Newer home owner here. I have these cracks on the foundation of the house. Behind them is a crawl space open to dirt. How should I fix or address these? Do they need to be fixed? I try and DIY as much as I can so don't be afraid to suggest a DIY solution. I'm also not above getting professional help, I just haven't had to thus far.
https://en.m.wikipedia.org/wiki/Lime_mortar
If its line mortar replace it with lime mortar otherwise you will have a foundation failure down the line. The lime mortar is a sacrificial material. If you use normal cement, moisture will instead pass through the rocks and eventually those will fail. Old houses are all about sacrificing materials to maintain the bones of the house.