Hi Gillian and thanks for your question about your maple leaves coming in red. There are multiple factors that can cause a maple to give red leaves. It can be a sign of environmental stress, depletion of soil nutrition, over- or under-watering, planting depth, or temperature.

As you took the pH of the different areas and the other trees seem normal, I'm going to guess it might be an iron deficiency. This doesn't mean the soil is lacking in iron, it is more likely that the lower pH doesn't allow the maple to access it. I would suggest your first thing to try is raising the pH around the tree with an addition of lime (see attached website for details).

You don't say why the pH in your different areas is so different, but the cause of that might be something to look at as well. Since it is the mature maple changing leaf color, it is not likely to be temperature or planting depth, so the other factor to consider is whether the tree got enough water last year through our dry, hot summer. Even our native trees need a couple of good soakings during the summer these days, so keep that in mind as we progress through July and August.

Hope this works for you!