In acid soils of a more tropical environment containing smectites, as well as in highly buffered subtropical soils containing Al hydroxide interlayered smectite (HIS) or vermiculite (HIV), the values of exchangeable Al, extracted by KCl solution (Al-KCl), may be exceptionally high, but not always toxic to the crops. This "exchangeable" Al does not come solely from electrostatic sites. This study aimed to evaluate the relationship between the Al extracted from the soil by KCl and ammonium oxalate solutions, and clay mineralogy. Soil samples collected in both the A and B horizons of 12 profiles coming from five Brazilian states were used, showing different soil mineralogical characteristics, with Al-KCl always higher than 4.0 cmol c kg-1 in the B horizon. Additional samples from two highly weathered soils of the State of Santa Catarina (one Hapludult from Rancho Queimado and one Hapludox from Curitibanos) containing lower values of Al-KCl were also included. Al in the KCl and ammonium oxalate extracts was determined titrimetrically (with NaOH 0.02 mol L-1) and by atomic absorption spectroscopy, respectively. Al extracted by KCl was closely related to the soil clay mineralogy. In the acid soils containing higher quantities of smectites, deficient drainage, and oscillation of the groundwater level, mineralogical analysis showed morphological evidence of clay destruction with subsequent release of Al that precipitates in amorphous forms. The high electrolytic concentration of the KCl solution partially dissolves such Al compounds, overestimating the values of exchangeable Al, especially in the subsurface horizons. In the more weathered acid soils of the subtropical environment, the values of Al-KCl were also high, but lower than those found in soils with smectites as the main clay mineral. Part of the Al-KCl in these soils seems to come from the dissolution of organic compounds and also from Al located in the interlayer of 2:1 clay minerals. Thus, Al extracted by KCl 1 mol L-1 did not adequately estimate exchangeable Al in most of the soils studied.
smectite; ferrolysis; "exchangeable" aluminum; acid soils; hydroxy-Al polymers
