Analysis of Available Trace Elements (Bioavailable Micronutrients) in Soil

Available trace element analysis measures the plant-usable fraction of micronutrients such as iron, manganese, copper, zinc, boron, and molybdenum in soil, providing the scientific basis for precise fertilization and crop deficiency diagnosis. Using standardized extraction procedures such as DTPA, hot-water, and ammonium oxalate methods combined with AAS, ICP-OES, or ICP-MS determination, accredited third-party laboratories deliver reliable data for agriculture, horticulture, and environmental assessment. This article explains the principles, sample requirements, test items, methods, applications, and frequently asked questions of soil available trace element analysis.

Technical Overview

Available trace elements, also called bioavailable or plant-available micronutrients, are the portion of iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), and molybdenum (Mo) held in the soil solution and in weakly bound, exchangeable, complexed, and easily reducible pools that plant roots can take up during a growing season. Unlike total elemental content, the available fraction is governed by soil pH, organic matter, calcium carbonate content, redox potential, texture, moisture regime, and microbial activity. A soil may contain abundant total zinc or iron and still induce deficiency symptoms in crops because the elements are fixed in unavailable mineral or oxide forms.

Because total concentration is a poor predictor of plant response, soil testing practice worldwide uses selective chemical extraction: a buffered or mildly acidic extractant simulates the solubilizing action of plant roots, and the extracted metals are then quantified by atomic absorption spectrometry (AAS), inductively coupled plasma optical emission spectrometry (ICP-OES), or inductively coupled plasma mass spectrometry (ICP-MS). Established procedures such as ISO 14870 (buffered DTPA extraction), NY/T 890 (DTPA extraction for Zn, Mn, Fe, and Cu), and the NY/T 1121 series for available boron and molybdenum provide a standardized, reproducible basis for fertilizer recommendations, deficiency and toxicity diagnosis, soil fertility grading, and environmental risk evaluation. Results are conventionally reported in mg/kg on an air-dried soil basis.

Testing Samples

Agricultural and Greenhouse Soils

Composite topsoil samples (typically 0-20 cm plow layer) from cropland, paddy fields, and vegetable greenhouses are the most common submission type. Sampling is best performed after harvest and before basal fertilization. A representative composite of 15-20 subsamples collected in a zigzag or S-shaped pattern is homogenized, quartered, and reduced to approximately 1 kg. Fertilizer bands, field edges, manure heaps, and wet spots should be avoided to prevent bias.

Orchard, Vineyard, and Horticultural Soils

For perennial fruit trees, tea gardens, and vineyards, samples are taken from the effective root zone, often 0-40 cm, and frequently from the drip-irrigation or fertilization zone. Calcareous orchard soils are prone to iron and zinc deficiency, making available micronutrient monitoring especially valuable for guiding foliar and soil corrections.

Forest, Grassland, and Ecological Monitoring Soils

Forest floor and mineral horizon samples from long-term ecological monitoring plots, nature reserves, and grassland restoration projects support nutrient cycling studies and site fertility evaluation. Sampling depth and horizon designation should follow the project monitoring protocol.

Contaminated, Reclaimed, and Amended Soils

Soils from industrial sites, mining areas, and fields receiving compost, biosolids, or other amendments can be tested to assess both the plant-available nutrient supply and the phytoavailable fraction of potentially toxic elements, complementing total-content risk screening under frameworks such as GB 36600-2018.

Testing Items

The core testing package for soil available trace elements typically includes the following items, with supporting parameters that strongly influence availability:

• DTPA-extractable (available) iron, manganese, copper, and zinc
• Hot-water-soluble (available) boron
• Ammonium oxalate-extractable (available) molybdenum
• Optional single-fraction tests such as water-soluble or exchangeable Mn, Fe, Cu, and Zn
• Supporting parameters: soil pH, organic matter, cation exchange capacity, calcium carbonate equivalent, electrical conductivity, and particle-size distribution, which are essential for interpreting availability results

Testing Methods

Sample Preparation

Samples are air-dried at room temperature (below 40 °C), gently crushed with a wooden or plastic roller to break aggregates, and passed through a 2 mm nylon sieve. To avoid cross-contamination of the target metals, non-metallic sampling tools (bamboo, plastic, or wooden scoops) should be used in the field, and galvanized, brass, or rusted tools must be avoided. Prepared samples are stored in clean polyethylene containers in a cool, dry location with full chain-of-custody labeling.

DTPA Extraction for Available Fe, Mn, Cu, and Zn

The DTPA method described in ISO 14870 and NY/T 890 is the internationally recognized procedure for neutral and calcareous soils (pH above approximately 6.5). A 10.00 g portion of air-dried soil is shaken for 2 hours at 20-25 °C with 20 mL of extractant containing 0.005 mol/L DTPA, 0.01 mol/L calcium chloride, and 0.1 mol/L triethanolamine (TEA), buffered at pH 7.3. The suspension is filtered or centrifuged, and Fe, Mn, Cu, and Zn in the clear extract are determined by flame AAS or, more efficiently, simultaneously by ICP-OES. The buffered extractant chelates the labile metals while the CaCl2 suppresses dissolution of carbonates, ensuring the measurement reflects the plant-available pool rather than total dissolution.

