Determination of Trace Elements in Fertilizers: Analytical Methods, Standards, and Quality Control

Trace element determination in fertilizers is essential for verifying micronutrient content, controlling heavy metal contaminants, and meeting mandatory regulatory limits in global agricultural markets. Using techniques such as ICP-OES, ICP-MS, and atomic absorption spectrometry, accredited third-party laboratories quantify essential micronutrients and hazardous elements in accordance with ISO, EN, and GB standards. This article provides a complete technical guide to sample types, testing items, digestion procedures, instrumental methods, and quality assurance for fertilizer trace element analysis.

Technical Overview

Trace elements in fertilizers are typically divided into two functionally distinct groups. The first group comprises essential plant micronutrients—iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl)—which crops require in small quantities, typically at levels ranging from a few milligrams per kilogram to several percent of the product. Deficiency of these elements causes physiological disorders such as chlorosis, poor fruit set, and reduced yield, while excessive concentrations can produce phytotoxicity and soil accumulation.

The second group consists of non-essential, potentially hazardous trace elements, primarily arsenic (As), cadmium (Cd), lead (Pb), chromium (Cr), mercury (Hg), nickel (Ni), and thallium (Tl). These contaminants enter fertilizers mainly through phosphate rock, industrial by-products, and organic waste streams. Regulatory frameworks such as China's mandatory standard GB 38400-2019 on hazardous substance limits in fertilizers, and the EU Fertilising Products Regulation, impose strict maximum concentrations on these elements, making accurate trace element determination a critical requirement for market access, label verification, and product safety.

Modern fertilizer trace element analysis combines rigorous sample preparation—microwave digestion, aqua regia extraction, or alkaline ashing—with multi-element instrumental techniques including inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), and atomic absorption spectrometry (AAS), supported by validated quality control procedures under ISO/IEC 17025 laboratory management systems.

Testing Samples

Compound and Blended Fertilizers (NPK)

Complex, compound, and bulk-blended NPK fertilizers are the most common samples submitted for micronutrient testing. Both total micronutrient content and water-soluble fractions are determined, because guaranteed analyses on product labels frequently specify water-soluble iron, manganese, zinc, copper, and boron that are immediately available for plant uptake.

Straight Nitrogen, Phosphate, and Potash Fertilizers

Urea, ammonium sulfate, ammonium nitrate, monoammonium phosphate (MAP), diammonium phosphate (DAP), single superphosphate (SSP), triple superphosphate (TSP), muriate of potash (MOP), and sulfate of potash (SOP) are analyzed for inherent micronutrients and, in particular, for heavy metal carryover from phosphate rock, which is the dominant source of cadmium, arsenic, and lead in mineral fertilizers.

Micronutrient and Chelated Fertilizers

Dedicated micronutrient products such as zinc sulfate, borax and boric acid, sodium molybdate, copper sulfate, and manganese sulfate require assay of the active ingredient content. Chelated products such as Fe-EDDHA, Fe-EDTA, Zn-EDTA, and amino-acid-chelated micronutrients require additional determination of the chelated fraction and, in some cases, ortho-ortho isomer content for iron chelates, in addition to total metal content.

Water-Soluble and Foliar Fertilizers

Fully water-soluble fertilizers, drip-irrigation grade products, and foliar sprays are tested to confirm declared micronutrient levels, verify complete solubility, and screen for heavy metals that could damage foliage or contaminate irrigation systems.

Organic and Organic-Inorganic Fertilizers

Compost, manure-derived, biogas residue, humic acid, and organic-inorganic compound fertilizers carry an elevated risk of heavy metal accumulation from feedstock materials and are routinely tested against the stricter hazardous element limits applicable to organic input categories.

Raw Materials, Intermediates, and Soil Conditioners

Phosphate rock, sulfuric acid, ammonia, fillers, coating agents, and soil conditioners are analyzed as incoming materials control, allowing manufacturers to trace contamination to its source before production.

Testing Items

Typical trace element testing programs for fertilizers cover the following analyte groups:

• Essential micronutrients: total and water-soluble Fe, Mn, Zn, Cu; total and water-soluble B; Mo; Cl
• Chelated fractions: water-soluble chelated Fe, Zn, Mn, Cu in chelated micronutrient fertilizers
• Hazardous heavy metals: As, Cd, Pb, Cr, Hg, and, where required by the destination market, Ni, Tl, Cu, and Zn limits
• Related nutrients and quality parameters often tested alongside trace elements: Ca, Mg, S, Si, Se, Co, and sodium
• Speciation and availability indices, such as citrate-soluble or EDTA-extractable micronutrient fractions

For regulatory compliance testing in China, GB 38400-2019 defines maximum limits for As, Cd, Pb, Cr, and Hg across inorganic, organic, and organic-inorganic fertilizer categories, and these five elements constitute the core hazardous substance panel. Export-oriented programs may be extended to include the additional elements required by the destination market.

Testing Methods

Sample Preparation and Digestion

Representative laboratory samples are dried, ground, and homogenized to pass a 0.5 mm sieve before analysis. Digestion strategy depends on the analyte: closed-vessel microwave digestion with nitric-hydrochloric acid mixtures provides complete recovery for most metals; aqua regia reflux extraction is prescribed by ISO 17318 and GB/T 23349 for hazardous elements in fertilizers; and alkaline dry ashing with calcium hydroxide or calcium oxide fixative is used for boron to prevent volatilization losses. Water-soluble fractions are obtained by shaking a weighed portion with water at a specified ratio, followed by filtration before instrumental measurement.

ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry)

ICP-OES is the workhorse technique for multi-element micronutrient determination, offering simultaneous measurement of Fe, Mn, Zn, Cu, B, Mo, and heavy metals across concentration ranges from mg/kg to percent levels. Radial viewing tolerates the high dissolved-solids matrices typical of potash and NPK fertilizers, while axial viewing improves sensitivity for trace-level contaminants. Spectral interference correction and matrix-matched calibration standards are applied to ensure accuracy.

ICP-MS (Inductively Coupled Plasma Mass Spectrometry)

ICP-MS delivers ultra-trace sensitivity for Cd, Pb, As, Hg, Ni, and Tl, with detection limits at sub-microgram per kilogram levels. Collision/reaction cell technology is essential for resolving the argon chloride interference on arsenic (mass 75), which is significant in chloride-rich matrices such as muriate of potash. Internal standards, dilution control, and robust plasma conditions maintain accuracy in high-salt digests.

Atomic Absorption Spectrometry (AAS)

Flame AAS provides cost-effective determination of Fe, Zn, Cu, and Mn at elevated concentrations, while graphite furnace AAS (GFAAS) achieves the low detection limits needed for trace Pb and Cd. Deuterium or Zeeman background correction compensates for matrix absorption. GB/T 14540 specifies AAS procedures for Fe, Mn, Zn, and Cu in compound fertilizers.

Hydride Generation and Cold Vapor Techniques

Arsenic is determined by hydride generation AAS or atomic fluorescence spectrometry (AFS) after pre-reduction of As(V) to As(III) with potassium iodide and ascorbic acid. Mercury is measured by cold vapor AAS or AFS following oxidative digestion with potassium permanganate and potassium persulfate to stabilize the element. AFS methods are widely adopted in GB standards for their excellent sensitivity for hydride-forming elements.

Spectrophotometric and Other Classical Methods

Boron is reliably determined by azomethine-H or curcumin spectrophotometry in accordance with GB/T 14540, particularly where ICP-OES boron sensitivity is insufficient for low-level samples. Molybdenum can be measured by thiocyanate spectrophotometry, and chloride by potentiometric titration. X-ray fluorescence (XRF) spectrometry serves as a rapid screening tool for heavy metals in incoming raw materials.

Quality Assurance and Method Validation

Analyses are performed under ISO/IEC 17025 accredited quality systems. Certified reference materials, procedural blanks, duplicate digestions, and matrix spike recoveries are included in every batch, with control limits typically set at 85-115% recovery. Calibration verification standards, internal standard recovery monitoring, and documented limits of quantification ensure that reported values are traceable, defensible, and fit for regulatory submission.

Applications

Trace element determination in fertilizers supports decision-making across the entire product lifecycle:

• Manufacturer quality control and batch release against guaranteed nutrient analyses on product labels
• Mandatory compliance testing against hazardous substance limits in GB 38400-2019 for products sold in China
• Export certification and import clearance for markets governed by the EU Fertilising Products Regulation and other national frameworks
• Verification of micronutrient claims for specialty products such as chelated micronutrients, water-soluble fertilizers, and foliar feeds
• Raw material qualification and contamination source tracing in phosphate rock, fillers, and recycled organic inputs
• Agronomic research, fertilizer formulation development, and investigation of crop deficiency or toxicity incidents in the field
• Support for product certification schemes, government supervision and random sampling programs, and supplier audits

FAQ

Why is water-soluble content tested in addition to total micronutrient content?

Plants can only absorb nutrients in soluble form, so water-soluble content reflects the immediately plant-available fraction, whereas total content includes slowly available or unavailable bound forms. Label guarantees for micronutrients in most markets are based on water-soluble values, so both determinations are usually required for a complete assessment.

Which standard specifies heavy metal limits for fertilizers in China?

GB 38400-2019 is the mandatory national standard restricting hazardous substances in fertilizers, setting maximum limits for arsenic, cadmium, lead, chromium, and mercury across different fertilizer categories, with stricter limits applied to organic fertilizer products. Corresponding determination methods are specified in GB/T 23349 and related standards.

How much sample is required for testing?

A representative sample of approximately 200 to 500 grams is typically submitted, which is dried, ground, and reduced by quartering. Individual analytical portions of 0.2 to 1 gram are used per digestion, with additional mass reserved for replicate determinations and confirmatory testing.

What detection limits can be achieved?

With ICP-MS, detection limits for Cd, Pb, Hg, and Tl are typically at the 0.001 to 0.01 mg/kg level, comfortably below regulatory thresholds. ICP-OES and GFAAS achieve practical quantification limits of approximately 0.01 to 0.1 mg/kg for most heavy metals, while micronutrients are measured accurately from sub-mg/kg to percentage levels.

Why is boron digested differently from other trace elements?

Boron forms volatile compounds during conventional acidic ashing or evaporation, leading to severe losses and falsely low results. Alkaline dry ashing with a calcium oxide or calcium hydroxide fixative retains boron in the residue, after which it can be dissolved and determined accurately by spectrophotometry or ICP-OES.

What is the typical turnaround time for trace element testing?

Standard trace element panels are generally completed within five to ten working days from sample receipt, including digestion, instrumental analysis, and quality control review. Urgent services can often be arranged, while complex panels combining multiple digestion schemes may require additional time.