Paddy field soil testing is the systematic analysis of flooded rice-cultivation soils to evaluate fertility, redox-sensitive elements, and contaminants such as cadmium and arsenic. Accredited third-party laboratories analyze paddy soil samples against national and international standards, including GB 15618-2018, ISO, and ASTM methods, to support precision fertilization, food-safety compliance, and contaminated-land management. This guide explains the sample types, testing items, analytical methods, and practical applications of paddy field soil analysis.
Technical Overview
Paddy field soil testing refers to the sampling, preparation, and laboratory analysis of hydromorphic soils used for flooded rice cultivation. Paddy soils, classified as hydragric anthrosols, develop under repeated cycles of flooding, puddling, and drainage, which distinguish them chemically and physically from upland agricultural soils.
The alternating anaerobic and aerobic conditions in paddy fields drive pronounced redox oscillation. Under flooding, redox potential can fall below −200 mV, dissolving iron and manganese oxides, mobilizing arsenic, and shifting soil pH toward neutrality. Upon drainage and re-oxidation, pH may drop and cadmium solubility can increase. Because these dynamics strongly influence nutrient availability and heavy-metal uptake by rice, paddy soil testing must address both conventional fertility parameters and redox-sensitive contaminants.
The primary objectives of paddy field soil testing include fertility grading, site-specific formula fertilization, food-safety risk screening of cadmium and arsenic, compliance with agricultural soil quality standards such as GB 15618-2018, and verification of soil remediation effectiveness.
Testing Samples
Sample type and collection method are selected according to the testing purpose, field hydrology, and the redox sensitivity of the target parameters.
Plow-Layer Composite Samples
The plow layer, typically 0–20 cm in depth, is the standard sampling zone for routine fertility and contamination screening. A composite sample is formed by collecting 15–25 subsamples along an S-shaped or grid pattern across the field, avoiding field edges, fertilizer bands, and manure piles. The composite is reduced by quartering to approximately 1 kg for laboratory submission.
Subsoil and Profile Samples
Subsoil samples from 20–40 cm or deeper are collected to assess nutrient leaching, salt accumulation, deep migration of heavy metals, and subsoil compaction. Full profile samples taken by genetic horizon support soil classification, reclamation planning, and detailed contamination investigations.
Field-Moist and In-Situ Samples
Parameters such as ammonium nitrogen, nitrate nitrogen, ferrous iron, divalent manganese, and microbial activity change rapidly during drying. These analyses require field-moist samples transported chilled at 4 °C and analyzed promptly. Redox potential, pH, and temperature in flooded fields are preferably measured in situ using platinum-tipped electrodes.
Testing Items
A comprehensive paddy field soil testing program typically covers the following parameter groups, which can be selected individually or combined into fertility, food-safety, or remediation packages.
- Physical properties: particle-size distribution (texture), bulk density, total porosity, water-holding capacity, and aggregate stability.
- Fertility chemistry: pH, electrical conductivity, redox potential, organic matter, cation exchange capacity, base saturation, exchangeable calcium and magnesium.
- Primary nutrients: total nitrogen, phosphorus, and potassium; alkali-hydrolyzable nitrogen; available phosphorus; available potassium; slowly available potassium; ammonium and nitrate nitrogen.
- Secondary nutrients and micronutrients: available sulfur, available silicon (silicate), iron, manganese, zinc, copper, boron, molybdenum, and chloride.
- Heavy metals: cadmium, lead, arsenic, mercury, chromium, nickel, copper, and zinc, with optional extension to cobalt, vanadium, antimony, thallium, and hexavalent chromium.
- Organic contaminants: organochlorine and organophosphorus pesticides, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and petroleum hydrocarbons.
- Biological indicators: microbial biomass carbon and nitrogen, urease activity, catalase activity, and dehydrogenase activity.
Testing Methods
Sampling and Sample Preparation
Sampling follows recognized protocols such as NY/T 1121.1, HJ/T 166, and the ISO 18400 series. The preferred sampling window is after harvest, once the field has drained and before basal fertilization. Stainless-steel or bamboo tools are used for trace-metal sampling to prevent cross-contamination. After air-drying at room temperature, samples are crushed and sieved: 2 mm for pH, electrical conductivity, and available nutrients; 0.25 mm for organic matter and total nitrogen; and 0.149 mm for heavy-metal digestion.
