The highly weathered soils developed in the humid tropics have low nutrient reserve in the coarse fractions. To evaluate K and Mg total contents in minerals in the sand and silt fractions, soil samples developed from different parent materials and weathering degree from MG, ES, RS and RR states, Brazil, were selected to be analyzed by X-ray diffraction (XRD), differential thermal analysis, thermogravimetry and electron microscopy. The total contents of K and Mg of the whole soil and its fractions were determined by HF extraction. To estimate the mineral contribution in the total contents of K and Mg, the silt fraction was submitted to mineral selective extractions. The sand fraction of the more weathered soils has very low K and Mg contents, with predominance of quartz. For the younger soils, this fraction showed higher K and Mg levels in the presence of primary minerals, such as mica, feldspar and amphibole. Mica particles and kaolinite flakes were the main sources of K and Mg for the sand and silt fractions of the highly weathered soils. The mica minerals were analyzed by electron microscopy, with K2O contents ranging from 33.3 to 62.8 and 27.4 to 98.1 g kg-1 and MgO contents from 76.5 to 116.8 and 19.7 to 68 g kg-1 for sand and silt fractions, respectively. Because of low mica content in the silt fraction of the more weathered soils (less than 1%), and the high quartz level, even after kaolinite extraction, the mica was not detected by XRD. Only one soil showed a small mica peak.
mineral reserve; mica; kaolinite; weathered soils