I subbed some of the BP for soda....as with all the apples and brown sugar the cake has an acid base.....and I think it tastes better. Acid makes the soda "work".....and I like the taste it gives better than BP.
No not always better....but lots of cakes calling for fruit call for both BP and soda...particular old time recipes. BP used to contain an aluminum compound and be a bit bitter....so often with an acid ingredient cake like brown sugar or fruit or chocolate, the leavening could be partly soda and cut down on the bitter flavor. The BP I use is double acting but doesn't contain aluminum sulfate....but when I can I sill sub some soda for some of the BP...
Linda C

OK, thank you, Linda. I'm going to print your reply & put it in my reference file so I don't ask you again next year. :)

Happy New Year!
Kathleen

I was always taught that chocolate cakes, apple spice cakes and muffins etc using butter milk take baking soda and either less or no BP at all......and you must rush to get those things into the oven. And cakes made with egg whites like angelfood and other white cakes take cream of tartar.
It's likely not engraved in stone...but that's a rule of thumb.
My grandmother never measured....and often her cakes were bitter from baking powder. She used an aluminum salts powder Calumet....my mother used a single acting brand, Royal.....she always said that "that other st7uff is nasty!"LOL!