Physicochemical and Nutrient Analysis
pH is determined potentiometrically in a 1:2.5 soil-to-water suspension per NY/T 1121.2, ISO 10390, or ASTM D4972. Electrical conductivity is measured in a soil extract per ISO 11265. Organic matter is determined by potassium dichromate oxidation with ferrous sulfate titration per NY/T 1121.6, or by loss on ignition per ASTM D2974. Total nitrogen is measured by the Kjeldahl method per NY/T 53 or ISO 11261, total phosphorus by alkali fusion and molybdenum-antimony spectrophotometry per NY/T 88, and total potassium by flame photometry per NY/T 87. Available phosphorus in neutral and acidic paddy soils is extracted with sodium bicarbonate solution (Olsen method) per NY/T 1121.7 and ISO 11263, while available potassium is extracted with neutral ammonium acetate and quantified by flame photometry per NY/T 889.
Heavy-Metal and Contaminant Analysis
Sample decomposition employs aqua regia extraction per ISO 11466 or HJ 803 and HJ 804, or total digestion with hydrofluoric acid mixtures per HJ 680. Cadmium and lead are determined by graphite furnace atomic absorption spectrometry per GB/T 17141. Arsenic and mercury are quantified by atomic fluorescence spectrometry per GB/T 22105.1 and GB/T 22105.2. Chromium is measured per GB/T 17137 or HJ 491, copper and zinc per GB/T 17138, and nickel per GB/T 17139. Multi-element screening is commonly performed by ICP-MS or ICP-OES per HJ 680. Organic contaminants are analyzed by gas chromatography-mass spectrometry or liquid chromatography-tandem mass spectrometry following solvent extraction and clean-up.
Data Evaluation and Quality Assurance
Heavy-metal results are evaluated against the risk screening values of GB 15618-2018 for paddy fields, which are pH-dependent; for example, the cadmium screening value ranges from 0.3 mg/kg for strongly acidic soils to 0.8 mg/kg for alkaline soils, and arsenic values for paddy land range from 20 to 30 mg/kg. Exceedances trigger detailed investigation against risk intervention values. Nutrient results are interpreted using established soil fertility grading systems to guide fertilizer recommendations. Reliable testing is ensured through ISO/IEC 17025-accredited laboratory procedures, certified reference materials, duplicate analyses, method blanks, and matrix spike recovery checks.
Applications
Paddy field soil testing results directly support decision-making across rice production and land management.
- Site-specific formula fertilization, including optimized nitrogen, phosphorus, potassium, zinc, and silicon application rates for target yields.
- Food-safety risk control, predicting cadmium and arsenic accumulation in rice grain and guiding water-management strategies and low-accumulating variety selection.
- Contaminated farmland investigation and remediation, including evaluation of passivation agents, liming, topsoil replacement, and agronomic control measures.
- High-standard farmland construction, long-term soil health monitoring, and soil organic carbon stock accounting.
- Environmental baseline assessment of production areas for organic and green food certification programs.
FAQ
When is the best time to sample paddy field soil?
The optimal time is after harvest, once the field has drained and before the application of basal fertilizer. At this stage, nutrients and pH reflect the post-crop condition, and sampling logistics are practical. For fertility monitoring, testing every two to three years is generally sufficient, while contamination monitoring may follow annual or project-specific schedules.
What sampling depth should be used?
The standard depth is the plow layer at 0–20 cm. For contamination assessment, paired samples from 0–20 cm and 20–40 cm are recommended to evaluate vertical migration. Deeper profile horizons are sampled only for detailed investigations or reclamation design.
Should samples be analyzed fresh or air-dried?
Routine fertility and heavy-metal tests use air-dried, sieved samples, which are stable and reproducible. Mineral nitrogen, ferrous iron, and biological activity must be analyzed on field-moist samples kept at 4 °C, since drying irreversibly alters these redox-sensitive parameters.
How are heavy-metal results interpreted?
Results are compared with the pH-dependent risk screening values of GB 15618-2018 for paddy fields. Concentrations below screening values indicate generally acceptable risk, while exceedances require further assessment against intervention values and may necessitate risk-control or remediation measures.
How long does paddy field soil testing take?
Routine fertility panels are typically completed within five to ten working days. Comprehensive heavy-metal and organic contaminant suites, including confirmation of exceedances, generally require ten to fifteen working days at accredited third-party laboratories operating under ISO/IEC 17025 quality systems.