In Brazil, Pinus species are grown on approximately 1.87 million hectares, usually on chemically poor soils. The present study investigated the mineralogy of sand, silt and clay fractions and estimated the mineral reserve of K by different chemical extraction methods in naturally K-poor soil under Pinus taeda L., in the State of Paraná, Brazil. In a 500 m² area, five trees with large diameter (dominant trees) were selected, and trenches opened (depth 1.6 m) under the canopy of each tree. All soil profiles were classified as dystrophic Typic Red-Yellow Latosol and were similar in horizon morphology and sequence. The average horizon depths were O = 0.04 m, A1 = 0-0.09 m, A2 = 0.09-0.24 m, BA = 0.24-0.43 m, B1 = 0.43-0.66 m, and B2 = 0.66-1.60+ m. Samples collected from each horizon were subjected to physical (granulometry) and chemical analyses (pH, organic carbon, potential acidity, Al3+ and exchangeable bases, available P, total and non-exchangeable K) and sandy, silt and clay fractions were studied by X-ray diffraction (XRD). The mineralogy of the sand and silt fractions was fairly uniform, with total predominance of quartz and occurrence of only discrete XRD reflections of mica. The clay fraction also showed limited occurrence of micaceous minerals. The sequential treatments for removal of Fe oxides, gibbsite and kaolinite effectively determined mica concentrations in the clay fraction, which facilitated the identification of biotite and muscovite by XRD. The low concentrations of non-exchangeable K obtained at different concentrations of boiling HNO3 (maximum of 91 mg kg-1) and total K extracted with concentrated HF (maximum of 202.7 mg kg-1) were consistent with the poverty of the soil fractions in primary mineral sources of the nutrient. The significant and positive correlations between non-exchangeable soil K and the nutrient concentrations and contents in the trees indicate the importance of making K reserve forms available in the nutrition of this species, when grown in highly weathered soils with low exchangeable K levels.
mica; nitric extraction; non-exchangeable K; total K
