Bioavailable Element Analysis and Determination: Methods, Standards, and Applications

Bioavailable element analysis, also known as available element determination, measures the plant-available and environmentally mobile fractions of nutrients and potentially toxic elements in soils, sediments, and fertilizers rather than their total concentrations. By combining selective chemical extractants such as DTPA, EDTA, Mehlich-3, and Olsen solutions with instrumental techniques like ICP-OES and ICP-MS, this testing provides the scientific basis for precision fertilization, soil fertility evaluation, and risk-based contaminated land management. This article presents a comprehensive technical overview of available element analysis, covering sample types, testing items, extraction and determination methods, quality control, and typical applications.

Technical Overview

Available element analysis determines the fraction of an element in a solid matrix that can be released into soil solution, taken up by plant roots, or absorbed by organisms under natural environmental conditions. The total concentration of an element, obtained through total digestion with acids such as hydrofluoric acid or aqua regia, provides limited information about its actual agronomic or toxicological effectiveness, because most elements exist in mineral lattices, insoluble precipitates, and strongly adsorbed forms that are not biologically accessible.

The "available" fraction is an operationally defined quantity: it is the amount of an element extracted by a specified reagent under controlled conditions of concentration, pH, temperature, extraction time, and soil-to-solution ratio. Single extraction methods, such as DTPA, EDTA, Mehlich-3, ammonium acetate, and sodium bicarbonate (Olsen) solutions, simulate rhizosphere acidification, chelation, and ion-exchange processes to estimate short-term availability. Sequential extraction schemes, such as the Tessier five-step procedure and the European BCR three-step protocol, further divide the total pool into water-soluble, exchangeable, carbonate-bound, iron-manganese oxide-bound, organic matter-bound, and residual fractions, enabling a deeper understanding of element mobility, transformation, and long-term release potential.

The availability of an element is governed by soil pH, redox potential, organic matter content, cation exchange capacity, texture, carbonate content, and microbial activity. Consequently, the selection of an appropriate extraction method must consider soil properties and the intended purpose of the analysis, and results from different extraction procedures are not directly interchangeable. Accredited third-party laboratories follow validated national and international standards, including the ISO 17402 guidance on bioavailability assessment and Chinese national and industry standards such as NY/T 890, NY/T 889, the NY/T 1121 series, and HJ 804, to ensure that results are accurate, reproducible, and comparable.

Testing Samples

Agricultural Soils

Representative soil samples from cultivated fields, greenhouses, orchards, vegetable bases, and grassland are the most common sample type. Samples are typically air-dried at room temperature, gently crushed, and passed through a 2 mm nylon sieve to remove stones and root debris while avoiding contamination from metal tools. For redox-sensitive parameters such as mineral nitrogen in paddy soils, fresh samples refrigerated and analyzed promptly are required.

Contaminated Site Soils and Sediments

Soils from industrial sites, mining areas, former factory lands, and agricultural plots affected by irrigation or atmospheric deposition, as well as river, lake, and marine sediments and dredged materials, are analyzed to assess the mobile and bioavailable fractions of potentially toxic elements for human health and ecological risk assessment and for the evaluation of remediation effectiveness.

Fertilizers and Soil Conditioners

Water-soluble fertilizers, micronutrient fertilizers, chelated trace element products, controlled-release and slow-release fertilizers, organic fertilizers, composts, biochars, and soil amendments are tested for water-soluble, chelated, and citrate-soluble nutrient fractions, which define the actual nutritional effectiveness of the product rather than its label-only total nutrient content.

Organic Wastes and Growing Media

Livestock and poultry manures, sewage sludge, biogas residues, mushroom residues, and commercial growing substrates are analyzed to determine available nutrient supply capacity and the leachable or phytoavailable fractions of regulated heavy metals.

