How to Fertilize a Black Locust Tree

1). Perform a soil test to assess the levels of potassium and phosphorus in your soil. Potassium is found in organic materials and fertilizers; phosphorus is contained in fertilizer and bone meal. Compare your results to the chart that comes with the test to assess what nutrients your tree may need.
2). Incorporate bone meal into your soil if the test indicates the tree needs phosphorus. Add ½ cup per 1 inch of trunk diameter. Work the bone meal into the top 5 inches of soil with a shovel.
3). Add compost to enrich the soil with potassium. Compost is a rich source of organic material with a high amount of potassium, but it is also high in nitrogen. Only add it if the soil is severely depleted of potassium. Mix in 3 inches of compost to a depth of 6 inches.
4). Water after adding any soil amendments to start the nutrients leaching through the soil to the roots of the tree. Water slowly so the water has time to seep into the soil. Irrigate until the soil is saturated to a depth of 6 inches.