This method, supported by field-based correlations, was initially developed to predict the P responsiveness of soils used for sugar cane in east-coastal Queensland (Kerr and von Stieglitz 1938). Colloquially termed BSES-P, this test has also been used to assess the ‘available’ P status of many surface soils in Queensland, including for land use surveys (e.g. Powell 1977).
Soil extracts for analysis are obtained by shaking air-dry soil of <2 mm at a ratio of 1:200 with 0.005 M sulfuric acid for 16 h, according to the method of Kerr and von Stieglitz (1938). Orthophosphate in the cleared extract is determined manually by the colorimetric method of Truog and Meyer (1929) and Truog (1930).
Because of the extended period of soil/extractant contact and wide soil/solution ratio, the method favours the quantity component of the labile pool of soil P. As it also favours extraction of Ca-bound P, many forms of which are not available to plants, the test can overestimate P availability, especially in soils containing basic forms of these minerals, such as hydroxyapatite.
There is no prior adjustment of soil pH, or any attempt to remove possible interferences.
Extraction Solution – 0.005 M Sulfuric Acid
Initially prepare a stock solution of 2.0 M sulfuric acid (H2SO4) by diluting 111.1 mL H2SO4 (18 M) to 1.0 L with deionised water. Standardise a known dilution of this stock solution (e.g. ≈0.05 M H2SO4 in CO2-free deionised water) against a known weight of sodium borate decahydrate (Na2B4O7.10H2O) in a similar manner to that described in Method 7A1. Alternatively, standardise using a purchased sodium hydroxide solution of known concentration, with methyl red as indicator.
If the stock solution is exactly 2.0 M H2SO4, dilute 100 mL of this to 40 L with deionised water. This solution of 0.005 M H2SO4 is stable for long periods if kept free of dust and chemical contamination. Equilibrate to 25°C prior to use.
Ammonium Molybdate – Sulfuric Acid Solution
Dissolve 25.0 g ammonium molybdate [(NH4)6Mo7O24.4H2O] in 200 mL distilled (preferred) or deionised water. Separately dilute 280 mL of 18 M H2SO4 by adding carefully to distilled or deionised water. Cool and make volume to 800 mL. Now add slowly the cool molybdate solution to the dilute H2SO4 and allow to return to room temperature before use. See Note 1 of Method 9B2 on the reason for preferring distilled water.
Stannous Chloride Solution
Dissolve 25.0 g stannous chloride (SnCl2.2H2O) in 100 mL 10 M HCl and dilute to 1.0 L with distilled water. Store in a glass bottle fitted with a delivery cock near the bottom so as to provide the solution in drops. Protect the solution from air by covering with paraffin oil.
Phosphorus Primary Standard
As for Method 9A2.
Phosphorus Secondary Standard
1 L contains 5.0 mg of P
Take 50 mL P Primary Standard and dilute with Extracting Solution (0.005 M H2SO4) to 500 mL in a volumetric flask. This solution should be freshly prepared each time working standards are made.
Phosphorus Working Standards
Add 1.0, 2.5, 5.0, 10.0, 15.0, 20.0, 30.0, 40.0, 50.0 and 60.0 mL of P Secondary Standard to separate 500 mL volumetric flasks. When diluted to 500 mL with Extracting Solution (0.005 M H2SO4), these solutions contain 0.01, 0.025 … 0.60 mg P/L. For a 1:200 soil/solution ratio these standards contain P equivalent to soil concentrations of 2.0, 5.0 … 120 mg P/kg.
Weigh 1.0 ± 0.01 g air-dry soil (<2 mm) into 250 mL extracting bottles and add 200 mL Extracting Solution (0.005 M H2SO4), stopper and mechanically shake end-over-end for 16 h at 25°C. Centrifuge or filter (No. 42 or 50 paper), discarding the first portion.
Pipette 20 mL aliquots of the particle-free extracts, standards, and a blank into separate 125 mL Erlenmeyer flasks. Add 1.0 mL of Ammonium Molybdate-Sulfuric Acid Solution and mix thoroughly. Add 4 drops of Stannous Chloride Solution, shake thoroughly and allow colour to develop for at least 5 min.
After a fixed time interval between 5 min and a maximum of 20 min, measure absorbance of samples, standards and the blank (0.005 M H2SO4 plus colour reagents) at 660 nm. Prepare a calibration curve of absorbance against concentration of the standards and read concentrations of samples and blank.
Dilute extracts of high P concentrations with 0.005 M H2SO4 Extracting Solution.
Acid-extractable P (mg P/kg) = [Sample value – Blank]
Report acid-extractable P (mg P/kg) on an air-dry basis.