The objective of this study was to evaluate a fluxing salt residue (slag) produced 1n large quantities by a Mg reclamation process as a potential K‐Mg nutrient source (41% KC1, 33% MgO and 3.2% MgCl2) for cereal crops 1n the Pacific Northwest. Field studies were conducted at Sandpoint, Idaho in 1983 and 1984 to evaluate this fluxing salt residue on the following three criteria: (1) potential phytotoxicity problems associated with plant growth, (2) response (yield) of spring wheat (Triticum aestivumL.), spring barley (Hordeum vulqareL.) and oats (Avena sativaL.), and (3) comparison with commercial K and Mg fertilizers.