Soil amendments can be tricky; this is more due to the fact that soils and fertility contain so many variables. The best starting point is to have a complete soil analysis taken, and if your an irrigator a complete water analysis will also be helpful. Knowing only the pH or phosphorus and potassium levels is not enough to determine proper fertilization.
Once you have your completed soil analysis back, here is a list of items that can give you an idea if SuperCal SO4 will increase your yields.
1. When the soil pH is over 8.2 and maybe even if it is less.
2. When the subsoil pH is lower that 5.
3. When the soil particles disperse when water is added.
4. When water puddles on it.
5. When there is water logging in the soil.
6. When the soil crusts after irrigation or rain.
7. When there is excessive cracking of the soil after irrigation or rain.
8. When the soils contain clays that swell and contract.
9. When the soil contains clay that is very dusty when dry.
10. When intense rain falls on soil that is not acid and where nearly all solutes may be leached from the soil.
11. When no-till is used.
12. When organics (manure, composts, etc.) are simultaneously applied to the soil with gypsum.
13. When crops require the development of fruits or seeds.
14. When sulfur is deficient.
15. When exchangeable sodium percentage (ESP) is over 3 and defiantly if it is over 9.
16. When irrigation water contains substantial amounts of bicarbonate.
17. When irrigation water contains less than 200 parts per million of salts in solution (<0.3 mmho/cm).
Some of these problems can be corrected with as little as 100 pounds per acre, while others may take a few hundred pounds per acre. If your farm has one or more of the above conditions SuperCal SO4 deserves to be part of your regular fertility program.
Taken as an excerpt from Soil Conditioner and Amendment Technologies Volume I. This book and others on soil management can be found here.