Mine probably wouldn’t grow in 6.5 either. That is the top 10"- the point is that the soil doesn’t have to all be strongly acidic. Whitcomb’s research proved a long time ago that pin oaks quickly respond to a surface application of aluminum sulfate even when the pH hasn’t been altered below a very thin layer of soil on the surface.
I discovered a similar thing happens with blueberries by accident. For years I assumed my blueberries were thriving in nearly neutral soil until it occured to me that I’d never tested the soil below. That’s when I found out that 10" down the pH dropped a full point.
Years after posting this on the NAFEX site, my son pointed out to me that Michael Phillips had given me credit for discovering this in one of his books. That was fun because it made my son proud, although he’s the only one I know who noticed. He’s never read a single Phillip’s book or any book on gardening or horticulture but he searched my name on the internet.
This can be useful information for blueberry growers if they have older plants that have an iron deficiency. A surface app of sulfur barely incorporated in the soil with some wood mulch over it may be adequate treatment. If it’s there the roots will find it. That is knowledge with a broad range of use.
I agree that blueberries are often helped by growing in raised beds- here they commonly grow in marshes on hillocks. It is also often easiest to grow them in an artificial mix rather than use a neutral or alkaline existing soil- especially if that soil has a lot of clay. Excellent drainage is as crucial to blueberries as ample free iron. But finer soil is also more resistant to pH change.
