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?

2 Answers 2


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).

  • 1
    Sorry for the dumb question, but why is ammonium reduced to nitrate rather than oxidised?
    – Dan
    Commented Dec 15, 2020 at 16:28
  • 2
    @DanSheppard, dumb questions don't exist. Oxidation is certainly used by nitrifying bacteria, see here. The bacteria gain energy with this process.
    – benn
    Commented Dec 15, 2020 at 17:07
  • Strong bases and acids like Potassium and chloride will always ionize in water . Sulfate is a little different with 2 H ions; the first H always ionizes in water, the second one ionizes at pH 4. Similarly for phosphoric acid with 3 H ions ; the first one always ionizes i water , the second H ionizes at pH 4 , the third H ionizes at pH 8. Commented Dec 16, 2020 at 16:33

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.

  • No offense, but I don't understand your reasoning. You say for example that K2SO4 is a strong acid and strong base. You say that when K is absorbed by plants it leaves a strong acid (sulfate). My definition of an (Arrhenius) acid is that it can donate H+, how can SO42- donate H+? Please explain it in layman terms, because I do not understand how this would work.
    – benn
    Commented Dec 18, 2020 at 17:05
  • The SO 4 doesn't do anything , it just hangs around as an ion. The plant absorbed the K and electronic balance must be maintained ( + must equal -). So the K+ ion is replaced with a H+ ion. Then the ions in the damp soil are 2 H+ and one SO4- - . Commented Dec 18, 2020 at 19:35
  • Okay, thanks. Still wondering where the H+ come from. And why are you so sure there are plants that need only K and not SO4?
    – benn
    Commented Dec 18, 2020 at 20:33
  • Plants need many things including K. . I have seen a technical article on human stomach acid which is strong hydrochloric acid. It describes that there is sodium chloride present and special cells pick up the Na ion ,sent it into the blood and replace it with a H+ ion in the stomach to make strong acid (; I don't understand it ) . I expect plants do something similar. An agronomist told me plant root hairs are very versatile , for example :The can solubilize ferric oxide ( not water soluble ) and absorb it using enzymes Commented Dec 19, 2020 at 0:30
  • I happen to be biologist, I read literature where they explain uptake of K in plants. It seems that most K is entered the plant via voltage gated channels, which do not replace the K+ with H+, like you suggest. There are also symporters that do exchange K+ for H+, but also that exchange K+ for Na+, Ca++, etc. It depends on what's redundant in the plant at that moment. Like said the non exchange (VG channel) method is mostly use, so no help there with pH drop.
    – benn
    Commented Dec 19, 2020 at 10:21

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