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

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. 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, non-exchangeable 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. 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. 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 non-exchangeable 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.