SOIL METHOD CODE EXPLAINER

Transcription

1 SOIL METHOD CODE EXPLAINER Summary details of Rayment and Lyons (2011) methods and method codes commonly included in ASPAC Inter-laboratory Proficiency Programs for soils. Refer to Rayment and Lyons (2011) for more details on these and other methods. Note that Rayment and Lyons (2011) supersedes the previously used text of Rayment and Higginson (1992). References to these publications are: Rayment, G.E. and Higginson, F.R. (1992). Australian Laboratory Handbook of Soil and Water Chemical Methods. 330 pp. Reed International Books Australia P/L, trading as Inkata Press, Port Melbourne. Rayment, G.E. and Lyons, D.J. (2011). Soil Chemical Methods Australasia pp. CSIRO Publishing, Melbourne. 2A1 3A1 Air dry moisture content Electrical conductivity (EC) of 1:5 soil/water extract 4A ph of 1:5 soil/water suspension 4B ph of 1:5 soil/0.01m calcium chloride extract 5A Chloride - 1:5 soil/water extract This method is required to adjust soil chemical results based on air-dry samples to an oven-dry (105 o C) basis. When the air-dry moisture content (M%) is known, the correction from air-dry to oven-dry is as follows: Oven-dry result = [Air-dry result x (100 + M%)] 100 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 1% of gypsum is present. Such soils would have an EC of about 2 ds/m. Soil EC x (Method 3B1) approximates percent total soluble salts, while approximate soil ionic strength (Method 3C1) at 0.1 bar (I 0.1 ) can be calculated as follows: I 0.1 = [0.0446*EC 1: ], where I 0.1 has units of mm, and EC 1:5 has units of 25 o C. 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. 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 CaCl 2, 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 CaCl 2 to a 1:5 soil/water suspension to achieve 0.01M CaCl 2 prior to measurement of ph as for 4B1. Codes 4B3 and 4B4 are identical to 4B1 and 4B2, respectively, except the soil/cacl 2 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. 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 AgNO 3 in conjunction with an Ag/AgNO 3 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 Soil Method Code Explainer 2010 GER.doc Page 1 of 6

2 6A1 Organic carbon - W & B 6B1 6B2 Total organic carbon - Heanes wet oxidation Total organic carbon - high frequency induction furnace (no soil pre-treatment) 6B3 Total carbon - high frequency induction furnace (with prior physical removal of charcoal and chemical removal of carbonates) 6G1 7A1 7A2 Soil organic matter by losson-ignition Total soil nitrogen - semi micro Kjeldahl, steam distillation. Total soil nitrogen semi micro Kjeldahl, <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. This measure of soil organic carbon (OC W&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 dichromatesulphuric 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 (105 o C) basis. Nowadays this method is less preferred than 6B methods. The chemical basis of this procedure is similar to that of method 6A1, except that external heating on a hot-plate digestor is included. Expensive apparatus is not required, while interference from carbonate is negligible. Correction for positive Cl - interference in saline soils (>0.5% Cl) is recommended. The method uses finely-milled air-dry sample, with weights varying with expected C concentrations. The method specifies reporting as %C on an oven-dry (105 o C) basis. This method for total soil OC involves production, purification and measurement of CO 2 evolved when soil carbon is ignited in a stream of O 2. Because all C compounds are converted to CO 2, 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 CO 2 released. The difference between the original volume of gas in the burette and the volume produced after ignition equals the volume of CO 2 evolved from the sample, after correction for gas temperature and pressure. Sub-method 6B2b is similar to Method 6B2a, except the CO 2 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 (105 o C) basis. Following quantitative action / pre-treatment to account for or to physically remove (if present) charcoal and to chemically remove carbonate with excess 5% H 2 SO 3 solution on a hot plate in a fume cabinet, the residual, re-dried soil sample is analysed for soil C by a suitable method, preferably Method 6B2b. The method involving carbonate removal and soil C analysis uses finely-milled air-dry sample, with weights varying with expected C concentrations. The method specifies reporting as %C on an oven-dry (105 o C) basis. This simple test involves the ignition of finely-milled air-dry sample initially to 105 o C and then to 550 o C. One hundred times the difference in sample weight in grams between these two temperatures, i.e, [100(Weight 105C Weight 550C )] = Loss on ignition 550C (LOI 550C ), which is assumed to approximate % Organic Matter. Method 7A1 provides a good estimate of total soil N in soils with little NO 3 : 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 NH 3 by steam distillation. The NH 3 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 (105 o C) basis. Method 7A2 is similar to Method 7A1, except that total N in the Kjeldahl digest is quantified by automated colorimetric procedures based on the Berthelot indophenol reaction. Method 7A2a relates to an automated colour, continuous Page 2 of 6

