Evaluating the Slow-Release Fertilizer Effects of Calcium Potassium Phosphates from Mechanochemically Treated Calcium Carbonate Materials with KH2PO4

Developing slow-release fertilizers, carefully adjusting their solubility profiles, is an important way to improve nutrient use efficiency and potentially reduce environmental impacts associated with nutrient losses and accumulation in water bodies, such as potential eutrophication. In this work, CaCO3 from industrial byproducts (spent lime, i.e., carbokalk, and eggshells) was mechanochemically activated with KH2PO4 and citric acid as an additive. X-ray diffraction indicated the formation of calcium potassium phosphate phases, along with residual CaCO3, depending on milling conditions. However, citric acid addition suppressed crystalline phosphate formation, likely influencing species organization and favoring poorly crystalline or amorphous phosphate phases, possibly through Ca complexation. Phosphorus percolation from milled samples remained low at early stages (0.2–4% after 2 days) and reached only 21–35% after 7 days, whereas potassium showed faster release, with 0.2–10% at 2 days and up to ∼100% percolation after 7 days. Greenhouse trials with Brachiaria brizantha over three harvest cycles suggested improved nutrient management, evidenced by a more gradual biomass increase and nutrient uptake, in contrast to the early peak and subsequent decline observed in positive controls using KH2PO4 and triple superphosphate (TSP). Thus, mechanochemical processing offers a smarter product capable of gradually releasing its fertilizer content, offering a promising route toward efficient and sustainable fertilization strategies.