I'm also concerned about our soils wearing out, and just not providing enough mineral as it is. So, better to get maximum nutrition out of grains. Remember, according to blood types, I'm supposed to eat a lot of grain anyway! Oddly enough it seems very natural to me. If I haven't had a hearty bread to eat, I feel like I haven't really eaten.
It depends on your set up, and soil type and many other factors. how much land you have and other things... but there are many ways to address that. one is having a circular system, like humanure. it can be done safely, but yo HAVE to do it right, or can have issues, and meds and heavy metals can be issues so this isnt always good. essentially you could ensure that little to nothing actually leaves the field like that....
There is also using deep rooted perennials and trees and bushes. These are the things that pull the things from deeper in the subsoil. even breaking down rock long term. use a range of things to try to cover all the bases. Take organic matter from non garden areas from these types of things and put it where your garden or compost is.
Using animals also concentrates many key things in their bones and such, wastes useful as well, but so are the rest of their bodies when the time comes. Or various things like clams and snails that live within my fish systems....
also one night it occurred to me, different plants including weeds concentrate various things. You could purposely sow those plants, let them be and collect them for the compost, thus concentrating these things into the gardens.... This could be a science onto itself really. Obviously you cant pull things from the soil that arent there, but depending on your growing style, you could certainly concentrate key things where they are more needed.
there really is NO need for outside inputs if you set up a circular system. if you have it now, you can keep it. If you have to little of something you can concentrate it where its needed. most systems are not circular and eventually the lopsidedness will show just like any other topic. Even if you start out depleted, you can pull those thing up from the subsoil with trees given enough time, or composting parts of already established things. Un fortunately it isnt easy to find the value for all of these things, or you could set up a simple math problem and regulate it well. but if your concerned about it, just make sure you use things for compost from those types of plants, heck thats how those things got to be there for your annuals to use to begin with....