Farmland Soil Physical and Chemical Properties Analysis: Testing Items, Standard Methods, and Applic

Farmland soil physical and chemical properties analysis is a core laboratory service that quantifies soil texture, structure, moisture, pH, salinity, organic matter, nutrients, and heavy metals to support precise fertilization, high-standard farmland development, and agricultural soil pollution risk assessment. By applying recognized standards such as NY/T 1121, GB 15618, ISO, and ASTM methods, growers, agricultural enterprises, and land administrators can make data-driven decisions that improve crop productivity, protect soil health, and ensure sustainable land use. This article explains the scope of farmland soil physicochemical testing, common sample types, key analytical parameters, standard test methods, and practical applications.

Technical Overview

Farmland soil physical and chemical properties analysis refers to the systematic sampling, pretreatment, and laboratory determination of the physical, chemical, and nutritional characteristics of cultivated soil. The physical fraction typically covers particle-size distribution (texture), bulk density, water content, porosity, aggregate stability, and hydraulic conductivity, which govern root penetration, aeration, and water retention. The chemical fraction covers pH, electrical conductivity (EC), soluble salts, organic matter, cation exchange capacity (CEC), exchangeable bases, macronutrients (N, P, K), secondary nutrients (Ca, Mg, S), micronutrients (Fe, Mn, Cu, Zn, B, Mo), and heavy metals of environmental concern such as cadmium, mercury, arsenic, lead, and chromium.

The service is driven by both agronomic and regulatory needs. On the agronomic side, results feed directly into soil testing and formulated fertilization programs, salinity management, and soil improvement schemes. On the regulatory side, agricultural land contamination screening and classification are performed against GB 15618, the risk control standard for soil contamination of agricultural land, while environmental monitoring often follows HJ series method standards. A typical analytical workflow includes sampling design, field sampling, air-drying and sieving pretreatment, instrumental or wet-chemical determination, quality control, and interpretation of results in a formal test report. All work should be performed by CMA/CNAS-accredited third-party laboratories or equivalent accredited laboratories to guarantee data legality, traceability, and comparability.

Testing Samples

Surface (Plow-Layer) Composite Samples

The plow layer is the primary sampling zone because it is where fertilization, tillage, and most root nutrient uptake occur. For dryland crops the standard depth is 0–20 cm, and for paddy fields it is generally 0–15 cm. A composite sample is formed by collecting sub-samples at 5–20 points arranged in an S-shaped or diagonal pattern within a uniform plot, pooling them, and reducing the composite to approximately 1 kg by quartering. Sampling should avoid field edges, fertilizer bands, manure piles, and recently fertilized rows.

Soil Profile Samples

Profile samples are collected horizon by horizon from excavated pits or power augers, commonly to 1.0–2.0 m depth. They are used to study salt migration in irrigated areas, water movement, gleyization, hardpan formation, and deep nutrient leaching, and they provide the basis for soil classification and land capability assessment.

Special-Purpose Samples

These include rhizosphere soil for nutrient availability studies, greenhouse and protected-cultivation soil where salinity and nitrate accumulation are frequent problems, saline-alkali soil for reclamation monitoring, and pre-treatment and post-treatment samples from remediation or soil improvement projects to verify effectiveness.

• Air-dry samples in a clean, ventilated place away from direct sunlight and contamination sources.
• Crush gently and sieve through 2 mm for pH, EC, available nutrients, and texture; through 0.25 mm for organic matter; and through 0.149 mm for total nutrient digestion.
• Label, seal, and store samples in polyethylene or glass containers, and transport them to the laboratory promptly with chain-of-custody records.

Testing Items

Physical Properties

• Particle-size distribution (sand, silt, clay) and soil texture classification (USDA 12-class system)
• Soil water content and field capacity
• Bulk density and total porosity
• Water-stable aggregates (>0.25 mm) and aggregate stability
• Saturated hydraulic conductivity

Basic Chemical Properties

• pH (active acidity/alkalinity) and lime requirement for acidified soils
• Electrical conductivity, total soluble salt content, and salt composition (CO3²⁻, HCO3⁻, Cl⁻, SO4²⁻, Ca²⁺, Mg²⁺, Na⁺, K⁺)
• Soil organic matter and soil organic carbon
• Cation exchange capacity and exchangeable Ca, Mg, K, Na; base saturation percentage

Nutrient Status

• Total nitrogen, phosphorus, and potassium
• Available nitrogen (alkali-hydrolyzable N, nitrate-N, ammonium-N)
• Available phosphorus (Olsen-P or Bray-P depending on soil pH)
• Available (exchangeable) potassium and slowly available potassium
• Secondary nutrients: exchangeable Ca and Mg, available sulfur
• Micronutrients: available Fe, Mn, Cu, Zn (DTPA-extractable) and available B (hot-water extractable)

Heavy Metals and Environmental Indicators

• Cadmium, mercury, arsenic, lead, chromium (including hexavalent chromium), copper, nickel, and zinc, typically digested by aqua regia and screened against the risk screening values of GB 15618

Testing Methods

Sampling follows NY/T 1121.1 (soil testing, Part 1: sampling) and relevant technical specifications, using grid-based designs adjusted to field size and variability. The table of core methods below reflects the mainstream national, ISO, and ASTM procedures used by accredited third-party laboratories.

