There would be no better place to ask these questions than at Broken Arrow. A pH of 6.8 is not so high as to require much of an adjustment - perhaps none at all. In any case, no nursery is any more knowlegeable about kalmias than Broken Arrow.

There would be no better place to ask these questions than at Broken Arrow. A pH of 6.8 is not so high as to require much of an adjustment - perhaps none at all. In any case, no nursery is any more knowlegeable about kalmias than Broken Arrow.

If it were a blueberry shrub, I would suggest 3/4 cup of sulfur, half in the bottom of the hole, and half scattered on top of the soil, but under the mulch, right after planting. That would be enough sulfur to lower the pH by a point or maybe 2 points, maximum. Bear in mind that sulfur has to be metabolized by bacteria that live in the soil, so it takes two years for the process to be completed. Here in Wisconsin, where I live, our native soil is high in iron, so I don't ever add iron sulfate or supplemental iron. The shrubs can get the iron they need from the soil, when the pH is in the correct range.

Use the sulfur and iron sulfate when you plant. Mix some in the soil mix when you plant but spread a little over half on top of the soil. It will slowly work its way down. The iron sulfate is quick acting and the sulfur is long lasting. Make sure you plant correctly with good drainage.

Here is a link that might be useful: How To Plant (Rarefind Nursery)

Iron sulfate won't acidify the soil.

For details on how iron sulfate acidifies the soil read:

Iowa Extension

Minnesota Extension

Oregon State Extension

Penn State Extension

Purdue Extension

However, never use Aluminum Sulfate. It is toxic to rhododendrons and azaleas. It is useful for hydrangeas, but not most other plants.

Sulfur is the best material to use to acidify the soil. But it is slow acting but lasts a very long time. Iron sulfate is much faster acting, but doesn't last as long. The combination of the two works very well.