Martinca: I tested sulfate of potash NPK 0-0-50 for the past 8 years .. this has 18% sulfur. also Muriate of potash (potassium chloride) at NPK 0-0-50 for 1 year, also Monopotassium Phosphate NPK 0-52-34 for 1 year. My clay is exceedingly high in magnesium so I also tested red lava rock as a source of potassium. Here's the info:

The Langbenite natural mineral with NPK 0-0-22, sold cheap on Amazon for $15 per 5 lb., it has 22% potassium, 10% magnesium, 22% sulfur, and only 3% chloride (chloride is the salty part and plants break out in rust when the chloride is high and potassium is low). The salt index of sulfate of potash is 43, salt-index of muriate of potash is 116, and salt-index of monopotassium phosphate is 8.4, data taken from University of IL. extension.

High potassium fertilizer at 5 lb. for $15, with NPK 0-0-22

The above Langbenite is cheaper and lower in salt than SOLUBLE sulfate of potash, see below link: "Potassium sulfate (K2SO4) is usually manufactured by reacting the chloride with sulfuric acid. Sulfate potash magnesia (K2SO4 · MgSO4) is essentially a mixture of potassium sulfate and kieserite with 18% K and 11% Mg. This is a useful fertilizer to apply to nonchloride crops when there is a need for Mg."

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/potassium-fertilizers

Here's the results in my garden with the above sources of potassium:

With extremely high potassium: sulfate of potash NPK 0-0-50 with 18% sulfur used on my HIGH MAGNESIUM CLAY. I used a large dose of 2 TBS (diluted in water) to reverse blackspots on Golden Celebration. Jess in South Africa with acidic clay also duplicated my result of reserving blackspots with the same dose on a large bush.

To induce blooming using NPK 0-0-50 (potassium at 50), I use 2 TBS along with gypsum for more calcium & sulfur. Here's the result of 40+ blooms on Frederic Mistral in spring flush. Even with 18% sulfur I had to use vinegar to dissolve it in my alkaline tap at pH 9.

If you don't have much acidic rain, then best choice is sulfate of potash NPK 0-0-50 (potassium of 50), if used at 1 tablespoon per bush, it will induce blooming, but not to the extreme of cluster-blooming like above pic. Its 18% sulfur made leaves dark green and shiny.

Monopotassium Phosphate NPK 0-52-34 has zero sulfur, so it was even harder to dissolve in tap-water, I had to use 1/4 cup of vinegar and 1 TBS of fertilizer per 3-gallon bucket of water. It did make Sonia Rykiel break out in more blooms, so instead of one bloom per branch, I get 2 to 3 blooms per branch. The quality of blooms is better with monopotassium phosphate NPK 0-52-34, but afterwards THIS CHEMICAL HARDENED MY CLAY. Phosphorus is even more INSOLUBLE than potassium, and this chemical has zero sulfur to soften my clay. I don't recommend this if someone has alkaline clay.

Muriate of potash (potassium chloride) is 3 times more SOLUBLE than sulfate of potash, data taken from an on-line agricultural source. This also has NPK 0-0-50 but no acidic sulfur. It's also very salty at salt-index of 116. I tested it during heavy rain. I tested it as granules, along with gypsum and it worked great in making blooms larger, see below result. If one surpasses 2 TBS of this fast-dissolved chemical, one gets brown edges on light colored blooms. Below blooms were achieved with 2 TBS muriate of potash along with 1 TBS gypsum sprinkled around the bush, done once a month.

Langbenite has 22% potassium, 10% magnesium, 22% sulfur, and only 3% chloride. The 10% magnesium helps to retain potassium in the soil, rather than being flushed out. Magnesium is alkaline & hard to dissolve in tap water, so please inform as to how well Langbenite dissolves in tap water. Thanks.

Since Langenite has 1/2 amount of potassium compared to sulfate of potash, 1 tablespoon might not be enough to induce blooming, please let me know if 1 TBS UP blooming plus making leaves dark-green and glossy. Langenite's 10% magnesium is comparable to my using sulfate of potash with 50% potassium in my high magnesium alkaline clay.