The soil in my vegetable garden is low in all nutrients except nitrogen. Even though I add slow release granulated fertilizer and composted manure the plants bear very little produce and what I do get is small. What's the best supplement(s) for this condition? I'm about ready to give up on vegetable gardening.
Thank you for your help.

The three primary plant nutrients are nitrogen, phosphorus, and potassium, or N-P-K on the fertilizer bag. If you have enough nitrogen then you might try a 0-10-10 or similar formula. Also a bag of Ironite will give you deep green plants without adding any appreciable amount of nitrogen. You might also want to practice crop rotation to keep from depleting the soil.

https://www.acehardware.com/departments/lawn-and-garden/lawn-care/lawn-fertilizers/7492911
https://en.wikipedia.org/wiki/Crop_rotation

3 additional things to consider are soil pH, soil drainage, and amount of sunlight received.