I need to improve the output of my vegetable garden. What can I add?

take a soil test to your home county extension agent. they can give specific recommendations as to what to add to your soil....Look in the yellow pages of the phonebook for your county extension agency.

Check for soil compaction, trace minerals, proper sun, proper drainage. It may be you're not lacking anything that can be added!

Manure! This is the best fertilizer I've ever used to increase production.

if you use a manure that has not been composted, you plants may burn, as the urine can change the ph of the soil. yup the pee comes with it. just drive by a cattle lot and sniff.

https://www.planetnatural.com/vegetable-gardening-guru/soil-quality/