Not much! You wonâÂÂt be able to just add the recommended rates to change a pH of 7 to a pH of 5. One, such a recommendation is for pre-planting amendments, and two, the pot medium brings in some additional dynamics that changes things, esp hydraulics flushing additives out of the medium.

I don't have experience adding sulfur as I acidify my water to manage pot media pH. But this is what I have found in my various readings and saved resources, plus a little more looking.

Sulfur acts a salt in solution, and as such, when added to a pot increases the salts roots are exposed to. So too much isn't just bad for pH, but can burn roots. Recommended maxim rates for established plantings are 300-400 lbs per acre. This translates to 0.006887 to 0.009183 lbs per sq ft (acre is 43,560 sq ft). Or, to a pot 12 inches in diameter, which is .785 sq ft, we are talking about some really small numbers - 0.005338 to 0.007222 lbs per pot.

When I google it, 0.005338 lbs is 0.085408 oz, and 0.007222 lbs is 0.115552 oz.

Per http://aces.nmsu.edu/pubs/_h/h-119.pdf, there are 9.4 ounces (weight) per cup of 90% sulfur.

Now, here is where I probably make a math mistake (if not already some above), I believe this comes down to 1/110th to 1/81th of a cup. (.11552 goes into 9.4 81 times). Googling 1/110th of a cup and 1/81th of a cup, to convert it to teaspoons comes to .43 to .58 teaspoons. So, 1/2 teaspoon is what I come to. Right? Double check that.

But you know what? Most people just scatter a small handful and call it good. If the pH drops too low before summer end, or the plants show signs of salt stress on the leaves, you can always flush the pot with your water, which apparently has a high pH if your bark/peat mix has a pH of 7. In fact, I would expect a fair bit of the sulfur to be flushed over time, AND, the above rates are for a soil depth of 6 inches. So, you can probably go to 1 tsp without any real concerns. Might even do that more than once per summer as it is continually being flushed out every time you water. But again, what do I know? I've never used it for an established planting. :-)

"But you know what? Most people just scatter a small handful and call it good."

That's me. I scatter a few tablespoonfuls of organic sulphur in the spring and then use an organic acidifying fertilizer (Dr Earth's) at least once a month.

I used about a half a cup of elemental sulfur on a 10 gallon pot. This worked fine for me. I would mention though that using peat and pine bark alone lowered the pH to between 6 - 6.5 without the sulfur. Error on the side of using less. It may make take some time to see the results. I'm finally seeing good results on my blueberries but it has taken 2 years of adjustments to get them there. Using sulfur was a key step.

Charina, the texture of soil is an important part of the calculation. Coarser the soil, the more potent the sulfur (or lime, if you are going the other way) Potting soil is extremely coarse so a little sulfur goes a long ways. Peat is also coarse, of course, so in soils where a lot of peat is applied the reduction in need of sulfur works from two angles, not only as the result of pH of peat. But equal quantities of a 4 pH peat and 6 pH soil shouldn't create a 5 pH medium.

I've poisoned trees in pots with too much fire wood ash in the mix I made. I was using compost and peat in the mix and the compost was neutral. The compost was much finer and had more influence on final pH, I believe.

Surface applications always tend to be safer. Although salts leach quickly the pH changing affect will be more gradual from the top down. Even if you miscalculate the affect will be less drastic than if it was incorporated. Especially if the water is gradually pushing pH in the other direction.

I must admit to being in the camp of few precise calculations during a lifetime in the soil. It has rarely cost me. The only careful measurements I usually take are in mixing pesticides or spreading fertilizer (including lime)..