Jay Angler wrote: "it isn't a soil building crop if you take any product from it. The whole plant must go to soil building." Does this sound right to all you permies?
I think at the extreme or theoretical level this is 'correct', but misleading. Yes. In a closed system all minerals are finite. Plants health is more than just minerals. Exodates from microbes, worm castings from the digestion of organic material, etc... go into the equation. In real terms your plants will benefit from the cycle. If you are truly 'building soil' then yes. It would need to be a closed system. However, that is not the way life and the natural world operates.
You may be interested in reading up on the Jena Study done in Germany. There are good youtube videos. The upshot is they planted test plots in the same field side by side. The more variety of plants that were included in a plot the higher the yields. It was mostly a linear relationship, but they found maximum yields above 16 unique plant varieties. Much like companion planting, it appears the overall health of the soil improves the more species you have contributing to the health of the soil. So if it were just about cycling one plant the yields would not see variation in yields. There is something beyond 'just the mineralization'.