Glauconite as a potential source of potassium in Brazilian agriculture - a review

Abstract

The high Brazilian dependence on potash fertilizers has encouraged the agricultural and forestry use of silicate rock powders. Dozens of rock powders, including glauconitic rocks produced in several states in Brazil are currently registered with the Ministry of Agriculture and Livestock (MAPA) as Soil Remineralizers, being generally known as alternative sources of K. The indication of these materials as potassium fertilizers is based on the high Brazilian dependence on K from abroad, which reached around 96% in the 2021 and 2022 harvests. Glauconitic rocks, such as verdetes, slates, and glauconitic siltstones occur in extensive areas in the State of Minas Gerais (Abaeté, Quartel Geral, Cedro do Abaeté, Sete Lagoas, Matutina, and São Gotardo), predominating in the Serra da Saudade formation as the sedimentary cover of the São Francisco Craton. In these rocks, K-K2O contents generally vary in the range of 5-12% m/m. It is soluble in water (0.1-1.4%) and 2% citric acid (0.7-2.3%). In these rocks, the degree of glauconite crystallinity is variable due to different formation conditions and degree of alteration by weathering, especially in rocky outcrops. The use of natural glauconitic rocks as potassium fertilizers has shown low agronomic efficiency due to their low solubility and reactivity. Physical and thermal treatments in the presence of fluxing, chemical (acid or alkaline attacks), and biological (Acidithiobacillus sp.) agents have shown potential for dissolving glauconites. However, studies showing the financial viability of these materials are necessary for both such treatments and their direct use as fertilizers.