Low Ca content and Al toxicity are important limitations for yield crops in Brazilian acids soils. Gypsum application may increase Ca contents and decrease Al toxicity. However, a better knowledge of the effects of gypsum on the ionic speciation of soil solution is very important. The ionic speciation of the soil solution (water extract 1:1) was evaluated in samples from a dystrophic clayey Rhodic Hapludox under no-till system, 5 years after surface application of gypsum (3; 6 and 9t ha-1). The ionic speciation was performed by using the Visual Minteq program in samples collected at the following depths: 0-0.05; 0.05-0.1; 0.1-0.2 and 0.2-0.4m. The main Al complexing agents in solution were with dissolved organic carbon (Al-DOC) and F (Al-F), while the ionic par AlSO4+ occurred in small proportion. Ca and Mg were complexed by DOC, and the effect was more pronounced for Ca, reinforcing the important participation the organic anions in the dynamic of these nutrients. Although sulphate had formed ionic pairs with Ca, Mg and Al (CaSO4(0), MgSO4(0) and AlSO4+), the free form (S-SO4-2) predominated; while phosphate strongly interacted with Al in solution, forming the chemical specie Al-HxPO4x.
ionic pair; complexation; dissolved organic carbon; aluminum; anions; cations
