Add 3 to 4 lb. Compro would seem to be the commercial compost of choice to add to soils that are extremely acid and are not to be planted with ericaceous plants, but it would be better to add another commercial compost to those soils in which ericaceous plants are to be grown to reduce the amount FeSO4 that would need to be added to drop the pH to a desirable level. Do not work the soil when it is wet. Try not to step on the bed once it is tilled. Consider double digging the herbaceous perennial bed or border.
Double digging is difficult and time consuming, but well worth the effort.
Test the soil for soil type, pH, and nutritional status. This is done after incorporating the organic matter, because adding organic matter changes the soil type and the soil type dictates the amount of pH modifying agents that need to be added to effect the desired change. Add and incorporate pH modifying agents and nutrients as dictated by the soil test. If it is desired to garden organically, incorporate an organic fertilizer such as cottonseed meal, blood meal, or tankage to supply nitrogen and other elements. In lieu of chemical fertilizers use bone meal and rock phosphate for phosphorus, and greensand, wood ashes, and granite dust for potassium.