I think its wishful thinking. I'll stay away from commenting on chemical fertilizers made from hydrocarbons to all kinds of more natural fertilizers other than to say there are a whole lot of options out there and some people feel pretty strongly about some of them.
Each vegetable has its own nutrient requirements as well as pH requirements. Some things do better in an acidic soil, some on an alkaline soil, and some need it fairly neutral. Also, they need differing amounts of the different nutrients. I suggest you get a soils analysis to determine where your soil stands. Probably the best way is to call your county extension agent and see what they say about how to do it. There might be a charge. I have not done it for a couple of years, but when I did it was free. A good soils analysis will tell you the pH of your soil and what nutrients it has. Mine usually gives recommendations of what to add and how much if I tell them which specific crop I am growing.
I divide vegetables into three categories based on nitrogen demand. Things that mostly consist of leaves are usually high nitrogen users. On your list, Brussels sprouts, broccoli, lettuce, garlic, onions and cabbage would fall into this category.
Some things don't really need much nitrogen. The legumes like beans and peas pretty much make their own if a certain bacteria is in the ground, and it usually is. It's not a bad idea to have some nitrogen in the ground to help them get a good start, but they don't need much. Too much can actually hurt them by causing them to grow huge vines but not put any real energy into producing seeds.
Then you have the others, like tomatoes and squash. They need some nitrogen to grow and produce, but they do best with a balanced approach.
The basic fertilizer formula gives you Nitrogen, Phosphorus, and Potassium. A 15-5-10 would have 15% Nitrogen, 5% Phosphorus, and 10% Potassium. There are other trace elements that certain plants need more than others. For example, I usually put a little Epsom salt in with the tomatoes to provide magnesium.
As you can see, it can get pretty complicated. I can make it even worse. Some nutrients are more water soluble than others. That means that certain elements will leach away faster than others if you get much rain. Your soil type enters into it too. Sands are pretty much inert as far as electrical charges and dont hold onto the nutrients very well. Clays are highly ionic and will hold some nutrients and keep them available to the plants.
I dont like complicated and I definitely think you can overthink it. I also think there is a pretty wide range of what works. So the way I approach it is to keep three types of fertilizers. My basic one is a balanced fertilizer, maybe a 10-10-10 or 13-13-13. Basically everything gets some of this. Then I have a low-nitrogen fertilizer, say a 5-20-10 for my legumes. Those are harder to find. The hardest for me to find are the high-nitrogen fertilizers that are suitable, say something like a 32-2-3. The reason I say a suitable one is hard to find is that most of them are a weed-n-feed type for lawns. Those are made for lawns and kill broad leafed plants or prevent seeds from sprouting. Im not putting something like that in my garden.
. I have a local source for free composted horse manure and another for free rabbit manure, all we have to do is go shovel it. Most things get mulched with compost, anything that looks like it needs a boost gets side dressed with some rabbit pellets, and the root veggies don't get anything. I'm not saying this would work for everyone, it's just what works for me.