3 automated colour finish 7A5 Total soil N Dumas hightemperature combustion 7B1 7C 9A 9B Water soluble nitrate - automated colour finish Mineral nitrogen with 2M KCl - steam distillation and automated colour finishes Total Bicarbonateextractable (Colwell) segmented flow analytical finish, while Method 7A2b codes an automated colour finish by flow injection analysis. No significant difference is expected in results due to the choice of colorimetric finish. The test commence with finelymilled air-dry sample, while both methods specify reporting results as %N on an oven-dry (105 o C) basis. This method utilises automated and/or microprocessor controlled instrumentation, which mostly is able to measure at least total C, N and S in the same sample. Dumas-N dry oxidation includes all forms of soil N, without the need for lengthy pre-treatments, although results can occasionally differ from those expected in soils with high levels of fixed ammonium-n (lower results) and when soils are organically rich (high results, due to incomplete combustion resulting in the formation of methane rather than CO 2 ). Typically, dry, finely-ground sample is subjected to high-temperature combustion (e.g ,250 o C) in a stream of purified O 2. An aliquot of the gases produced by combustion is carried by helium gas to a thermal conductivity cell for measurement of any N 2 generated, a process typically taking 3 5 min. A heated copper catalyst reduces NO x to N 2. Surrogate estimates can be obtained by NIR (method 7A6a) or MIR (method 7A6b) reflectance spectroscopy. The method specifies reporting results as %N on an oven-dry (105 o C) basis. 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 (submethod 7B1b) The methods specify reporting nitrate-n (mg N/kg) on an airdry basis. Note that in some highly weathered soils with a measurable anion exchange capacity, water may not extract all of the adsorbed nitrate-n. 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 (NH 4 + ) are measured by a modified Berthelot indophenol reaction, while the Griess-Ilosvay reaction is used for NO 3 -N (and NO 2 -N). The methods specify reporting results for NH 4 -N and NO 3 -N [plus NO 2 -N if present], respectively, as mg N/kg on an oven-dry (105 o C) 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. Method 9A1 is based on X-ray fluorescence, which is non-destructive. It uses finely-milled air-dry sample, subsequently pelleted with boric acid at high pressure, followed by X-ray fluorescence analysis calibrated with P-dosed silica standards prepared as dry, pressed-powder pellets. Total soil P by sodium carbonate fusion (method 9A2) is a destructive procedure that relies on complete fusion of finely-milled air-dry sample with sodium carbonate (Na 2 CO 3 ), followed by a manual colorimetric finish using molybdenum-blue absorbance at 882 nm. The methods specify reporting results as %P on a moisture-free basis. 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 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. Page 3 of 6

4 9C Olsenextractable 9E Bray 1-9F 9G 9I2 9I3 9I4 Dilute calcium chlorideextractable Acid-extractable Phosphorus buffer index, with Colwell P (PBI +ColP ) - pooled analytical finishes Phosphorus buffer index, with Olsen P (PBI +OlsenP ) - pooled analytical finishes Phosphorus buffer index, no This extractable P test on milled air-dry sample uses freshly prepared 0.5M sodium 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. This extractable soil P test is also referred to as fluoride-extractable P. It integrates the quantity and intensity components of labile P and hence is insensitive to changes in soil buffer capacity. The extraction of milled air-dry sample is simple and quick, but the very short extraction time (60 sec) makes it prone to between-operator errors. Such errors can be lessened by rapid commencement of centrifugation or filtration after extraction. The extracting solution is 0.03M ammonium fluoride in 0.025M hydrochloric acid and the published soil/extractant ratio (w/v) is 1:7. Method 9E1 codes the manual analytical finish based on molybdenum-blue and a preferred absorbance of 882 nm, whereas method 9E2 codes automated colorimetric equivalents (continuous segmented flow and flow injection analysis). The methods specify reporting results as mg P/kg on an air-dry basis. This test, which is highly correlated with soil solution P, is based on a 1:5 soil/0.005m CaCl 2 ratio and gentle, end-over-end extraction of milled air-dry soil for 18 h. Method 9F1 codes the manual analytical finish based on molybdenum-blue, with a preferred absorbance at 882 nm, whereas method 9F2 codes the automated colorimetric equivalent. The methods specify reporting results as µg P/kg on an air-dry basis. 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. This index of soil P sorption embraces adsorption as well as precipitation reactions. In addition, it utilises the Colwell-P test on the same soil sample as a measure of current soil P fertility (PBI +ColP ). Equilibrium 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 CaCl 2. In method 9I2a, orthophosphate-p in final particulate-free supernatant solutions is analysed by a molybdenum-blue analytical finish, at a preferred absorbance of 882 nm. Methods 9I2b and 9I2c code for analytical finishes based on ICPAES and a vanadate-p colour finish, respectively. PBI +ColP is calculated as {[Ps (mg P/kg) + Colwell-P (mg/kg)] / c (mg P/L) 0.41 }, where Ps = freshly sorbed P and c = final solution P concentration. The methods specify reporting results on an air-dry basis. This index of soil P sorption is identical to Method 9I2, except that it utilises the Olsen-P test on the same soil sample as a measure of current soil P fertility (PBI +OlsenP ). In method 9I3a, orthophosphate-p in final particulatefree supernatant solutions is analysed by a molybdenum-blue analytical finish, at a preferred absorbance of 882 nm. Sub-methods 9I3b and 9I3c code for analytical finishes based on ICPAES and a vanadate-p colour finish, respectively. PBI +OlsenP is calculated as {[Ps (mg P/kg) x Olsen P (mg/kg)] / c (mg P/L) 0.41 } where Ps = freshly sorbed P and c = final solution P concentration. The methods specify reporting results on an air-dry basis. This index of soil P sorption is identical to Methods 9I2 and 9I3, except there is no adjustment for current soil P fertility. That is, neither Colwell-P nor Olsen- Page 4 of 6

