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Potassium Reserves in the Clay Fraction of a Tropical Soil Fertilized for Three Decades

Published online by Cambridge University Press:  01 January 2024

Ruan F. Firmano
Affiliation:
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
Vander Freitas Melo
Affiliation:
Department of Soils and Agricultural Engineering, Federal University of Paraná, UFPR, Rua dos Funcionários 1540, 80035-050, Curitiba, PR, Brazil
Célia Regina Montes
Affiliation:
Center for Nuclear Energy in Agriculture (CENA), University of São Paulo (USP), Av Centenário, 303, 13418-900, Piracicaba, SP, Brazil
Adilson de Oliveira Junior
Affiliation:
Brazilian Agricultural Research Corporation, National Soybean Center, Rodovia Carlos João Strass (Orlando Amaral Access), 86001-970, Londrina, PR, Brazil
Cesar de Castro
Affiliation:
Brazilian Agricultural Research Corporation, National Soybean Center, Rodovia Carlos João Strass (Orlando Amaral Access), 86001-970, Londrina, PR, Brazil
Luís Reynaldo Ferracciú Alleoni*
Affiliation:
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
*
*E-mail address of corresponding author: alleoni@usp.br

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.

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Article
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Copyright © Clay Minerals Society 2020

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