Hi-Yield Aluminum Sulfate is used primarily to acidify soils. Helps to create an acid soil condition (corrects excess alkalinity) which is desirable to the growth of certain types of plants, such as Azaleas, Rhododendrons, Camelias, Gardenias, Hydrangeas (blue), Junipers, Yews, Cypress, Magnolia, Oak, Pine, and certain others. Can maintain or change the color of Hydrangea flowers. Blue Hydrangea flowers require Aluminum as an acidfier, to retain their blue color. They should not be fed a Phosphate fertilizer due to the fact that it has the tendency to "tie-up" Aluminum. Do not use Aluminum Sulfate on white or pink varieties unless a color to blue is desired.