This is a long article and I recommend doing a copy/paste to add this to your prepper documents.
https://modernsurvivalonline.com/diy-fertilizer/I found good suggestions on what to use as your DIY fertilizer components.
Some suggestions will NOT be available, post-event, so either plan on stocking these or using a substitute when they are not available.
DO NOTE the need to be able to test your soil to identify missing or inadequate nutrients. In a post-event world, that means you have stockpiled the needed test kits and/or you have a good agricultural chemistry background. You prepper library should have the necessary reference books for this subject.