Agricultural Soil Quality Analysis: Standards, Testing Parameters, and Evaluation Methods

Agricultural soil quality analysis is a systematic evaluation of the physical, chemical, and biological properties of farmland soil to determine its safety, fertility, and suitability for crop production. Guided by standards such as GB 15618-2018, ISO soil quality standards, and ASTM test methods, this analysis identifies heavy metal contamination, nutrient deficiencies, and degradation risks. Accredited third-party laboratories provide comprehensive soil testing services that support land-use classification, precision fertilization, and food safety assurance.

Technical Overview

Agricultural soil quality analysis refers to the systematic assessment of farmland soil through the measurement of physical, chemical, and biological indicators in order to evaluate soil health, fertility status, and contamination risk. Soil serves as the primary medium for food production, and its quality directly influences crop yield, agricultural product safety, and long-term land sustainability. Contamination from heavy metals such as cadmium, lead, arsenic, and mercury, along with pesticide residues and nutrient imbalance, poses significant risks to both ecosystems and human health through the soil-crop-food pathway.

The regulatory framework for agricultural soil evaluation in China is primarily defined by GB 15618-2018, the Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land, which establishes risk screening values and risk intervention values for contaminants. Based on analytical results, farmland is classified into three management categories: priority protection class (uncontaminated or slightly contaminated), safe utilization class (contamination between screening and intervention values), and strict control class (contamination exceeding intervention values). Supporting methodologies are provided by the NY/T 1121 series for soil testing, HJ/T 166-2004 for soil sampling technical specifications, and international references including ISO 10390, ISO 10694, and relevant ASTM standards.

Testing Samples

Cultivated Topsoil Samples

Surface soil samples are collected from the plow layer, typically at a depth of 0 to 20 cm for dryland fields and 0 to 15 cm for paddy fields. This layer is where most root activity, nutrient cycling, and contaminant accumulation occur. Multiple subsamples are collected in an S-shaped, X-shaped, or grid pattern across the field and composited into a representative sample of approximately 1 kg after mixing and quartering.

Deep Profile Soil Samples

For contamination investigation and migration assessment, profile samples are collected at layered depths, commonly 0–20 cm, 20–40 cm, 40–60 cm, and deeper intervals, using soil augers or profile pits. Deep sampling helps determine whether pollutants have leached beyond the root zone and may threaten groundwater.

Greenhouse and Facility Agriculture Soils

Protected cultivation soils, such as greenhouse and tunnel soils, are frequently analyzed due to intensive fertilization, secondary salinization, and acidification risks. These samples are assessed for electrical conductivity, nitrate accumulation, and heavy metal enrichment resulting from repeated agrochemical inputs.

Paddy Field and Orchard Soils

Paddy soils require special consideration of redox conditions, as elements such as cadmium and arsenic exhibit mobility behavior that varies significantly between flooded and drained states. Orchard and perennial crop soils are typically sampled at greater depths and evaluated for micronutrient status and long-term metal accumulation from historic pesticide use.

Testing Items

Comprehensive agricultural soil quality analysis generally covers the following parameter groups:

• Basic physicochemical properties: pH value, electrical conductivity (EC), soil organic matter (SOM), cation exchange capacity (CEC), soil texture and particle size distribution, bulk density, and soil moisture content.
• Nutrient indicators: total nitrogen, alkaline hydrolyzable nitrogen, available phosphorus, available potassium, slowly available potassium, and trace elements including available iron, manganese, copper, zinc, and boron.
• Heavy metal contaminants: the eight regulated elements under GB 15618-2018, namely cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn), with hexavalent chromium (Cr(VI)) evaluated in specific risk scenarios.
• Organic contaminants: organochlorine pesticides (such as HCH and DDT), organophosphorus pesticides, polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons, and where applicable, polychlorinated biphenyls (PCBs).
• Biological and fertility health indicators: microbial biomass carbon, soil enzyme activity (urease, catalase, phosphatase), and earthworm or respiration assays for biological quality evaluation.

