A field method for exchangeable (1_M KCl extractable) aluminum(_III) in soils

Abstract

A rapid method for estimation of exchangeable (1 M KCl extractable) aluminium(III) in soils is reported. It involves a 5 min extraction of a measured volume of soil, followed by incremental addition of the supernatant to the metallochromic reagent chrome azurol S (CAS) in hexamine buffer (pH 4.9). Formation of Al (CAS) (purple) from CAS (amber) is rapid. Iron(III) is masked by ascorbic acid. The intermediate colour of the resultant solution containing complexed and non-complexed CAS is compared with standards in polystyrene cuvettes. Tests on 17 soils with KCI-extractable aluminium levels in the range 50-850 mg kg-1 established that aluminium extracted in 5 min (X, mg kg-') is related to the 16 h value (Y, mg kg-1) by Y = 1.43X+ 32; r2 = 0.962**); an approximate conversion factor of 1.5 (Y = 1 .50 X) was applied to all measured values to estimate soil alumimium values.