5 extractable P adjustment (PBI unadj ) pooled analytical finishes P values are included in the calculation of PBI unadj. Sub-method 9I4a quantifies orthophosphate-p in final particulate-free supernatant solutions by a molybdenum-blue analytical finish, at a preferred absorbance of 882 nm. Sub-methods 9I4b and 9I4c code for analytical finishes based on ICPAES and a vanadate-p colour finish, respectively. PBI unadj is calculated as {[Ps (mg P/kg) / c (mg P/L) 0.41 }. The methods specify reporting results on an air-dry basis. 10A Total soil S Method 10A1 is based on X-ray fluorescence, which is non-destructive. It uses finely-milled air-dry sample, subsequently pelleted with boric acid at high pressure, followed by X-ray fluorescence analysis calibrated with S-dosed silica standards prepared as dry, pressed-powder pellets. Method 10A2 utilises automated and/or microprocessor controlled instrumentation, which mostly is able to measure at least total C, N and S in the same sample, inclusive of sulfate- and sulfide-s. The test is based on the detection of S, after the combustion of the sample in a stream of O 2, with the S released converted to SO 2 gas. The use of vanadium pentoxide (V 2 O 5 ) as an accelerant ensures the decomposition and release of inorganic S into the gas stream. The S released is measured quantitatively (as SO 2 ) using IR detection. The instrument is calibrated against a reference standard, with the total S reported as % S. The methods specify reporting results as %S on a moisture-free basis. 10B 10C1 12A1 12C Calcium phosphateextractable S (pooled) Potassium Chloride 40 S (KCl-40 extractable-s) DTPAextractable Fe, Cu, Mn and Zn Calcium chloride extractable B These tests all provide a measure of the extractable sulfate-s status of soils with normal to deficient levels of plant-available S. The methods included in this pooling-group are not guaranteed to extract all sulfate-s in soils containing gypsum, or in acid sulfate soils where sulfides minerals have significantly oxidised to sulfate-s. All four methods involve extraction of milled air-dry sample with 0.01M Ca(H 2 PO 4 ) 2 at ph 4.0 using a soil/solution ratio of 1:5 and an extraction time of 17 h. Specifically, method 10B1 uses manual Johnson and Nishita distillation to quantify sulfate-s in clarified soil extracts, whereas Method 10B2 codes for an automated, continuous flow Johnson and Nishita distillation finish. The remaining two analytical options involve ICPAES (Method 10B3) and ion chromatography (Method 10B4). All methods specify reporting results as mg SO 4 -S/kg on an air-dry basis. Method 10B4 gives lower results on soils containing appreciable amounts of soluble organic S, as it is specific to sulfate. This test extracts sulfate-s from soil solution, from easily exchanged and adsorbed surfaces, and from easily mineralised ester sulfates. It involves continuous equilibration of milled air-dry sample with 0.25M KCl at a soil/solution ratio (w/v) of 1:6.67 at a pre-established constant temperature of 40±0.5 o C for 3.0 h. Clarified extracts, re-equilibrated to laboratory room temperature, are best analysed by ICPAES at nm. Method 10C1 specifies reporting results as mg SO 4 -S/kg on an air-dry basis. This trace element test uses milled air-dry sample and involves extraction / equilibration for 2 h at a soil/solution ratio of 1:2 (w/v). The buffered extracting solution is 0.005M with respect to DTPA, 0.01M to CaCl 2 and 0.10M to triethanolamine (TEA), adjusted to ph 7.3±0.05 with either dilute HCl or TEA. The elements Fe, Cu, Mn and Zn in clarified extracts are analysed by ICPAES (preferred) or AAS as soon as possible after extraction. The analytical finish should have little effect on expected results. The method specifies reporting results as mg element/kg on an air-dry basis. 12C methods commence with milled air-dry sample and involve refluxing soil for 10 min with 0.01M CaCl 2 at a 1:2 soil/solution ratio (w/v). After replacing liquid lost during reflux, extracts are filtered hot, cooled and subsequently analysed for soluble B. Method 12C1 codes for measurement by an azomethine-h manual colorimetric procedure, whereas Method 12C2 codes for measurement by ICPAES at the preferred wavelength of nm. The methods specify reporting results as CaCl 2 -extractable B (mg B/kg) on an air-dry basis. Method 12C1 is appropriate when ICPAES instrumentation is unavailable. Page 5 of 6

