Determination of cadmium content in fertilizers is an essential quality control and regulatory testing procedure that safeguards soil health, crop safety, and the food supply chain by quantifying toxic heavy metal contamination. This technical article reviews the major regulatory limits, applicable standards such as ISO 17318, GB/T 23349, and GB 38400-2019, sample digestion procedures, and instrumental techniques including ICP-MS, ICP-OES, and graphite furnace atomic absorption spectrometry used by accredited third-party laboratories.
Technical Overview
Cadmium is a non-essential, highly toxic heavy metal that enters fertilizer production chains primarily through phosphate rock. Because Cd2+ substitutes isomorphously for Ca2+ in the apatite lattice, cadmium concentrations in phosphate ores range from less than 1 mg/kg in igneous deposits to well over 100 mg/kg in certain sedimentary phosphorites. Secondary contamination routes include zinc-smelting by-products used as micronutrient raw materials, livestock manure and sewage sludge used in organic fertilizers, and industrial by-products such as fly ash and phosphogypsum incorporated into soil conditioners.
Unlike organic pollutants, cadmium cannot be degraded in soil. It adsorbs onto clay minerals, iron and manganese oxides, and organic matter, with a soil residence time measured in decades. A pH-dependent plant-available fraction is taken up by crops, with rice, leafy vegetables, and root crops being notable accumulators. Chronic dietary exposure causes irreversible kidney tubular damage and bone demineralization, which is why food safety authorities worldwide enforce strict cadmium limits in cereals and why fertilizer-borne cadmium is regulated at the source.
- China: GB 38400-2019 sets a maximum limit of 10 mg/kg total cadmium for fertilizers placed on the market.
- European Union: Regulation (EU) 2019/1009 limits cadmium to 60 mg/kg P2O5 in inorganic macronutrient fertilizers containing 5% P2O5 or more, with provision for review toward lower thresholds.
- Other jurisdictions: national limits generally fall between 1 and 60 mg/kg depending on product category, with stricter thresholds applied to organic and recycled-nutrient fertilizers.
Testing Samples
Phosphate and Compound Fertilizers
Single superphosphate (SSP), triple superphosphate (TSP), monoammonium phosphate (MAP), diammonium phosphate (DAP), and NPK compounds carry the highest inherent cadmium risk because they are derived directly from phosphate rock and wet-process phosphoric acid. Testing frequency is typically highest for this category.
Organic Fertilizers and Biostimulants
Composted manure, crop straw, food-processing residues, and bio-solids-based products are tested because feedstock quality varies widely and heavy metals can concentrate during composting. Regulatory regimes treat these products under the same or similar cadmium limits as mineral fertilizers.
Water-Soluble and Specialty Fertilizers
Foliar feeds, fertigation grades, and controlled-release formulations present high-chloride or high-urea matrices that require careful digestion and matrix-matched calibration, particularly when derived from lower-grade wet-process acids.
Soil Conditioners and Liming Materials
Phosphogypsum, fly ash, phosphate rock dust applied directly to soil, and industrial liming by-products are screened for cadmium before land application to prevent long-term accumulation.
Raw Materials and Intermediates
Phosphate rock, wet-process phosphoric acid, ammonium phosphate slurries, micronutrient salts such as zinc sulfate from smelter origins, and recycled nutrients such as struvite and biomass ash are analyzed for supplier qualification and incoming inspection.
Testing Items
The core deliverable is the total cadmium content, reported in mg/kg on a dry-matter basis. Because fertilizer regulations are typically multi-element, cadmium is usually determined together with the companion toxic elements arsenic, lead, chromium, and mercury under the GB 38400-2019 and ISO 17318 suites. Optional scopes include the water-soluble cadmium fraction for environmental risk assessment and sequential extraction for bioavailability studies.
- Total cadmium (Cd), primary regulated parameter
- Full heavy metal suite: As, Pb, Cr, Hg alongside Cd
- Water-soluble and extractable cadmium fractions (optional, for risk profiling)
- Moisture content for dry-basis result conversion
Testing Methods
Sampling and Sample Preparation
Representative increments are collected per recognized fertilizer sampling practice such as ISO 8358 or equivalent national procedures, combined, and reduced by coning and quartering. The laboratory portion is ground to pass a 0.5 mm sieve, homogenized, and either dried at a maximum of 40 °C or analyzed as received with a parallel moisture determination. All tools and containers are acid-washed to prevent cross-contamination.
