Soil nutrient determination in China is governed by a structured hierarchy of national standards (GB and GB/T) and supporting industry standards (NY/T, LY/T, HJ) that define how soil samples shall be collected, prepared, and analyzed for pH, organic matter, nitrogen, phosphorus, potassium, and micronutrients. Understanding these standards is essential for agronomists, laboratory managers, and farmland operators who need defensible soil test data for fertilization recommendations, land quality assessment, and regulatory compliance. This article explains the standard framework, core testing items, prescribed analytical methods, quality-control requirements, and practical applications.
Technical Overview
Soil nutrient determination in China is organized within a multi-level standardization system. At the top are the Guobiao national standards: GB denotes mandatory national standards with legal force, while GB/T denotes recommended national standards. Beneath these sit sector-specific industry standards, most importantly the NY/T series issued for agriculture, the LY/T series for forestry soils, and the HJ or HJ/T series for environmental monitoring. Together, these documents specify sampling procedures, pretreatment, extraction chemistries, instrumental techniques, calculation formulas, and precision limits for every major soil fertility indicator.
For routine agronomic testing of cultivated soils, the NY/T 1121 series (Soil Testing) forms the operational backbone, with individual parts covering sampling, pH, organic matter, available phosphorus, cation exchange capacity, and additional parameters. Classical GB methods from the 1980s, such as GB 7173-1987 for total nitrogen (semi-micro Kjeldahl method) and GB 9834-1988 for organic matter (potassium dichromate oxidation titration), established the analytical foundation; following later standardization reforms, many of these were reaffirmed, revised, or functionally succeeded by GB/T and NY/T versions, so laboratories must verify the currently effective edition before issuing reports.
Chinese methods are broadly harmonized with international counterparts. Examples include ISO 10390 for pH, ISO 10694 for organic carbon by dry combustion, ISO 11261 for total nitrogen by modified Kjeldahl, ISO 13878 for total nitrogen by dry combustion, ISO 11263 for Olsen-extractable phosphorus, ISO 23470 for cation exchange capacity using cobaltihexamine chloride, and ISO 14870 for DTPA extraction of trace elements. This alignment allows accredited third-party laboratories to report GB-based data in contexts where international comparability is required.
Mandatory and Recommended National Standards (GB and GB/T)
GB and GB/T documents carry the highest authority in the standardization hierarchy and are adopted nationwide. They define the reference principles for parameters such as total nitrogen, organic matter, and available boron, and they interface with environmental quality standards such as GB 15618-2018, the risk control standard for agricultural land contamination, which sets the broader context for soil quality management.
Agricultural Industry Standards (NY/T)
The NY/T family provides the day-to-day method library for farmland fertility evaluation. In addition to the NY/T 1121 series, widely cited documents include NY/T 53-1987 for total nitrogen, NY/T 87-1988 for total potassium, NY/T 88-1988 for total phosphorus, NY/T 889-2004 for readily available and slowly available potassium, and NY/T 890-2004 for DTPA-extractable zinc, manganese, iron, and copper.
Forestry and Environmental Standards (LY/T and HJ)
Forest soils are covered by the LY/T series, for example LY/T 1228-2015 for nitrogen determination in forest soils, with companion standards for other nutrients. Environmental sampling and quality assurance are governed by HJ/T 166-2004, the technical specification for soil environmental monitoring, while HJ 634-2012 addresses ammonium and nitrate nitrogen extracted with potassium chloride solution.
Testing Samples
Cultivated and Arable Soils
The most common sample type is the plow layer of dryland fields, typically collected at 0 to 20 cm depth, with paddy fields often sampled at 0 to 15 cm. Following NY/T 1121.1-2006, a field unit is sampled along a grid, diagonal, or S-shaped pattern, 15 to 20 cores are composited, and the bulk sample is reduced by quartering. Sampling is best performed after harvest and before base fertilization so that results reflect the residual fertility available to the next crop.
Greenhouse and Intensively Managed Soils
Protected cultivation soils accumulate salts and nitrate nitrogen due to high input rates and reduced leaching. Sampling is usually performed at 0 to 25 cm, often supplemented with a 25 to 50 cm sublayer, and the test program is expanded to include electrical conductivity, water-soluble salts, and nitrate nitrogen to diagnose secondary salinization.
Forest and Grassland Soils
Forest soil programs under the LY/T system separate the organic litter layer from mineral horizons, commonly 0 to 20 cm and 20 to 40 cm. These samples contain coarse fragments and undecomposed organic material, so pretreatment includes more rigorous removal of stones and roots, and nutrient interpretation uses forestry-specific criteria rather than agronomic thresholds.