Physical Parameter Methods

• Texture: pipette method or hydrometer/sieve method based on Stokes' law (NY/T 1121.3; ASTM D6913 and D7928)
• Bulk density: ring knife (drive-cylinder) method (NY/T 1121.4; ASTM D2937)
• Water content: oven drying at 105 ± 5 °C to constant mass (ISO 11465; ASTM D2216)
• Aggregate stability: wet-sieving of water-stable aggregates

Chemical Parameter Methods

• pH: potentiometry with a glass electrode in a 1:2.5 (or 1:5) soil-to-water suspension (NY/T 1121.2; ISO 10390; ASTM D4972)
• EC: conductivity measurement of a 1:5 soil-to-water extract (ISO 11265)
• Organic matter: potassium dichromate oxidation with titration (NY/T 1121.6) or dry combustion with a carbon analyzer (ISO 10694)
• CEC and exchangeable bases: neutral 1 mol/L ammonium acetate exchange (NY/T 1121.5), with quantification by flame photometry, AAS, or ICP-OES

Nutrient and Metal Methods

• Total nitrogen: Kjeldahl digestion and distillation (NY/T 53; ISO 11261) or Dumas dry combustion
• Available nitrogen: alkaline hydrolysis diffusion method; nitrate and ammonium by UV spectrophotometry or indophenol blue colorimetry
• Total phosphorus: sodium hydroxide fusion followed by molybdenum-antimony anti-spectrophotometry (HJ 632)
• Available phosphorus: extraction with 0.5 mol/L NaHCO3 at pH 8.5 (Olsen) for neutral-to-alkaline soils or dilute acid-fluoride (Bray-1) for acidic soils, quantified by molybdenum blue spectrophotometry
• Total potassium: NaOH fusion with flame photometry; available potassium: 1 mol/L neutral ammonium acetate extraction with flame photometry or AAS
• Micronutrients: DTPA extraction (pH 7.3) with AAS or ICP-OES determination; available boron by hot-water extraction with curcumin or azomethine-H spectrophotometry
• Heavy metals: aqua regia digestion (HJ 803) with ICP-MS, ICP-OES, or AAS determination; Hg and As by atomic fluorescence spectrometry (GB/T 22105); Cd and Pb by graphite furnace AAS (GB/T 17141)

Quality assurance is maintained through method blanks, a minimum of 10% parallel duplicates, certified reference soils for matrix matching, multi-point calibration with verification standards, ion-balance and spike-recovery checks for soluble salts, and participation in proficiency testing schemes. Reports issued by CMA/CNAS-accredited laboratories carry legal validity for regulatory and commercial purposes.

Applications

• Precision and formulated fertilization: converting soil test values into crop-specific fertilizer recommendations and variable-rate application maps
• Soil fertility grading and productivity evaluation for cultivated land quality monitoring networks
• Contamination risk screening and detailed investigation of agricultural land against GB 15618 screening and intervention values
• High-standard farmland construction, land consolidation, and project acceptance testing
• Saline-alkali soil reclamation, acidified soil correction with lime, and evaluation of subsoiling or organic amendment effects
• Environmental site assessment for green food and organic production bases, including greenhouse soil salinity and nitrate control
• Remediation verification before and after soil improvement or pollution treatment projects
• Soil carbon stock accounting for carbon sequestration projects and sustainable land management reporting
• Agronomic research, field trials, and agricultural insurance or land-transaction due diligence

FAQ

How deep should farmland soil samples be collected, and how many points are needed?

For routine plow-layer testing, sample 0–20 cm for dryland and 0–15 cm for paddy fields. Combine 5–20 sub-sampling points per uniform plot using an S-shaped route, then quarter the composite to about 1 kg. Larger or heterogeneous fields should be divided into separate sampling units to avoid averaging away real differences.

How often should farmland be tested?

For fertility and fertilization purposes, testing every 2–3 years is generally sufficient because pH, organic matter, and available nutrients change gradually. Greenhouse soils, saline-alkali amelioration zones, and remediation sites should be monitored annually or per cropping season until values stabilize.

What is the difference between total and available nutrients?

Total nutrients represent the entire pool bound in the soil matrix, including forms crops cannot absorb in the short term, and they mainly reflect long-term soil reserves. Available nutrients are the extractable fractions held on exchange sites or dissolved in soil solution, and they directly guide immediate fertilizer recommendations. Both are complementary: total values indicate potential, available values indicate current supply.

Which parameters indicate a salt-affected or acidified soil problem?

Elevated EC, total soluble salt content, and exchangeable sodium percentage indicate salinity or alkalinity stress, which is common in irrigated and greenhouse soils. A pH below about 5.5, together with low exchangeable Ca and Mg and high exchangeable aluminum, signals acidification and the need for liming or other amendments.

How are heavy metal results evaluated?

Aqua-regia-digestible concentrations of Cd, Hg, As, Pb, Cr, Cu, Ni, and Zn are compared with the risk screening values for agricultural land in GB 15618. Results below screening values generally indicate acceptable risk; results above them trigger detailed investigation, possible category identification, and, where necessary, risk control or remediation measures.

How long does a complete physicochemical panel take?

A routine panel of pH, EC, organic matter, and available N, P, and K typically takes 5–10 working days after sample receipt. Extended panels including CEC, micronutrients, full profiles, and heavy metals with ICP-MS confirmation usually require 10–15 working days, depending on sample load and digestion requirements.

How should samples be handled before submission?

Use clean sampling tools and containers, avoid galvanized or brass tools when metals are tested, keep samples clearly labeled with depth, location (GPS coordinates), land use, and sampling date, and ship them in cool, sealed bags as soon as possible. For microbial or nitrate-related analyses, chilled transport at 4 °C is recommended to preserve the original state of the sample.