Too Basic

As a follow up to my last post on our struggling redwoods. I've performed some tests of the soil pH

So somewhat ruling out the pot size for now, the temperature and weather didn't really seem to be a factor. We had a really mild winter this year, almost no snow, and not too cold. Again both redwoods have been indoors, and sitting by some windows, so they probably never get much colder than around 60 degrees or so. The only other thing I haven't really checked, is the pH level of the soil, I read somewhere that they need a slightly acidic soil (pH 5.5 - 6.0).

From a a local garden supply store I bought a pH testing kit with 10 uses. Its pretty simple to use. Just put some soil into the test container, add a special capsule of grey powder and mix with water. I took a sample from B2, shown here before mixing.

Here is the result after about a minute of settling. The color didn't come through great, but it was a fairly dark green, looking to be around pH 7.0 - 7.5, which is too alkaline according to the info I have above.

According to the kit anyway, there isn't much you can do to change a soil that is alkaline to more acidic, though they do mention some: aluminum sulfate or iron sulfate as ways to lower pH.

Will any of this work for B2?