Analytes Certified for in 2020

Soil Testing

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Acid-extractable Phosphorus (9G2)

This test is often referred to as BSES-P. Soil extracts are obtained by shaking milled air-dry soil at a ratio of 1:200 (w/v) with 0.005M sulfuric acid for 16 h. Method 9G1 codes the manual analytical finish of Truog and Meyer, with absorbance at 660 nm. Method 9G2 codes automated analytical finishes (continuous segmented flow and flow injection analysis) based on molybdenum-blue and a preferred absorbance of 882 nm. The methods specify reporting results as mg P/kg on an air-dry basis.
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Nitrate Nitrogen (7C2)

Initially, milled air-dry soil is extracted for 1 h with 2M KCl at a 1:10 soil/solution ratio. For 7C1a to 7C1h methods, mineral-N components are quantified by steam distillations and subsequent titrations. For 7C2 methods, mineral-N fractions in the clarified soil extract are determined by automated colorimetric procedures. Ammonium ions (NH4+) are measured by a modified Berthelot indophenol reaction, while the Griess-Ilosvay reaction is used for NO3-N (and NO2-N). The methods specify reporting results for NH4-N and NO3-N [plus NO2-N if present], respectively, as mg N/kg on an oven-dry (105oC) basis. Specifically, method 7C2a relates to an automated colour, continuous segmented flow analytical finish, while Method 7C2b codes an automated colour finish by flow injection analysis. Little difference is expected in results due to choice of colorimetric finish.

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CaCl2 extractable B - ICP (12C2)

The soil is extracted with boiling 0.01M Cacl2 solution at a 1:2 soil:solution ratio for 10 mins.  It is then quickly filtered to avoid any re-fixation of the solubilised B.  The extract is then analysed by ICP-AES.

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Bicarbonate Extractable Phosphorus - Colwell (9B1)

This popular Australian P test on milled air-dry sample is suitable for acidic, neutral and alkaline soils. The extractant is freshly prepared 0.5M sodium bicarbonate @ pH 8.5. The wide soil/extractant ratio of 1:100 and an extended shaking time of 16 h favours readily available and more slowly available forms of soil P, while suppressing the solubility of basic calcium phosphates often found in neutral and alkaline soils. Method 9B1 describes a manual, molybdenum-blue colorimetric procedure with a preferred absorbance at 882 nm, whereas Method 9B2 refers to the same initial soil extraction, followed by an equivalent automated molybdenum-blue colorimetric finish (continuous segmented flow or flow injection analysis). The methods specify reporting results as mg P/kg on an air-dry basis.

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Electrical conductivity (EC) of 1:5 soil/water extract (3A1)

This test on milled air-dry sample at a soil/water ratio of 1:5 for 1 h is suitable for use on all soils, irrespective of whether acidic or alkaline. It usually underestimates the soluble salt status of soils containing natural or added gypsum, particularly if ³ 1% of gypsum is present. Such soils would have an EC of about 2 dS/m. Soil EC x 0.336 (Method 3B1) approximates percent total soluble salts, while approximate soil ionic strength (Method 3C1) at 0.1 bar (I0.1) can be calculated as follows: I0.1 = [0.0446*EC1:5 – 0.000173], where I0.1 has units of mM, and EC1:5 has units of dS/m @ 25oC.

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Exchangeable Bases - Ammonium Chloride (15A1)

Method for measuring exchangeable bases (Ca2+, Mg2+, Na+, K+) - 1M ammonium chloride at pH 7.0

 

Suited for use on all soils, irrespective of whether acidic or alkaline, but preferred on acidic to weakly alkaline soils not dominated by solid-phase carbonates. Method 15A1 has no pre-treatment to remove soluble salts, with alternatives to remove them chemically (15A2) or to adjust for the presence of soluble sodium (15A3).

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DTPA Trace Elements (12A1)

Soils are extracted with 0.005M DTPA, at a 1:2 soil:solution ratio, and shaken for 2 hr at 25oC.  Analytical finish is either ICP-AES or Flame AAS.