Testing Methods

Sample preparation follows standardized protocols, including air-drying, grinding, sieving (typically through 2 mm and 0.149 mm nylon mesh), and homogenization. All samples are handled with non-metallic tools to prevent cross-contamination of metal analytes. Analytical methods are selected according to the target parameter and applicable standards:

• Heavy metals: inductively coupled plasma mass spectrometry (ICP-MS) for multi-element determination at trace levels, inductively coupled plasma optical emission spectrometry (ICP-OES) for higher-concentration elements, flame and graphite furnace atomic absorption spectroscopy (AAS), and atomic fluorescence spectrometry (AFS) for mercury and arsenic, following HJ 803, HJ 680, GB/T 17141, and GB/T 22105 series methods.
• Organic contaminants: gas chromatography-mass spectrometry (GC-MS), gas chromatography with electron capture detection (GC-ECD) for organochlorine pesticides, and high-performance liquid chromatography (HPLC) for PAHs, in accordance with HJ 835, GB/T 14550, and HJ 783 methods.
• Basic properties: potentiometric determination of pH (NY/T 1121.2, ISO 10390), potassium dichromate oxidation for organic matter (NY/T 1121.6), and sedimentation or laser diffraction for particle size analysis.
• Nutrients: Kjeldahl digestion for total nitrogen, Olsen and ammonium acetate extraction methods for available phosphorus and potassium, and spectrophotometric techniques for micronutrients following NY/T series standards.

Quality assurance measures include method blanks, duplicate samples, matrix spikes, certified reference materials (CRMs), and calibration verification, ensuring that analytical data meet the accuracy and precision requirements for regulatory reporting.

Applications

Agricultural soil quality analysis results serve multiple practical purposes across the agricultural and environmental sectors:

• Land classification and compliance: determining whether farmland meets GB 15618-2018 risk screening values to support priority protection, safe utilization, or strict control designations.
• Soil pollution surveys: providing data support for detailed national and regional soil pollution investigations and dynamic monitoring programs.
• Precision fertilization and soil improvement: generating soil nutrient maps and prescription recommendations to optimize fertilizer use, correct acidity or salinity, and improve organic matter levels.
• High-standard farmland development: verifying soil quality requirements for land consolidation, reclaimed farmland acceptance, and supplemental cultivated land assessment.
• Remediation effect evaluation: confirming that treatment and remediation projects for contaminated farmland have achieved target cleanup levels.
• Food safety and origin certification: supporting green food, organic certification, and geographical indication product applications by demonstrating compliant soil conditions.

FAQ

How often should agricultural soil be tested?

For routine fertility monitoring, testing every 2 to 3 years is generally recommended. For contaminated land under safe utilization management, annual or crop-cycle monitoring of relevant contaminants is advised, with increased frequency following any remediation activity.

How many sampling points are required for one field?

Sampling density depends on field area and uniformity. As a general practice, each composite sample is formed from 5 to 20 subsamples, with grid densities of one point per several hectares for general surveys and tighter grids for detailed contamination investigations in accordance with HJ/T 166-2004.

What is the difference between risk screening values and risk intervention values?

Risk screening values define contaminant concentrations below which agricultural land pollution risk is generally negligible, while risk intervention values define concentrations above which the risk to agricultural product safety and human health is high, potentially requiring strict control measures or restricted cultivation. Concentrations between the two thresholds trigger refined risk assessment and safe utilization measures.

How much soil sample is needed for a full analysis?

A minimum of approximately 1 kg of prepared sample is typically sufficient for a full panel covering basic properties, nutrients, and the eight regulated heavy metals. Organic contaminant analysis may require additional sample volume depending on the number of target compounds.

What is the typical turnaround time for test results?

Standard analysis for basic properties and heavy metals usually requires 7 to 10 working days after sample receipt. Extended panels including organic contaminants, or projects requiring elaborate quality review, may require additional time for extraction, instrumental analysis, and data validation.

Which report standards should the results comply with?

Reports should present results with detection limits, analytical methods, and evaluation against applicable criteria such as GB 15618-2018 risk screening values, along with the relevant soil pH bracket, since screening thresholds for several metals vary with soil acidity. Projects with international requirements may additionally reference ISO or ASTM equivalents.