Q.adding nutrients to the soil
I usually grow a variety of vegetables such as tomatoes, eggplant, peppers, etc. with varing results. Usually tomatoes do the best. What nutrients will provide the best results on a regular basis for all the vegetables. I was thinking of bone meal, but what about iron, magnesium etc.
I recommend not thinking in terms of a limited number of individual plant nutrients, but use a complete organic fertilizer that supplies as much of the full range of mineral and biological amendments as possible.
Agricola's 4-8-4 is the best I know of:
Next best may be Dr Earth and Down To Earth All Purpose or vegetable blends: