POTASSIUM RESERVES IN THE CLAY FRACTION OF A TROPICAL SOIL FERTILIZED FOR THREE DECADES
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POTASSIUM RESERVES IN THE CLAY FRACTION OF A TROPICAL SOIL FERTILIZED FOR THREE DECADES RUAN F. FIRMANO1, VANDER FREITAS MELO2, CÉLIA REGINA MONTES3, ADILSON DE OLIVEIRA JUNIOR4, CESAR DE CASTRO4, AND LUÍS REYNALDO FERRACCIÚ ALLEONI1 * 1
Department of Soil Science, Luiz de Queiroz College of Agriculture (ESALQ), University of São Paulo (USP), Avenida Pádua Dias 11, Piracicaba, SP 13418-260, Brazil 2 Department of Soils and Agricultural Engineering, Federal University of Paraná, UFPR, Rua dos Funcionários 1540, Curitiba, PR 80035-050, Brazil 3 Center for Nuclear Energy in Agriculture (CENA), University of São Paulo (USP), Av Centenário, 303, Piracicaba, SP 13418-900, Brazil 4 Brazilian Agricultural Research Corporation, National Soybean Center, Rodovia Carlos João Strass (Orlando Amaral Access), Londrina, PR 86001-970, Brazil
Abstract—Highly weathered soils of the humid tropics generally provide a poor mineral reserve of potassium (K), but evidence has been found which indicates that even in such soils non-exchangeable forms of K can be made plant available and this warrants further investigation. The objective of this study was, therefore, to determine the extent to which K can be released from poorly available reserves over a long period of time. The focus was on an Oxisol in southern Brazil cultivated for 32 years with a rotation of soybeans (Glycine max L.), maize (Zea mays L.), wheat (Triticum aestivum L.), and oats (Avena strigosa L.) with and without K fertilization. Mineral sources of K were identified by X-ray diffraction and by sequential chemical extraction from the clay fraction. The amounts of K-bearing mineral species and the amounts of total and plant-available K were quantified, then the effects of the long-term K-fertilization regime on these values were evaluated. The clay fraction was dominated by hematite, gibbsite, and phyllosilicates such as kaolinite. These minerals were unaffected by the K deprivation in the cropping systems, but in the clay fraction the absence of K fertilization for 32 years reduced the structural order of the 2:1 phyllosilicates associated with K reserves. This effect was most prominent in the root zone of the soil. Deprivation of K for more than three decades decreased the crystallinity of 2:1 phyllosilicates, which could be better evaluated from XRD patterns after the removal of kaolinite and Fe (oxyhydr)oxides. The K-free cultivation reduced the amounts of total soil K by increasing the depletion of K from pools that typically are poorly accessible to plants. Keywords—Clay mineralogy . Extraction procedures . Soybeans . X-ray diffraction INTRODUCTION th
Potassium (K) is the 7 most abundant element in the Earth’s crust, forming 2.2% of its composition. In the lithosphere, igneous rocks and shale have the highest concentrations of K, while limestone and sandstones have lower concentrations (Zörb et al. 2014). Brazil is the 4th largest consumer of K-fertilizers in the world and imports ~90% of its demand, which grows annually. In rainy years, high yields can be achieved
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