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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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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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pH: 1:5 SOIL:0.01M CaCL2 Suspension (4B1)

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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DE-11

Microwave digestion - closed system with HF, and final medium HNO3 and/or HCl.  AAS-Flame with deuterium background correction, using air-acetylene.

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DE-30

Microwave digestion - closed system without HF, and final medium HNO3 and/or HCl. Spectrophotometry (manual) finish.

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(DE-09)