A study of potassium dynamics and mineralogy in soils from subtropical Brazilian lowlands

Abstract

Purpose The dynamics and availability of potassium (K) in soils depend on many factors, including the mineralogy of fractions and the soil geochemical conditions. The objective of this work is to quantify K in distinct lowland soil compartments in the south of Brazil, relating them to particle size fractions and mineralogical constitution. Material and methods The soil samples (0-20 cm depth) were collected in six different lowland areas of flooded rice fields in southern Brazil. A mineralogical characterisation was performed. A semi-quantitative analysis was performed on the clay fraction using X-ray diffractogram (XRD) decomposition technique. The K concentration in the different soil fractions was determined using the following methods: potassium total (Kt) by the hydrofluoric acid total digestion method, nonexchangeable K (Kne) extracted by sodium tetraphenyl borate (NaTPB) after a 24-h contact, exchangeable K (Ke) extracted by ammonium acetate at pH 7, and available K extracted by the Mehlich-1 method. Potassium desorption curves from the soil were obtained by successive extractions with NaTPB.Based on the contents found in the different soil forms and compartments, the contribution of each Kne and Kt particle size fraction could be mathematically evaluated. Results and discussion Silt fraction mineralogy may vary depending on the soil and the parent rock. The K-feldspar minerals were observed in the XRD of three soils, explaining the high Kt contents. The following mineral species were observed in the clay fraction: kaolinite, illite, smectite/vermiculite or smectite/vermiculite with a hydroxy-Al interlayer. The Kt and Kne (R 2 =0.99) values are correlated, and both show a low correspondence with the Ke (R 2 <0.20). The silt and clay contributed with values from 9% to 37% and 63% to 91% of the Kt, respectively, for the different soil samples. The Kne contribution, however, ranged from 10% to 62% and 62% to 90% for the clay and silt fractions, respectively. Conclusions Lowland soils in the south of Brazil include large non-exchangeable K reserves that are associated with the presence of K-feldspar and mica in the silt fraction, as well as smectite, vermiculite and poorly crystallised illite in the clay fraction. For this reason, large quantities of nonexchangeable K are available for desorption by a NaTPB extractor, which represents a potential source of release of K to plants. The use of mineralogy to delineate the K dynamics in soil compartments was quite effective and should be adopted in fertility diagnosis and potassium fertilisation programmes for rice crops.

Disponibilidade de potássio a arroz irrigado por alagamento em solos de várzea

“…Entretanto, esses valores não foram associados com o K absorvido pelas plantas (Figura 1a). Essa baixa correlação pode estar relacionada com a reatividade dos coloides, ou seja, com os teores de matéria orgânica e de argila desses solos, que podem funcionar como tampão de K. Os solos que possuem alto poder tampão de K têm capacidade de manter constantes os teores de K na solução do solo, seja por meio de adsorção específica + adsorção não específica, quando se aduba com K, ou por meio de dessorção, que repõe o K na solução do solo, quando esse elemento é perdido do solo por lixiviação ou absorção pela planta (Santos et al 2013 (Britzke et al 2012) e eventuais diferenças no manejo e adubações realizadas pelos agricultores antes da coleta de solo. O K disponível representou 1-44 % do K total do solo, indicando grande variação no comprometimento das reservas de K do solo e na capacidade de suporte dos solos em manterem a disponibilidade às plantas, no caso de não haver adubações potássicas de manutenção.…”

Baseline and Quality Reference Values for Natural Radionuclides in Soils of Rio de Janeiro State, Brazil

“…The highest median 40 K value was observed in the Gleysol, which may be ascribed to the presence of primary minerals such as mica and K-feldspar, leading to high total K contents and high non-extractable K in lowland soils (Britzke et al, 2012). The highest medians for 226 Ra and 228 Ra were found in the Leptsol, a shallow and young soil derived from granite and gneisses that are part of those rocks for which above-normal levels of natural-series radionuclides were detected (Leal and Lauria, 2016).…”

Forms and Distribution of Potassium in Selected Maize Growing Soils of Haveri District of Karnataka, India

“…The organic matter in soils contains a negligible amount of K because it is not a constituent of biomolecule, hence is easily and quickly leached from leaves due to its high solubility. However, the fraction of soil K directly available to plants (i.e., in soil solution) is usually a small proportion (0.10-0.20 per cent) of the total soil K, whereas both the immediately available water-soluble fraction and the exchangeable fraction comprise 1.00-2.00 per cent and the soilunavailable fractions 96-99 per cent (Britzke et al, 2012).…”