Analytes Certified for in 2020
This method is required to adjust soil chemical results based on air-dry samples to an oven-dry (105oC)
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
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.
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.
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.
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.
Exchange acidity (hydrogen and aluminium) by 1M potassium chloride
This is a preferred method for estimating the acidic cation status of acidic sub-tropical and tropical soils.
Exchangeable bases - 1M ammonium acetate at pH 7.0.
This rapid method for exchangeable cations in non-saline acidic through to slightly alkaline soils has no pre-treatment for soluble salts. It should yield similar data to those of method 15A1, except it can overestimate exchangeable Ca in soils containing calcium carbonate.
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.
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.
Empirical extraction at 40oC in 0.25M KCl, for 3 hours. Total S in the extract is measured by ICP-AES.
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.
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.
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.
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 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 finely-milled air-dry sample, while both methods specify reporting results as %N on an oven-dry (105oC) basis.
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 Clâ€‘ 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 (105oC) basis.
Tests for water-soluble chloride (Cl) on milled air-dry sample are suitable for use on all soils. For method 5A1, Clâ€‘ 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, Clâ€‘ 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 Clâ€‘ 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.
Microwave Digestion - closed system without HF, and final medium HNO3 and/or HCl. ICP-AES finish.
Extraction with water. Spectrophotometric (automated) finish.