Acid Digestion
Total decomposition is achieved by closed-vessel microwave digestion with nitric, hydrochloric, and hydrofluoric acids, followed by boric acid complexation of residual fluoride. Aqua regia digestion is widely accepted as a pseudo-total approach for fertilizer compliance under ISO 17318, while dry ashing at 450–550 °C with subsequent hydrochloric acid dissolution remains a validated alternative in older national procedures. Method blanks accompany every digestion batch.
Graphite Furnace Atomic Absorption Spectrometry (GFAAS)
GFAAS is the classical reference technique in GB/T 23349 and ISO 17318. Cadmium is measured at 228.8 nm with Zeeman or deuterium background correction, using an ammonium dihydrogen phosphate and magnesium nitrate chemical modifier, pyrolysis near 600–750 °C, and atomization at approximately 1500–1700 °C. Matrix-matched calibration yields a typical method LOQ of 0.05–0.1 mg/kg, comfortably below the 10 mg/kg regulatory limit.
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
ICP-OES offers high multi-element throughput for routine batch control. Sensitive cadmium lines include 214.439, 226.502, and 228.802 nm. High total dissolved solids from fertilizer digests require appropriate dilution and internal standardization with yttrium or scandium. Typical LOQs of 0.1–0.5 mg/kg are fully adequate for limit verification against GB 38400-2019.
Inductively Coupled Plasma Mass Spectrometry (IC-MS/ICP-MS)
ICP-MS provides the lowest detection limits, typically below 0.01 mg/kg, using isotopes 111Cd and 114Cd with indium or rhodium internal standardization. Helium kinetic energy discrimination removes molybdenum oxide polyatomic interferences relevant to micronutrient fertilizers. Isotope dilution ICP-MS is optionally applied when certification-level accuracy is required.
Rapid Screening
Energy-dispersive X-ray fluorescence (EDXRF) enables fast, non-destructive screening of phosphate rock and finished goods for material segregation, with all positive or borderline results confirmed by wet-chemical digestion and spectrometric determination.
Quality Assurance and Validation
Accredited third-party laboratories support each determination with method blanks, duplicate samples with relative percent difference within 20%, certified reference materials of fertilizer matrix from national metrology institutes, and spike recoveries between 85% and 115%. Calibration verification standards bracket the regulatory limit, and the expanded measurement uncertainty near the limit is typically reported at 15–25% (k = 2).
Applications
Cadmium determination data underpin regulatory compliance, supply-chain control, and environmental protection programs across the fertilizer value chain.
- Manufacturer quality control and raw material supplier qualification
- Market access compliance testing against GB 38400-2019, EU Regulation 2019/1009, and destination-market regulations
- Verification of inputs for organic and recycled-nutrient certification schemes
- Import and export inspection and customs clearance documentation
- Farmland soil protection and remediation programs tracking contaminant loading
- Agronomic research, lifecycle assessment, and phosphate beneficiation studies
FAQ
What is the regulatory limit for cadmium in fertilizers?
In China, GB 38400-2019 limits total cadmium to 10 mg/kg. In the European Union, Regulation (EU) 2019/1009 applies a limit of 60 mg Cd/kg P2O5 to phosphate-containing inorganic macronutrient fertilizers. Always confirm the applicable limit for the specific product category and destination market.
Which analytical method is the most sensitive?
ICP-MS offers the lowest limits of quantification, often below 0.01 mg/kg. GFAAS and ICP-OES remain fully compliant reference methods for verifying the commonly applied regulatory thresholds.
How low must the detection limit be?
Best practice requires the method LOQ to be no higher than one-third of the regulatory limit, meaning approximately 3 mg/kg or better for the Chinese limit. Routine laboratory methods achieve 0.01–0.5 mg/kg.
Why is hydrofluoric acid used in digestion?
Cadmium bound within the apatite and silicate lattices of phosphate matrices is not fully released by nitric and hydrochloric acids alone. HF ensures total decomposition; aqua regia digestion is accepted as a pseudo-total method but may under-recover cadmium by 5–15% in certain matrices.
How much sample is required and how long does testing take?
A representative sample of 100–200 g is generally sufficient. Standard turnaround at accredited third-party laboratories is five to seven working days, with express options available.
Do organic fertilizers need cadmium testing?
Yes. Organic and recycled-nutrient fertilizers are subject to the same cadmium limits, and their feedstock variability makes routine testing especially important for compliance and certification.