Potassium and magnesium in clay minerals of some Brazilian soils as indicated by a sequential extraction procedure

The total contents of potassium (K) and magnesium (Mg) in selected Brazilian soils (Oxisols, Ultisols, Inceptsols, and Alfisols) developed from different parent materials were investigated to estimate the contribution of clay minerals to K and Mg reserves. Total K and Mg contents in the Na-saturated clay were determined by HF total digestion. The contribution of each mineral species to the total contents of K and Mg, was estimated by a sequential mineral extraction procedure, following the order: Na-saturated clay-1) amorphous aluminum and iron oxides (ammonium oxalate); 2) crystalline iron oxides (dithio- nite - citrate -bicarbonate); 3) amorphous alummosilicate and gibbsite (0.5M NaOH); 4) kaolinite (5M NaOH)-, 5) mica (NaHSO4), and 6) feldspar (HF total digestion). In order to monitor mineralogical changes, after each extraction, the samples were studied by x-ray diffraction (XRD), analytical transmission electron microscopy (ATEM), differential thermal analysis and thermo-gravimetric analysis. Total amounts of K and Mg in the clay of the strongly weathered soils were very low, indicating a low reserve of these nutrients. Due to the high proportion of kaolinite in the clay fraction of these strongly weathered soils, this mineral accounts for more than 50% of the total K and Mg. The relative proportion of kaofinite and its contribution to the K and Mg reserves increased with weathering. In strongly weathered soils, the contribution of micaceous minerals to the total K and Mg ranged from 17 to 75% and from 8 to 59%, respectively, whereas in the younger soils the range was between 51 and 83%, and 35 and 82%, respectively. Only after the removal of the iron oxides and of the kaolinite was the mica concentration in the clay fraction high enough to be detected by XRD in some of the strongly weathered soils. Dioctahedral illite was the dominant micaceous mineral due to its high resistance to weathering. Microanalysis of individual mica particles by EDAX-ATEM revealed KO and MgO concentrations between 62 and 121 a kg(-1) and between 5 and 10 g kg(-1), respectively.