Actually, ash is a relatively weak base- while the solution may test to a high pH, it'll only take a wee bit of acid to knock it back down. As your plants showing you, this'll work- but it's a long, drawn out fix.
Benefits of commercial pH up-

Higher mol weight- it will nuetralize a much greater amount of acid per application, and retain buffering ability much longer in the soil.

Replicability- wood ash is inconsistant- esp in it's chemical properties. Trust me- I've used ash as a buffer on gigantic scale. It's only advantages are price and availability ( as you can often dispose of ash waste from other operations, killing two birds.)

If yer married to ash, don't use the particulate from the coal bed of the fireplace, or the base of the fire- the fly ash is much more consistant, more soluble, and more potent.


By soil lighteners, she's ( I think) talking about perlite and the like.