Geological and Geochemical Survey Samples

Surface soil and sediment samples collected during land quality geochemical surveys and multi-purpose geochemical mapping programs are analyzed for available nutrient and element fractions to support land resource evaluation, agricultural zoning, and environmental baseline studies.

Testing Items

Available element analysis covers nutrient elements, potentially toxic elements, and elemental fractionation, with the main testing items summarized below.

• Available macronutrients: alkaline hydrolyzable nitrogen, ammonium nitrogen, nitrate nitrogen, available phosphorus (Olsen-P, Bray-P), available potassium, slowly available potassium, exchangeable calcium, magnesium, and sodium, available sulfur, and available silicon.
• Available micronutrients: DTPA- or EDTA-extractable iron, manganese, copper, and zinc; hot-water-soluble or calcium chloride-extractable boron; ammonium oxalate-extractable molybdenum; and water-soluble chlorine.
• Bioavailable potentially toxic elements: DTPA-, EDTA-, or dilute calcium chloride-extractable cadmium, lead, chromium, nickel, cobalt, copper, zinc, arsenic, mercury, and thallium for risk screening and passivation evaluation.
• Elemental fractionation: water-soluble and exchangeable, carbonate-bound, iron-manganese oxide-bound, organic matter and sulfide-bound, and residual fractions determined by BCR or Tessier sequential extraction.
• Fertilizer availability parameters: water-soluble nutrient content, chelated trace element content, water-insoluble residue, citrate-soluble phosphorus, and nutrient release rate of controlled-release products.

Testing Methods

Single Extraction Procedures

The extraction reagent is selected according to soil properties, target elements, and the intended interpretation of results. Commonly applied procedures include the following.

• DTPA-TEA-CaCl2 solution at pH 7.3 for extractable iron, manganese, copper, zinc, cadmium, lead, and nickel in near-neutral and calcareous soils, as specified in NY/T 890-2004 and HJ 804-2016.
• Mehlich-1 (dilute hydrochloric and sulfuric acid) and Mehlich-3 (acetic acid, ammonium nitrate, ammonium fluoride, and EDTA) multi-element extractions for acidic to neutral soils, allowing simultaneous determination of phosphorus, potassium, calcium, magnesium, and micronutrients.
• 0.5 mol/L sodium bicarbonate solution at pH 8.5 (Olsen method) for available phosphorus in neutral and calcareous soils, and dilute ammonium fluoride-hydrochloric acid (Bray-1) for acidic soils.
• 1 mol/L neutral ammonium acetate extraction for exchangeable and available potassium, calcium, magnesium, and sodium, as specified in NY/T 889-2004, with slowly available potassium determined by boiling nitric acid extraction.
• Alkaline hydrolysis diffusion for hydrolyzable nitrogen, and potassium chloride or dilute calcium chloride extraction for mineral nitrogen (ammonium and nitrate).
• Hot water or boiling dilute calcium chloride extraction for available boron, and acid ammonium oxalate extraction at pH 3.3 for available molybdenum.
• 0.01 mol/L calcium chloride extraction as a weak, plant-relevant estimator of the unbuffered mobile fraction of trace elements and heavy metals.

Sequential Extraction Procedures

The BCR three-step protocol extracts the acid-soluble (exchangeable and carbonate-bound), reducible (iron-manganese oxide-bound), and oxidizable (organic matter and sulfide-bound) fractions in sequence, with the residual fraction obtained by residual digestion. The Tessier scheme provides a five-step separation of the exchangeable, carbonate-bound, iron-manganese oxide-bound, organic-bound, and residual forms. Fractionation data quantify element mobility, transformation pathways, and long-term release risk.

Instrumental Determination

Extracts are determined by techniques matched to the target elements and required detection limits: inductively coupled plasma optical emission spectrometry (ICP-OES) for multi-element nutrient determination, consistent with ISO 22036; inductively coupled plasma mass spectrometry (ICP-MS) for trace and ultra-trace elements such as cadmium and thallium; flame and graphite furnace atomic absorption spectrometry (FAAS and GFAAS) for single elements; molybdenum-blue spectrophotometry for phosphorus; flame photometry and ion chromatography for potassium, ammonium, and nitrate; and hydride generation atomic fluorescence spectrometry (HG-AFS) for arsenic, selenium, and mercury.