Saline-Alkali, Acidified, and Degraded Soils
Problem soils require profile sampling that may extend to 100 cm for salt movement assessment. In saline-alkali regions, water-soluble salt totals, chloride, and sodium adsorption considerations are added to the nutrient panel; in acidified subtropical soils, exchangeable acidity and exchangeable aluminum are frequently determined alongside routine nutrients.
Testing Items
A standard soil nutrient report is organized in three tiers: routine macronutrient indicators, secondary and micronutrients, and supporting physico-chemical properties. The exact panel is selected according to land use, crop type, and the diagnostic purpose of the survey.
Routine Macronutrient and Fertility Indicators
- Soil pH, measured potentiometrically as the master variable governing nutrient availability
- Soil organic matter and organic carbon, the key indicators of soil quality and nitrogen supply
- Total nitrogen, total phosphorus, and total potassium as reserve pools
- Alkali-hydrolyzable (available) nitrogen, available phosphorus, and available potassium as the directly plant-responsive fractions
- Slowly available potassium released from 2:1 clay minerals
Secondary Nutrients and Micronutrients
- Exchangeable calcium and magnesium, critical for acid soils and fruit crops
- Available sulfur and available silicon, increasingly important in high-yield systems
- DTPA-extractable iron, manganese, copper, and zinc
- Hot-water-soluble boron and ammonium-oxalate-extractable molybdenum
- Chloride, monitored in saline and tobacco-producing regions
Supporting Physico-Chemical Properties
- Cation exchange capacity, which expresses nutrient retention capability
- Mechanical composition (texture) and bulk density
- Water-soluble salt content and electrical conductivity for salinity diagnosis
Testing Methods
Sampling and Sample Preparation
After compositing and quartering in the field, samples are transported to the laboratory, air-dried at ambient or slightly elevated temperature below 40 degrees Celsius, gently crushed, and sieved. A 2 mm sieve fraction is used for pH, available phosphorus, available potassium, and exchangeable measurements, while a finer 0.25 mm fraction is reserved for organic matter and total nutrient digestion. Careful labeling, chain-of-custody records, and avoidance of contamination from tools or galvanized containers are explicit requirements of the sampling standards.
Soil pH
pH is determined potentiometrically with a calibrated glass electrode in a soil-water suspension, most commonly at a 1:2.5 ratio with a stabilization period before reading, per NY/T 1121.2-2006 and consistent with the principles of ISO 10390. Measurement in potassium chloride solution may be added to characterize exchangeable acidity in acid soils.
Organic Matter and Organic Carbon
The classical national method is potassium dichromate-sulfuric acid oxidation with external heating, followed by back-titration of excess dichromate with ferrous ammonium sulfate, as established in GB 9834-1988 and NY/T 1121.6-2006. Organic matter is calculated from oxidized organic carbon using the conventional factor of 1.724. Where higher throughput and direct carbon data are required, dry combustion with an elemental analyzer under ISO 10694 provides a recognized alternative, and ISO 14235 covers sulfochromic oxidation.
Nitrogen
- Total nitrogen: semi-micro Kjeldahl digestion with a mixed catalyst, distillation, and boric acid titration per GB 7173-1987 and NY/T 53-1987, equivalent in principle to ISO 11261; dry combustion under ISO 13878 is accepted as an instrumental alternative
- Available nitrogen: alkali-hydrolysis diffusion method, in which soil is treated with sodium hydroxide in a sealed diffusion dish, evolved ammonia is absorbed in boric acid, and the absorbed ammonia is titrated with standard acid
- Mineral nitrogen: ammonium and nitrate nitrogen extracted with potassium chloride solution and quantified by spectrophotometry or flow analysis, supported by HJ 634-2012 for environmental contexts
Phosphorus
Available phosphorus is extracted with 0.5 mol/L sodium bicarbonate at pH 8.5 (Olsen method) for neutral and calcareous soils, as prescribed in the corresponding part of the NY/T 1121 series and aligned with ISO 11263. For acidic soils, particularly forest soils, dilute acid-fluoride (Bray-type) extractants are applied. In both cases, phosphorus in the extract is quantified by molybdenum-antimony antispectrophotometry. Total phosphorus is determined after sodium hydroxide fusion followed by the same colorimetric finish, per NY/T 88-1988.
Potassium
Readily available potassium is extracted with neutral 1 mol/L ammonium acetate and measured by flame photometry or atomic absorption spectrometry per NY/T 889-2004. Slowly available potassium is released by boiling 1 mol/L nitric acid extraction, and total potassium is determined after alkali fusion by flame photometry per NY/T 87-1988. The three-pool interpretation allows differentiation between immediately available, reserve, and structural potassium.
