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.

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 (I_0.1) can be calculated as follows: I_0.1 = [0.0446*EC_1:5 – 0.000173], where I_0.1 has units of mM, and EC_1:5 has units of dS/m @ 25^oC.

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).

Phosphorus buffer index, with Colwell P (PBI+ColP) pooled analytical finishes (9I2A)

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.