How can one neutralize acid in soup?
Like with to much salt in a soup or stew, throw a peeled whole potato or 2 in the soup and cook till tender remove and discard. this will cut it a lot.
Or as with spaghetti sauce you can add a spoon full of sugar to cut the acids effect.
Figure out how to add more base ingredients. Most veggies are base (I think).
Beyond that, my mother would add a pinch of sugar or brown sugar. It gave it a, "What is that mystery ingredient?" kind of a flavor.
Also, try adding more stock or water to dilute, or more whole ingredients, like cooked lentils.
Try a pinch of baking soda, though not too much. Will make it bitter. Tastes good in addition to a little sugar.
I am wondering what kind of soup this is? All of the suggestions are good, but unless a soup has tomatoes or you have added vinegar, I don't know how it would be acidic in the first place. However, another suggestion for mellowing the flavor is to add some milk. This would work for a veggie soup, chicken noodle, or something like that. My babysitter used to add milk to get the kids to drink more milk, but it also makes the soup milder in flavor.
I tried the baking soda idea for a dessert I made with blackberries which were very tart, and it turned the color blackish instead of the deep reddish purple it should have been. Tasted okay, but ugly! So do a small test first with any plant materials, maybe?
Just to explain what happened with kimhis blackberry dessert -- you have gotten a chemical reaction to the base, baking soda. The blackberries are obviously an indicator, and turn dark. Some things react with acids, some with bases. Tea turns lighter with acids. Red Cabbage is another indicator food. In the science lab we use litmus paper, and other types of papers and solutions.
Just as aside, from work I have done with my students in the lab, most cleaning products are base; most foods are acidic or neutral.
This is really off the topic of soup, but I thought people might be interested.