How long will SO4 last?
The lasting is determined on what you are trying to accomplish. Normal recommendation is to use annually until the desired affect is accomplished. Dropping or reduction of a high pH will be accomplished by the addition of an acidifying compound WITH the application of SO4, and then it will still be a long process with monitoring needed. Addition of needed nutrients such as C and S in the sulfate form will be needed on either an annual basis or every other year, depending on the amount required.
