Aluminum in complexes with soil organic matter is usually determined by selective extraction techniques. A common extractant for this Al form is sodium pyrophosphate. Due to its lack of selectivity for some soil samples, unbuffered Al extractants, such as CuCl2 and LaCl3, have been recommended. The selectivity of sodium pyrophosphate (Al p) and of unbuffered Cu (AlCu) and La (AlLa) chlorides to extract aluminum bound to soil organic matter (Al-humus) was evaluated in soil samples from 31 pedons (Spodosols and Quartzipsamments) representative of the sandy coastal plains of the state of São Paulo. The results were compared with those obtained by ammonium oxalate and KCl and the extraction capacity and selectivity of the reagents interpreted as commonly used in the literature. Pyrophosphate was not totally selective for some samples containing inorganic aluminum. CuCl2 and LaCl3, in turn, were more selective for Al-humus in the same samples and their order of extractability was: Al p>AlCu>AlLa . Based on the different extraction capacities and the relations between the reagents used the analytic evidences were converged in order to analyze and interpret parameters such as Al saturation in the soil organic matter and stability of C-metal interactions. The stability of the interaction C-metal and humus Al saturation of all horizons studied was greater in the Bs, Bhs and Bsm spodic horizons. Based on the results it was concluded that all reagents studied can be used as Al extractants since they allow establishing relations and interpretations that may help understanding the complex chemistry of Al-humus on the studied Restinga soil. These extractors are therefore adequate and promising to study and understand processes involved in the genesis of tropical Spodosols derived from sandy coastal plain sediments.
podzolization; spodic horizons; Al-humus complexes; Al-humus saturation degree
