Adding elemental Sulphur is a common practice to lower pH of soil. Ammonium Sulfate can also have the same effect.
I read that Potassium Sulfate and Magnesium Sulfate may not have the same acidifying effect. Do you know why?
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Good question, I understand the confusion about sulphur and sulphate. But chemically they are different and have different chemical reactions. Even different sulphate salts do not react the same, and often reduction in pH is not caused by the sulphate group solely.
For elemental sulphur, it will be degraded by microorganisms in soil. By this reaction H+ (proton) ions will be made which causes pH to drop.
Ammonium sulphate on the other hand, will also be degraded but especially the ammonium (NH4+ ion) will be reduced to nitrate and again H+ ions (also called nitrification). This process is the main cause of pH drop.
Potassium sulphate and magnesium sulphate do not react, but will just dissolve in water. No H+ ions will be released, so pH stays the same.
Another sulphate salt which is also used to acidify soil is aluminium sulphate. Here a reaction occurs (without microorganisms). Water reacts with the aluminium sulphate where OH- is bound to the Al3+ ion and will floc, and H+ will be released again (to form sulpheric acid in solution).
Ammonium sulfate is a strong acid and a weak base , in very basic terms. "strong " means fully ionized ; "weak" means partially ionized so not providing as much acid (H) or base (OH) as a strong component. And in this case the weak base will be absorbed by growing plants leaving only the strong acid ( sulfate ,sulfuric ). Potassium sulfate is strong acid and strong base , but again the K will be picked up by growing plants , leaving the acid. Mg SO4 is a strong acid and relatively weak base . All have an acidifying affect but ammonium sulfate is strongest. Of course there are many other components in your soul which will often have an effect. My sandy ( silica ) soil is very acidic but the silica is neutral , that leaves any the acidic components of the organic materials ( tannic acid , etc) to dominate. In many soils you have limestone particles and clays to balance the organic acids giving a more neutral or alkaline pH .This explanation is like summarizing " Gone With the Wind " in a couple sentences.