Agronomic Value of Carnallite-Derived Potassium and Magnesium Fertilizers: Nutrient Availability, Soil Interactions, and Crop Performance

Carnallite is a hydrated potassium–magnesium chloride mineral that can supply K and Mg, but its agronomic value cannot be inferred from stoichiometry or dissolution alone. This critical narrative review examines processing-dependent nutrient inventories, product quality, soil exchanges, K–Mg nutrition, chloride exposure, and crop-response evidence. We evaluated the supplied 2010–2026 candidate bibliography thematically; we cannot reconstruct its retrieval and screening history. We distinguished direct carnallite-processing studies, agronomic analogies, mechanistic research, and untested hypotheses by their relationship to the question, not by automatic methodological quality grades. Evidence from polyhalite or independent nutrient supplementation does not validate carnallite-derived products. A proposed evaluation framework specifies product-specific K and Mg dose matching, independent nutrient-response controls, defined chloride and salinity contrasts, and separate assessment of agronomic efficiency and source comparisons. Consider field use or commercialization only after chemical and physical product characterization and replicated trials confirm efficacy and acceptable environmental performance in relevant soil–crop systems.