Contribution of clay fraction minerals of subtropical soils to the specific surface area and cation exchange capacity

The specific surface area (SSA) is related to several properties that define the soil physico-chemical behavior. In this research, 23 soil samples developed from different parent materials found in the southern Brazil were used. They were evaluated for the contribution of the main soil constituents to the effective cation exchange capacity (CECe) and SSA. The clay fraction minerals were identified by X ray diffraction (XRD) and quantified by thermal analysis; the CECe, the SSAt (total) by the ethylene glycol monoethyl ether method (EGME), the SSAe (external) by the BET-N-2 method, and the ASEi (internal) by the difference of the values obtained with the two methods. The clay fraction samples were determined before and after the following sequential selective dissolution procedures: removal of organic matter (clay-NaOCl); removal of organic matter and free iron oxides (clay-NaOCl + DCB); removal of kaolinite, gibbsite and low cristalinity aluminosilicate minerals (clay-NaOCl+DCB + NaOH 5 mol L-1). The mineralogy of the clay fraction of the soils varied considerably, as verified by X ray diffraction, thermogravimetric analysis, and SSA and CECe values. For most soils kaolinite is the predominant mineral in the clay fraction, followed by expandable 2:1 clay mineral or iron and aluminum oxihydroxides. SSA and CECe of the natural clay fraction were not correlated due to the occlusion of the clay surface by organic matter. Iron oxide aggregate effects reduced SSAe by 21%. For the studied samples, the 2:1 clay minerals were present in concentrations varying from 3 to 65% of the silicate fraction and presented average values of 1.054 mmol(c) kg(-1) and 202 m(2) g(-1) for CECe and SSEt, respectively. SSAi contributed with 58% of SSAt of the clay fraction, mostly due to 2:1 clay type minerals.