There's also the pesky detail that if minerals in the soil are taken up into plants, and plants are then eaten by animals, then animals need to go back into the soil we grow our crops in or the the soils get depleted of minerals. That's why most salt is iodized, because we've leached all the iodine out of our croplands and never put it back. There is only so much fossil fertilizer in the world. Eventually we are going to have to accept that we are part of nature instead of separate beings above it and doing things to it. Factory farming sucks and needs to end, but we can't "Just fucking leave animals in peace." we are not separate from them. They are us and we are them.

The acceptable amount = refrain from hurting animals "as much as possible and practicable". That takes care of all the gotchas and the well actuallys.