Assessment of potentially reactive pools of aluminum in Andisols using a five-step sequential extraction procedure

A five-step sequential extraction procedure was developed for the partitioning of soil aluminum (Al) into five fractions: exchangeable, weakly organic bound, strongly organic bound, inorganic non-crystalline and crystalline. The results obtained by the sequential extraction method for Al were compared with those estimated from single extractions using identical operating conditions applied in each individual sequential fraction. The Al content in the extracts was measured by inductively coupled plasma optical emission spectrometry. The results indicated that the first four steps [potassium chloride (KCl), copper chloride (CuCl₂), sodium pyrophosphate (Na₄P₂O₇), acid ammonium oxalate ((NH₄)₂C₂O₄)] in the sequential procedure could be as effective as single extraction methods at estimating exchangeable Al, weakly organic bound Al, strongly organic bound Al, and non-crystalline Al. However, the crystalline Al content by sequential procedure was not in agreement with single extraction procedures. Further, the sequential extractions resulted in more consistent estimates of the aluminum/silicon (Al/Si) molar ratio and allophane and crystalline Al contents than single extractions method. Results of X-ray diffraction on the soil samples confirm the presence of appreciable amounts of at least three types of crystalline minerals, including gibbsite, vermiculite or halloysite. Our result implies that the sequential method can be more reliable in estimating the various Al forms in Andisols.