Physical, chemical and mineralogical characteristics of soils from the meadows of Sousa (Paraíba State - Brazil) were studied, as well as the effect of alkaline and alkaline-earthy cations on humic substance stability. Furthermore, criteria of environmental distinction used by the small farmers and different forms of land use in the studied region were identified. Profiles from the classes Ustifluvents (Neossolo Flúvico), Haplustalfs (Luvissolo), Natrustalfs (Planossolo nátrico), and Usterts (Vertissolo crómico) were therefore selected, sampled and analyzed. Besides sodium, magnesium also had an effective participation in clay dispersion, mainly in the Usterts. Fe2O3 contents were low in all profiles, with a probable prevalence of less crystalline forms. The calcium-sodic mineralogy of the silt fraction is in agreement with the relative high content of calcium, magnesium and sodium. The outstanding presence of vermiculite/smectite and ilite was observed in the soil clay fraction of all soils. The high iron content in the clay fraction of Usterts suggests, besides the presence of hematite, the occurrence of 2:1 expansible mineral, rich in iron, possibly nontronita. The pre-treatment for carbonate elimination (HCl 0.1 mol L-1), applied during the humic substance fractionation, caused increases of 300 and 340 % for the humic and fulvic acid fractions, respectively, and a reduction of 60 % in the humin fraction, suggesting a participation of calcium and of magnesium humates and fulvates in organic matter stabilization.
semi-arid; alkalinity; smectites; organic matter; environmental stratification
