complementary indices such as the single-factor pollution index, the Nemerow comprehensive index, and the geo-accumulation index may be applied for zonal quality mapping. Where results fall between screening and control values, a detailed risk assessment is triggered, incorporating crop uptake testing of edible parts against food contaminant limits (GB 2762), exposure pathway analysis, and site-specific conditions to determine the final land category and management measures.
Applications
Agricultural land soil environment assessment supports a wide range of regulatory and commercial purposes:
- Delineation of soil environmental quality categories and formulation of safe utilization plans for contaminated farmland.
- Detailed soil pollution investigations and source apportionment around mining, smelting, electroplating, and historical industrial sites, as well as sewage-irrigated districts.
- Arable land quality grading, high-standard farmland construction, and black soil or permanent basic farmland protection projects.
- Producing-area environmental evaluation supporting green food, geographic indication, and organic certification programs.
- Baseline investigation and post-remediation effect evaluation for passivation, phytoremediation, soil replacement, and planting structure adjustment projects.
- Environmental due diligence for land transfer, agricultural investment, insurance claims, and environmental impact assessment of agricultural development projects.
- Long-term soil environment monitoring networks and trend analysis for regional soil quality management.
FAQ
Which standard applies to agricultural land: GB 15618-2018 or GB 36600-2018?
GB 15618-2018 applies to agricultural land, including paddy fields, dry farmland, orchards, and tea plantations, and uses risk screening and control values oriented toward food safety. GB 36600-2018 applies to construction land and uses a human-health-based screening framework. The applicable standard depends on the current and planned land use.
What happens if a test result exceeds the risk screening value?
Exceeding a screening value does not automatically mean the land is unsafe; it triggers a detailed risk assessment. This includes strengthened sampling density, testing of crop edible parts against GB 2762 food contaminant limits, and evaluation of exposure pathways. If the risk control value is exceeded, strict control measures such as restricting edible crop cultivation, planting structure adjustment, or alternative land use are required.
At what depth and density should samples be collected?
For routine assessment, composite samples are taken from the 0–20 cm cultivated layer at 5–20 subsample points per unit. Detailed investigations typically apply grid spacing of 100 m × 100 m to 400 m × 400 m, and profile samples to 40–60 cm or deeper where vertical migration or point-source contamination is suspected.
Why test pH together with heavy metals?
pH is the reference parameter because GB 15618-2018 sets screening values in four pH bands, reflecting the strong influence of soil acidity on metal mobility and bioavailability. A cadmium concentration that is acceptable in a calcareous soil may exceed the screening threshold in an acidic soil of the same use type.
Is total concentration sufficient, or should bioavailable fractions be analyzed?
Total concentration is required for standard compliance evaluation. Speciation analysis using sequential extraction is recommended as a supplementary tool when results approach thresholds or when remediation by passivation agents is planned, because plant uptake correlates more closely with the acid-soluble fraction than with total content.
What is the typical project timeline?
Most assessments are completed within 4 to 8 weeks, covering sampling plan design, field work, laboratory analysis (10–20 working days for inorganic parameters, longer for extended organic suites), QA/QC review, and reporting with risk evaluation conclusions and management recommendations.
