Searle and Lee (1984) noted that AgTU+ was as effective as 1 M KCl at removing Al3+ from soil. Slightly higher values of exchange acidity (Al3+ + H+) relative to 1 M KCl were reported by Pleysier and Juo (1980) when soil was extracted with 0.01 M AgTU+ for 2 h at a soil/extract ratio of 1:6. This method, broadly based on that reported by Blakemore et al. (1987), uses portion of the original supernatant soil extract from Method 15F1. Any Al3+ present is determined by ICPAES (preferred) or by AAS.
Table 15.15. Examples of dilutions and concentrations for Na+ and K+ Working Standards – 0.01 M AgTU+.
0.01 M AgTU+
As for Method 15F1 (same batch used for soil extracts).
Saturated Potassium Chloride (required particularly for AAS analysis) As for Method 15F1.
Aluminium Primary Standard
1 L contains 0.1799 g of Al3+
Dissolve 0.1799 g pure aluminium (Al) wire in about 5 mL of 10 M HCl, with the addition of a single small crystal of a soluble Hg salt to catalyse the reaction. Make to 1 L with deionised water in a volumetric flask. Alternatively, dilute a reliable, commercial stock standard Al3+ solution.
Aluminium Working Standards
Accurately dispense 0, 5, 10, 15, 20 and 25 mL Al Primary Standard into 500 mL volumetric flasks. Add 2.5 mL saturated KCl to each and make to volume with 0.01 M AgTU+. Concentrations of these solutions correspond to 0, 1.799, … 8.995 mg Al3+/L, equivalent to 0, 1, 2, 3, 4 and 5 cmolc/kg of Al3+ for undiluted AgTU+ extracts, based on a 1:50 soil/extract ratio. Store in the dark and discard should any precipitate develop with time.
Take a 20 mL (approximate) portion of the original supernatant soil extracts from Method 15F1 and add 0.1 mL saturated KCl. The saturated KCl is necessary to suppress the ionisation of Al3+ during measurement by AAS. It is also present in the Al Working Standards.
Measure Al3+ in Working Standards and in the prepared 0.01 M AgTU+ – saturated KCl soil extracts by ICPAES or AAS, without further dilution.
Follow manufacturer’s recommendations with respect to instrument parameters. One spectral line for Al by ICPAES is 308.215 nm, although this should be checked to ensure there are no spectral interferences. If so, another choice may be superior. For AAS, a N2O/C2H2 flame and the spectral line 309.3 nm for Al are preferred. A method blank should be included and adjustments made as necessary.
Report exchangeable Al3+ (cmolcAl/kg), expressed on an oven-dry basis. Use the air-dry moisture to oven-dry moisture ratio to convert to an oven-dry concentration. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.