As I stated earlier, it apparently isn't necessary for sulfur to be well incorporated for it to have a dramatic affect on the availability of iron. Dr. Carl Whitcomb conducted several experiments with iron deficient pin oaks. He discovered that a surface application rapidly corrected the problem even though only the very surface of the soil had been altered. Feeder roots got the iron that was needed from the surface even as the majority of the root system continued to function in an unaltered soil.