Data are limited for the phytotoxicity of metal mixtures in field applications. Naturally aged and leached soils from a decommissioned Ontario gold mine were used to establish phytotoxicity thresholds for mixtures of As, Co, Cu, Zn, and Ni. Growth studies were conducted with barley (Hordeum vulgare L.) and tomato (Lycopersicon esculentum), using seedling emergence, shoot length, and shoot/root dry weight as growth response endpoints. Structural Equation Modelling and Multiple Linear Regression were applied to evaluate growth-inhibiting parameters of non-contaminant factors (pH, clay, and OC). Barley and tomato emergence were explained by the direct effect of OC. Clay had the most significant indirect effect through Mg (tomato) and Cu (barley) root uptake. Soil physicochemical properties critically modified phytotoxicity of metals via uptake of essential cations, with OC acting as the master soil variable for phytotoxic mechanisms. This study will aid to estimate phytotoxicity thresholds for risk assessment of metal-contaminated soils worldwide.