Quality Assurance and Quality Control

Validated methods are supported by procedural blanks, duplicate extractions with relative deviation control, matrix spikes with recovery typically within 85 to 115 percent, multi-point calibration with certified standard solutions, method detection limit verification, and the use of certified reference materials for available-form analysis where available. Sample preparation uses non-contaminating tools and nylon sieves, and extraction conditions including oscillation speed, time, and temperature are strictly controlled to guarantee inter-laboratory comparability.

Applications

Available element analysis is applied across agriculture, environmental management, and product quality control.

• Precision fertilization: soil test-based fertilizer recommendations for nitrogen, phosphorus, potassium, and micronutrients that reduce over-fertilization, prevent trace element deficiency, and improve crop yield and quality.
• Soil fertility and land quality evaluation: classification of nutrient supply capacity for cultivated land quality grading, high-standard farmland construction, and land quality geochemical surveys.
• Environmental risk assessment: estimation of the phytoavailable and leachable fractions of cadmium, lead, arsenic, and other potentially toxic elements for human health and ecological risk screening under bioavailability-based approaches.
• Remediation monitoring: verification of the effectiveness of immobilization and stabilization remedies, where a reduction in the DTPA- or EDTA-extractable fraction demonstrates successful passivation of heavy metals.
• Fertilizer product development: characterization of water-soluble, chelated, and slow-release nutrient fractions for product formulation, registration testing, and quality control.
• Scientific research: studies of rhizosphere chemistry, element transformation under flooding or liming, long-term field trials, and soil carbon-nutrient interactions.

FAQ

What is the difference between total and available element content?

Total content is the entire amount of an element in the sample obtained by complete digestion, while available content is the operationally defined fraction extracted by a specified reagent that approximates what plants or organisms can actually absorb. A soil may have high total cadmium but a low DTPA-extractable fraction, meaning low immediate environmental risk, which is why availability data are essential for realistic agronomic and risk decisions.

How should samples be collected and preserved?

Collect representative samples using stainless steel or plastic tools, typically 500 g to 1 kg per composite sample. For most available nutrient and trace element analyses, air-dry the sample at room temperature, crush gently, and pass it through a 2 mm nylon sieve. For mineral nitrogen and redox-sensitive parameters, keep samples refrigerated in sealed bags and deliver them quickly, because drying alters these fractions.

Which extraction method should be selected?

The choice depends mainly on soil pH and analytical purpose: DTPA suits near-neutral and calcareous soils for micronutrients and heavy metals, Mehlich-1 and Bray-1 suit acidic soils, Olsen suits available phosphorus in neutral and calcareous soils, and ammonium acetate is standard for exchangeable cations. Accredited third-party laboratories can recommend the most appropriate method based on sample background and data use.

Are results from different extraction methods comparable?

No. Available element content is an operational definition tied to the specific extractant and conditions, so results obtained by different methods cannot be directly compared. The test report must always state the extraction method, and long-term monitoring programs should keep the method consistent to maintain data continuity.

What is the typical turnaround time?

Single extraction analyses with ICP-OES or ICP-MS determination are generally completed within five to seven working days after sample receipt. Sequential extraction schemes and low-level trace determinations requiring ICP-MS or HG-AFS may require additional time because of multiple extraction stages and stricter contamination control.

Can available element analysis support regulatory compliance?

Yes. Bioavailability-based results are increasingly used to refine risk assessments of agricultural and contaminated soils, to demonstrate the effectiveness of stabilization remedies, and to verify fertilizer nutrient claims, provided the analysis is performed by accredited third-party laboratories using validated standard methods with complete quality control documentation.