Halifax Fertilizer 15-0-15 50% XCU 15-0-15 50% XCU Fertilizer 50 lb - This fertilizer helps aid lawns that are lacking in nutrients. It contains 15% potassium that is known for increasing growth while promoting stress resistance. This fertilizer contains 50% XCU. XCU slow-release fertilizer provides gradual, steady nutritional uptake for up to 10 weeks of plant response.