I believe that hydrangeas have that quality, but how does that help vegetable growers? Also determining the degree of acidity and alkalinity might not be known very well.

I guess that is true but it is usually a good idea to send out a sample to find out everything else anyway. I pay about 8 bucks per sample to test for the usual stuff,P,K, Mag, cal,ph.

It's true, but breeding is stretching the expression of the colors in some modern Hydrangeas so people in high/low pH gardening areas can more easily get the color they want.

It's still neat, though not ideal as a pH indicator besides a basic understanding of what may be going on in your neighborhood/yard based on how they're growing.

Some Hydrangas, macrophyllas, will blossom blue in an acid soil and pink in an alkaline soil but not all. Some of those that did express that trait have also been bred to not do that.

Someone needs to breed a very accurate one that changes colors every 0.5 pH units. They could call it Litmus Hydrangea.

Or Phydrangea.

Fwiw, the expression is tied to aluminum availability. At low pH aluminum is abundant for plant uptake and it leads to the "blue" expression. At high pH, Al is generally tied up/unexchangable in soil and leads to the "pink" expression.

Technically you could saturate a high pH soil with high levels of aluminum and get a blue (or blue-ish) expression even though the pH is still high. That said, this would be a very rare thing to find in a natural soil and would be a result of pollution. Also, fwiw, once this plant available aluminum in the soil is "used up" the new flowers would flush pink and some of the older flowers may revert to pink.

Aluminum and pH influence is neat...especially for a highly toxic plant (and human) metal.

also, lol Phydrangea

Perhaps pHydrangea then. :-D

Thanks nc, I have one and did not know it was related to aluminum availability.