**5. Conclusions**

The findings of this chapter, which are associated to acidity conditions ranging from an unsuitable to a more favorable one for cropping, indicate that sulfate adsorption has no or little positive effect on calcium adsorption on kaolinite and gibbsite. On the other hand, as both calcium and sulfate concentrations in the solution phase are increased toward the precipitation threshold of calcium sulfate, calcium adsorption on those minerals is inhibited. Further, for a higher initial calcium loading (10 mM), the equilibrium pH enhance from 4.0 to 6.9 decreases calcium adsorption to kaolinite and gibbsite. Besides indicating low calcium adsorption on gibbsite, the experimental data suggest that the application of higher amounts of sulfate-concentrated products such as gypsum on lime-amended oxisols tends to reduce the effectiveness of kaolinite, the main clay-sized mineral responsible for the cation exchange capacity of those soils, to impair calcium leaching. The results also suggest that hematite and goethite are the main oxisol clay-sized minerals responsible for the calcium sulfate co-adsorption reported to oxisol samples. This hypothesis should be evaluated in future research. From the data presented in this chapter, it seems that sulfate at the pH conditions considered dominates the gibbsite behavior, while on kaolinite the calcium is the more relevant ion. The basal plane of sapphire as studied with second harmonic generation yields results that are in close agreement with those of the kaolinite particles, which suggests that the kaolinite behavior might be explained by its gibbsite plane. The role of the ion pair in solution and of sulfate even on negatively charge surfaces has been discussed, and the relevance of studying these features also with respect to atmospheric sciences (freezing of water and ice nucleation on particle surface) was addressed. In particular the link between aqueous chemistry and ice nucleation should be used to understand processes the atmosphere.