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Colwell extractable K (18A1)

The Colwell extraction was developed for measuring available P, but can also be used for determining available K. The extractant is freshly prepared 0.5M sodium bicarbonate @ pH 8.5, with a soil:extractant ratio of 1:100 and a shaking time of 16 hr.  The sodium cation is considered the primary extractor, displacing the similar potassium cation from exchange sites.

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KCl-ext.S (10D1)

Empirical extraction at 40oC in 0.25M KCl, for 3 hours.  Total S in the extract is measured by ICP-AES.

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Ca(H2PO4)2 extractable S (10B3)

Sulfate sulphur is extracted in the absence of activated charcoal from air-dry soil <2 mm particle size, by 0.01M Ca (H2PO4)2 at pH 4.0 using a soil/solution ratio of 1:5 and an extraction time of 17 h at 25 degrees C. This extracted sulphur is then determined in an aliquot of particle-free soil extract by ICP-AES.

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Mehlich3 ICP (18F1)

Analysis of Mehlich 3 extract using inductively-coupled atomic emission spectroscopy (ICP-AES)

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Water soluble nitrate - automated colour (7B1)

This method uses the same 1:5 soil/water suspension described for method 3A1. The filtered or centrifuged aliquot is subjected to automated colorimetric analysis based on the Griess-Ilosvay reaction, either by continuous segmented flow analysis (sub-method 7B1a) or by flow injection analysis (sub-method 7B1b) The methods specify reporting nitrate-N (mg N/kg) on an air-dry basis. Note that in some highly weathered soils with a measurable anion exchange capacity, water may not extract all of the adsorbed nitrate-N.
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Extractable Phosphorus - Olsen (9C1)

This extractable P test on milled air-dry sample uses freshly prepared 0.5M sodium bicarbonate @ pH 8.5 as the extractant. The soil/extractant ratio of 1:20 and a relatively short extraction time of 30 min favours readily available forms of soil P, while suppressing the solubility of basic calcium phosphates often found in neutral and alkaline soils. Method 9C1 involves a manual, molybdenum-blue colorimetric finish with a preferred absorbance at 882 nm. Method 9C2a defines an equivalent automated molybdenum-blue continuous segmented flow analytical finish, while Method 9C2b codes an automated colour finish by flow injection analysis. No significant difference is expected in results due to the analytical finishes described. The methods specify reporting results as mg P/kg on an air-dry basis.
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Organic carbon - W & B (6A1)

This measure of soil organic carbon (OCW&B; expressed as %C) usually yields a lower figure than the true total organic carbon value. The method uses finely-milled air-dry sample. It involves wet oxidation by a dichromate-sulphuric acid mixture and relies only on heat of reaction. Soil weight should take account of the expected concentration of OC, and it is expected that allowance will be made for positive soluble Cl‑ interference in soils containing >0.5% Cl. The method specifies reporting on an oven-dry (105oC) basis. Nowadays this method is less preferred than 6B methods.

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PBI - Unadjusted (9I4A)

This index of soil P sorption embraces adsorption as well as precipitation reactions. Equilibrated soil extracts are obtained by shaking milled air-dry soil continuously for 17 h at a ratio of 1:10 (w/v) with a P equilibrating solution initially containing the equivalent of 1000 mg P/kg in 0.01M CaCl2. The residual P in the extract is determined using the Murphy and Riley colorimetric finish.  In other related methods (9I2a, 9I3a) the PBI value is modified to allow for the soil fertility level.  In this method, there is nosuch adjustment. The methods specify reporting results on an air-dry basis.

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pH: 1:5 SOIL:0.01M CaCL2 Suspension (4B2)

This pH test on milled air-dry sample is suitable for use on all soils, irrespective of whether acidic or alkaline. Values are usually unaffected by fertilisation prior to sampling, as changes to the soil’s ionic strength is masked by the calcium chloride. Code 4B1 indicates direct use of 0.01M CaCl2, at a soil/solution ratio of 1:5, with mechanical shaking for 1 h prior to pH measurement using calibrated electrodes positioned in the unstirred supernatant after settling of the suspension. Code 4B2 provides a similar measurement outcome but relies on the addition of 0.21M CaCl2 to a 1:5 soil/water suspension to achieve 0.01M CaCl2 prior to measurement of pH as for 4B1.

