Mechanically activated glauconitic sandstone: potential for alternative potassium fertilizer

Potassium (K) is an essential nutrient for all living organisms. Its high mobility in soils often limits its availability to plants, constraining growth. To meet the increasing demand for food, K-based fertilizers are routinely applied to enhance the yields of agricultural crops. However, the use of conventional KCl fertilizers can lead to soil salinization, their supply is vulnerable to geopolitical and economic fluctuations, and the production of soluble KCl is energy- and environmentally intensive. Therefore, potassium-rich aluminosilicate minerals, such as micas, glauconite, and K-feldspar, have been considered as alternative potassium fertilizers. This study investigates the usability of mining waste composed of glauconitic sandstone as a raw material for potassium fertilizer, with a focus on mechanical activation as a method to enhance potassium leachability. Bulk sandstone and enriched glauconite samples were subjected to mechanical activation to evaluate changes in particle size, morphology, specific surface area, and potassium solubility. Mechanical activation significantly enhanced the release of plant-available K, with up to 80% of total K released into solution in 240 minutes, with 55% of K extracted in  first 30 minutes, indicating high process efficiency. Glauconite sandstone enrichment improved the K content by 14% but also led to a nearly seven-fold increase in bulk Cr concentrations reaching 291 ppm, approaching regulatory limits for fertilizers agricultural use. While mechanical activation offers a scalable, energy-efficient alternative to conventional K-fertilizers, the relatively low bulk K₂O content (<5%) and elevated Cr levels limit the practical application of the Estonian glauconitic sandstone as a K-fertilizer. These findings highlight both the potential and constraints of mechanical activation for sustainable potassium fertilizer production.