Minerals are leeched from the field into the plant/animal matter, which you then eat, which you then poop, which then enters a waste treatment plant, which then enters a tributary which then enters the ocean. There is no way for these minerals to make their way back to the field in question, so you have a 'one way' (aka 'open loop') process. If you want a truly sustainable method, you need to 'close the loop' and find someway to get the important minerals and other elements back to the fields so that plants can continue to grow there. Or figure out how to make plants not need the specific minerals.