6 15A1 15D3 15G1 17B 18F1 Exchangeable bases (Ca 2+, Mg 2+, Na +, K + ) -- 1M ammonium chloride at ph 7.0 Exchangeable bases - 1M ammonium acetate at ph 7.0 (rapid method) Exchangeable Al 3+ by 1M potassium chloride Pseudo-total elements reverse aqua regia digestion Mehlich 3 multielement (P, Ca, Mg, Na, K, Fe, Cu, Mn, Zn, B, S, Al) empirical test with analysis by ICPAES 18F2 Mehlich 3 extractable P colour finish. END; GER, 8 Nov 2010 Method 15A1 provides a good estimate of exchangeable bases for acidic and neutral, non-saline, non-calcareous and non-gypsiferous soils, with results little affected by the choice of analytical finish for cations. There is no adjustment or pre-treatment to remove soluble salts or to suppress carbonate dissolution. Milled, air-dry soil sample is equilibrated with 1M NH 4 Cl at ph 7.0 for 1 h with mechanical shaking at a soil/solution ratio (w/v) of 1:20. The subsequent suspensions are clarified and analysed for exchangeable bases by ICPAES, by AAS, or by flame emission spectrometry for Na + and K +. The method specifies reporting exchangeable bases (Ca 2+, Mg 2+, Na +, K + ) as cmol i /kg on an oven-dry (105 o C) soil basis. This rapid method for exchangeable cations has no pre-treatment for soluble salts. It should yield similar data to method 15A1, except it can overestimate exchangeable Ca 2+ in soils containing calcium carbonate. Milled, air-dry soil sample is equilibrated with 1M NH 4 OAc at ph 7.0 for 30 min using mechanical shaking at a soil/solution ratio of 1:10. Suspensions are clarified prior to analysis for exchangeable bases (Ca 2+, Mg 2+, Na +, K + ) by ICPAES, by AAS, or by flame emission spectrometry for Na + and K +. The method specifies reporting exchangeable bases (Ca 2+, Mg 2+, Na +, K + ) as cmol i /kg on an air-dry soil basis. Exchangeable Al 3+ is one output of Method 15G1 for Exchange Acidity (H + + Al 3+ ) by 1M KCl. Milled, air-dry soil is equilibrated with 1M KCl for 1 h at a soil/solution ratio of 1:10. The filtered extract is initially titrated to ph 8.0 with 0.02M NaOH to derive exchangeable acidity. Exchangeable Al 3+, if present (soil needs to be acidic) is quantified by back-titration to ph 8.0 with 0.02M HCl, following the addition of 4% NaF solution. The addition NaF causes the ph to increase due to the formation and release of NaOH in the presence of Al hydroxides. The method specifies reporting exchangeable Al 3+ as cmol i /kg, expressed on an oven-dry (105 o C) basis. 17B methods have multi-element scope, inclusive of (pseudo) total K, P and S. Method 17B1 uses very-finely milled air-dry sample, with reverse aqua regia block digestion, followed by instrumental analysis. Similar analytical conditions apply to Method 17B2, except microwave assisted digestion is employed. The methods specify reporting results as either mg element/kg or % element on a moisture-free basis. This universal test is widely used internationally for rapid, cheap, soil fertility assessments (cations, macro- and micro-nutrients). The method uses milled air-dry sample and involves vigorous shaking for 5 min with Mehlich 3 extracting solution (mixture of 0.2M CH 3 COOH M NH 4 NO M NH 4 F M HNO M ph 2.5 ± 0.1) at a soil/extractant ratio (w/v) of 1:10. Suspensions are clarified prior to multielemental analysis, typically by ICPAES. Expect little difference in results when AAS is used for soil cations and nutrient heavy metals, provided possible ionisation interference is suppressed with lanthanum solution or similar. The method specifies reporting Mehlich 3 extractable elements as mg element/kg on an air-dry weight basis (required for proficiency testing across Australasia). Method 18F2 codes an optional colorimetric analytical finish for Mehlich 3 extractable P, particularly for laboratories without access to ICPAES or when soil P calibrations were based on a colorimetric finish. Typically, the P result (mg P/kg on an air-dry basis) differs from that obtained by ICPAES. Page 6 of 6