Sodium pyrophosphate is the most widely used extractant of Al bound to soil organic matter (Al-humus). However, its selectivity has been questioned, so that some authors have proposed the use of substitutional extractants. To evaluate the effectiveness of unbuffered Cu (CuCl2), La (LaCl3) and K (KCl) chlorides to extract Al-humus, soil material from 31 representative pedons of Spodosols and Quartzipsamments from sandy coastal plains of São Paulo State was evaluated. The results were compared with those achieved by sodium pyrophosphate. CuCl2 was more efficient than LaCl3 and KCl and LaCl3 less than pyrophosphate; i.e., extracted part of the Al associated to inorganic poorly crystalline compounds and stable organic complexes, the latter possibly inactive in terms of soil acidity and ionic exchange. On the other hand, KCl and LaCl3 removed the reactive Al forms. However, Al extracted by LaCl3 (AlLa) was best correlated with organic carbon (Corg) and soil CEC, indicating that AlLa is related to the main component responsible for the development of charges in the studied soil. The extractors allowed analyzing the Al-humus interaction degree, as well as the influence of soil pH in this interaction. The most stable Al-humus complexes were only found in the well-drained Bs, Bhs and C horizons, where the lowest effectiveness of all soil materials analyzed was found for LaCl3 and mainly KCl. The more labile organic matter bounded Al was found in the A horizons. In general, the stability of Al-humus interaction of the studied horizons showed the following sequence: A < Bh < C < Bhm < Bs/Bhs/Bsm, of which the latter and a few C are the only well-drained subsurface horizons, indicating that the presence of water (Bh horizons) and litter turnover (A horizons) are favorable for the maintenance of the more labile Al species.
Spodosols; sodium pyrophosphate; CuCl2; LaCl3; KCl; Al-humus complexes
