Acid-extractable Phosphorus (9G2)

Air dry moisture content (2A1)

This method is required to adjust soil chemical results based on air-dry samples to an oven-dry (105^oC)
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

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.

Water soluble nitrate - automated colour (7B1)

Organic carbon - W & B (6A1)

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 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 (105^oC) basis. Nowadays this method is less preferred than 6B methods.

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

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.