Are your plants yellowing despite a correct watering program? You may need to add iron to your soil. Copperas Iron Sulphate from Hi-Yield relieves iron deficiencies in the soil and increases acid levels. For trees: apply 1/2 lb. per inch of trunk around drip line. For shrubs: apply 1 lb. per 2 ft. of height. Caution: Will stain concrete and pavement!