Cation Exchange Capacity and Exchangeable Bases
Cation exchange capacity is determined by saturating exchange sites with ammonium using the EDTA-ammonium acetate method specified in the NY/T 1121 series, then quantifying the adsorbed ammonium. Exchangeable calcium and magnesium are measured in the ammonium acetate extract by atomic absorption or ICP-OES. For international comparability, ISO 23470 offers the cobaltihexamine chloride procedure, and effective CEC concepts from ISO standards can be applied in acid soil studies.
Micronutrients
- Iron, manganese, copper, and zinc: DTPA extraction at pH 7.3 followed by AAS or ICP-OES per NY/T 890-2004 and ISO 14870
- Boron: hot-water reflux extraction followed by curcumin or azomethine-H spectrophotometry, with GB 12298-1990 as the classical reference
- Molybdenum: ammonium oxalate-oxalic acid (Tamm) extraction with detection by graphite furnace AAS or catalytic methods
- Sulfur: phosphate or calcium chloride extraction with ICP-OES or turbidimetric finish
- Silicon: dilute acid or citrate buffer extraction with molybdenum blue colorimetry
Quality Assurance and Quality Control
Compliant laboratories operate under documented quality systems, typically holding CMA and CNAS accreditation. Standard practice includes method blanks, duplicate determinations on at least 10 to 20 percent of samples, calibration verification with certified reference materials, and control charts for critical parameters such as pH repeatability. HJ/T 166-2004 provides the overarching technical framework for sampling representativeness, cross-contamination control, and data validation, and participation in interlaboratory proficiency testing is expected for accredited third-party laboratories.
Applications
Soil Testing and Formulated Fertilization
The primary application is the national soil testing and formula fertilization program, in which available nutrient values are compared against regional critical levels to generate field-specific fertilizer recommendations. This reduces over-application of nitrogen and phosphorus, improves potassium balance, and lowers both input costs and environmental losses.
Farmland Quality Assessment and Fertility Grading
GB-based soil data feed into farmland quality surveys, high-standard farmland construction projects, and fertility grading schemes inherited from the national soil survey framework. Graded nutrient levels support land capability classification, soil improvement prioritization, and long-term monitoring of cultivated land assets.
Environmental and Food Safety Linkage
Nutrient testing complements contaminant screening under GB 15618-2018. Excessive nitrate accumulation in greenhouse soils, phosphorus loading near water bodies, and salinization trends are all diagnosed through the same national sampling and analytical infrastructure, enabling integrated agronomic and environmental decision-making.
Commercial, Research, and Certification Support
Standardized results underpin fertilizer efficacy trials, evaluation of soil conditioners and amendments, organic and good-agricultural-practice certification audits, and carbon accounting programs that track changes in soil organic carbon stocks over time. Researchers also rely on the uniform methods to build comparable regional and national soil nutrient databases.
FAQ
What is the difference between GB, GB/T, and NY/T standards for soil testing?
GB standards are mandatory national standards, GB/T standards are recommended national standards, and NY/T standards are agricultural industry standards. In soil nutrient work, GB documents historically established reference principles, while the NY/T 1121 series now provides the detailed operational methods used daily by agronomic laboratories. All three carry formal standing, and the currently valid edition governs report issuance.
How deep should soil samples be collected, and how many cores are composited?
For cultivated dryland, the standard plow layer is 0 to 20 cm; paddy fields are commonly sampled at 0 to 15 cm, and orchards or greenhouse trials may extend to 40 cm or deeper. Typically 15 to 20 cores are composited per sampling unit and reduced by quartering to obtain a representative 0.5 to 1 kg laboratory sample.
Why do national standards prescribe different methods for available phosphorus in different soils?
Extraction chemistry must match soil properties. The sodium bicarbonate (Olsen) extractant suits neutral and calcareous soils by suppressing calcium phosphate dissolution artifacts, whereas acid-fluoride (Bray-type) extractants better reflect availability in acidic soils. Using the wrong extractant produces biased fertilizer recommendations.
What is the difference between total and available nutrient determination?
Total nutrients, obtained by fusion or digestion, measure the entire reserve pool including mineral-bound forms, and mainly indicate long-term potential. Available nutrients, obtained by mild chemical extraction simulating root uptake, measure the short-term plant-responsive fraction and are the basis of fertilizer rate calculations.
Can GB-based results be compared with international test data?
In most cases, yes. Key GB and NY/T methods are methodologically equivalent to ISO counterparts such as ISO 10390 (pH), ISO 11261 (total nitrogen), ISO 11263 (Olsen phosphorus), and ISO 14870 (DTPA micronutrients). Accredited third-party laboratories can note method equivalence in reports to support cross-border comparability.
How frequently should farmland be tested?
General guidance is to test routine fertility indicators every two to three years, with more frequent monitoring for greenhouse soils, high-value crops, salt-affected fields, or demonstration plots under intensive management. Sampling should always occur at the same point in the crop rotation to keep year-over-year data comparable.
