A field test is described for determination of 0.02 M CaC12 extractable ('soluble') aluminium in soils. The method uses a 5 min extraction (2 min shaking followed by 3 min settling period). An aliquot of extract is then added to the metallochromic reagent chrome azurol S, CAS (amber). Aluminium reacts rapidly with CAS in hexamine buffer (pH 4.9) to form a 1 : 1 complex, Al(CAS) (purple). Intermediate colours arise in solutions where CAS is partly converted to the aluminium complex. The developed colour may be compared visually (or spectrophotometricaliy in the laboratory) with standards. Alternatively the CAS concentration may be selected to give the purple colour at a predetermined concentration of aluminium in the extractant solution, corresponding to the toxicity threshold for a specific crop and soil. CAS-reactive Al (5 min extraction) was correlated with total Al (1 h extraction; AAS) soluble in 0.02 M CaCl2, by Alt = 1.62[CAS-Al]+0.5 for soils with Alt < 60 mg kg-1 (n = 22; r2 = 0.99).