Hot-Water Extraction for Available Boron

Available boron is extracted with boiling deionized water according to the NY/T 1121 series, typically refluxing soil with water at a 1:2 ratio for 5-10 minutes. After cooling and filtration, boron is determined colorimetrically with azomethine-H at 420-430 nm or directly by ICP-OES. Because ordinary borosilicate glassware can release boron, quartz or boron-free apparatus and high-purity reagents are mandatory throughout the procedure.

Ammonium Oxalate Extraction for Available Molybdenum

Available molybdenum is extracted with an acidified ammonium oxalate-ammonium oxalate (Tamm) buffer at pH 3.3, using a 1:10 soil-to-solution ratio and an extended shaking period, following the NY/T 1121 series. Molybdenum in the extract is quantified by catalytic polarography or, increasingly, by ICP-MS or ICP-OES, which offer lower detection limits and simpler operation. The oxalate complex preserves the molybdate adsorbed on iron and aluminum oxides that represents the plant-responsive pool.

Alternative Multi-Element Extractants

For acidic soils, 0.1 mol/L hydrochloric acid (Mehlich-1), Mehlich-3 (a multi-acid solution containing EDTA and ammonium fluoride), 0.05 mol/L EDTA, or ammonium bicarbonate-DTPA (AB-DTPA) may be more appropriate and can extract macro- and micronutrients in a single step. The choice of extractant must match soil properties and locally calibrated interpretation thresholds, because concentrations obtained by different procedures are not directly interchangeable.

Instrumental Determination and Quality Control

Instrumental determination is performed by flame AAS for single elements or by ICP-OES/ICP-MS for simultaneous multi-element measurement, with typical method detection limits of 0.005-0.05 mg/kg depending on the element and matrix. Quality control in accredited third-party laboratories includes reagent blanks, calibration verification standards, duplicate analyses with relative deviation limits (commonly within 10-20%), matrix spike recoveries of 85-115%, and analysis of certified reference soils with certified extractable values. Typical turnaround time is 5-10 working days after sample receipt.

Applications

Data from available trace element analysis translate directly into practical decisions across agriculture and environmental management:

• Soil-test-based fertilizer prescription, including micronutrient recommendations for zinc-responsive cereals and rice, iron-sensitive fruit trees on calcareous soils, and boron-sensitive crops such as oilseed rape, cotton, citrus, and apple
• Diagnosis of latent deficiency (hidden hunger) before visible symptoms appear, and confirmation of suspected toxicity or imbalance
• Precision agriculture and variable-rate application, using gridded field sampling to map spatial variability of available micronutrients
• Nutrient monitoring in greenhouses and intensive horticulture, where repeated cropping and high nutrient demand rapidly deplete the available pool
• Environmental risk assessment of contaminated or amended soils by comparing the plant-available fraction with total-content screening values, helping evaluate realistic phytoavailability rather than worst-case totals
• Land reclamation, mine restoration, and evaluation of compost, biochar, or biosolid amendments for their effect on micronutrient supply
• Scientific research on soil chemistry, nutrient cycling, long-term field trials, and soil quality indicator systems

FAQ

Q: What is the difference between available and total trace element analysis?

A: Total analysis uses complete digestion (for example, hydrofluoric acid or aqua regia procedures such as HJ 803) and measures the entire elemental reservoir, while available analysis uses a mild selective extractant that measures only the fraction plants can realistically absorb. The two are complementary: total content indicates the long-term reserve and contamination status, whereas available content predicts short-term crop response and fertilization need.

Q: Which extractant is suitable for my soil?

A: The DTPA method is designed for neutral and calcareous soils, while acidic soils are usually tested with Mehlich-1, Mehlich-3, or dilute acid extractants. Boron is extracted with hot water and molybdenum with ammonium oxalate regardless of soil pH. Laboratories select the procedure based on soil pH and regional calibration databases.

Q: How should results be interpreted?

A: Indicative critical levels for deficiency commonly cited include DTPA-Zn below about 0.5 mg/kg, DTPA-Fe below 2.5-4.5 mg/kg, DTPA-Mn below 1-5 mg/kg, DTPA-Cu below 0.1-0.2 mg/kg, hot-water B below 0.25-0.5 mg/kg, and available Mo below 0.10-0.15 mg/kg. However, thresholds vary with crop species, yield target, climate, and soil type, so interpretation should always use locally calibrated values or agronomic expert judgment.

Q: How are samples collected and how long can they be stored?

A: Collect 15-20 subsamples from the plow layer in a zigzag pattern using clean non-metallic tools, composite them, and submit about 1 kg in a clean polyethylene bag with information on location, GPS coordinates, depth, land use, and sampling date. Correctly air-dried and sieved samples are generally stable for several months when kept cool, dry, and sealed.

Q: Can one extraction measure all micronutrients at once?

A: DTPA and Mehlich-3 extracts cover Fe, Mn, Cu, and Zn simultaneously, and Mehlich-3 also covers macronutrients. Boron and molybdenum, however, require their own dedicated extraction chemistry, so a complete micronutrient assessment normally involves two or three separate procedures.

Q: Why is pH tested alongside available micronutrients?

A: Soil pH is the single most influential factor controlling micronutrient availability: iron, manganese, zinc, copper, and boron availability decreases sharply as pH rises in calcareous soils, while molybdenum availability increases. Reporting pH, organic matter, and related parameters together allows meaningful agronomic interpretation and correct fertilization advice.