A study was conducted to evaluate a waste rock powder collected from the Seaham quarry in New South Wales, Australia, as a source of potassium (K) in soil. The K supplying capacity of different size fractions of the mineral powder was evaluated by employing five chemical extractants as well as growing maize and holy basil in sand culture experiments. The K release by chemical extractants increased with decreasing particle size of the mineral powder. The amount of K released by different extractants followed the order: water < 0.01 M calcium chloride < 0.01 M citric acid < 1 N ammonium acetate < 1 N boiling nitric acid. The cumulative K release from the mineral powder in successive extraction procedure was recorded higher with organic and mineral acids, which suggested that the material was a slow release K source. A significant positive correlation was observed between K release by different chemical extractants and the biomass yield (r = 0.93; p < 0.05) and K uptake (r = 0.96; p < 0.05) by plants. Among the different chemical extractants, 1 N ammonium acetate and 1 N boiling HNO3 showed higher correlations (r = 0.91 and 0.96, respectively) with plant K uptake values. Both the chemical and biological methods were able to extract only a portion (12–20%) of total K present in the mineral powder. The results indicated that the mineral powder could be used as a slow release K fertilizer in soils. Further studies in long term applications with exhaustive crops under field conditions are needed to assess its feasibility as a source of K in agriculture.