Codes 4B3 and 4B4 are identical to 4B1 and 4B2, respectively, except the soil/CaCl2 suspensions are stirred during measurement. Method 4B5 codes for an MIR surrogate measurement. There is merit in separate use of both water and calcium chloride to measure soil pH.

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pH of 1:5 soil/water suspension (4A1)

This test on milled air-dry sample involves mechanical shaking with deionised water in a closed system for 1 h at a soil/water ratio of 1:5 prior to pH measurement using calibrated electrodes, while stirring the soil/water suspension. The method is suitable for use on all soils, irrespective of whether acidic or alkaline. Values may be lower than expected on recently fertilised soils due to a temporary increase in soil solution ionic strength.
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Total organic carbon - high frequency induction furnace, volumetric (6B2)

This method for total soil OC involves production, purification and measurement of CO2 evolved when soil carbon is ignited in a stream of O2. Because all C compounds are converted to CO2, the C from carbonates, charcoal, undecomposed wood, etc, will be included, as no soil pre-treatment is specified. In the volumetric sub-method 6B2a, concentrated KOH solution is used to absorb the CO2 released. The difference between the original volume of gas in the burette and the volume produced after ignition equals the volume of CO2 evolved from the sample, after correction for gas temperature and pressure. Sub-method 6B2b is similar to Method 6B2a, except the CO2 produced by ignition is measured via infrared / thermal conductivity detection. Both 6B2a and 6B2b use finely-milled air-dry sample, with weights varying with expected C concentrations. Surrogate estimates can be obtained by NIR (method 6B4a) or MIR (method 6B4b) reflectance spectroscopy. The methods specify reporting as %C on an oven-dry (105oC) basis.

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Total soil nitrogen semi micro Kjeldahl, steam distillation. (7A1)

Method 7A1 provides a good estimate of total soil N in soils with little NO3: otherwise slightly lower apparent total soil N concentrations are likely. The method initially involves semi-micro Kjeldahl digestion. Subsequently, NaOH solution is added to release NH3 by steam distillation. The NH3 released is quantitatively absorbed in dilute boric acid, with total soil N determined by titration with standard acid. The method uses finely-milled air-dry sample and specifies reporting results as %N on an oven-dry (105oC) basis.

 

 

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Not Specified or mixed (NS)

The laboratory either did not specify the method used or uses a mix of methods when submitting to the certification body

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Chloride - 1:5 soil/water extract (5A1)

Tests for water-soluble chloride (Cl) on milled air-dry sample are suitable for use on all soils. For method 5A1, Cl‑ in clarified 1:5 soil/water extracts is determined by potentiometric titration with AgNO3 in conjunction with an Ag/AgNO3 electrode array. For method 5A2a, Cl‑ in clarified 1:5 soil/water extracts is determined by an automated, continuous flow colorimetric procedure based on the formation — in the presence of ferric ions and free thiocyanate ions — of highly coloured ferric thiocyanate in proportion to the Cl‑ concentration. Method 5A2b is similar, except it pertains to the use of flow injection analysis (FIA). For 5A1 and 5A2 methods, it is assumed there are no chemical interferences of significance. Moreover, Method 5A2a has proven more precise than method 5A1, particularly at soil concentrations <50 mg Cl/kg. Other analytical finish options involve chemically-suppressed ion chromatography (5A3a), single-column electronically suppressed ion chromatography (5A3b), and direct measurement by ICPAES (Method 5A4). The methodology specifies reporting results on an air-dry basis.

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DE-23

Microwave Digestion - closed system without HF, and final medium HNO3 and/or HCl. ICP-AES finish.

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BB-31

Extraction with water. Spectrophotometric (automated) finish.

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CA-37

Finely divided dry sample.  Dumas combustion.