I used to keep a pony on land that had been used to grow organic potatoes prior to it being put down to grassland. The owner of the stables was told not to graze sheep on the land for at least 7 years as there was too much copper in the soil - how can that be healthy?
Erm, I have not said it is, so please do not imply I have??
Your point therefore is mutted as such, but what your comment does indicate / show - is possibly the farmers growing the potatoes have rendered the land not fit for certain sheep grazing use by making it somewhat toxic through using compounds that contain high levels of copper through potentially treating blight for example.
To be certain of the toxicity levels though - the owners would need to test to land and not simply go off word of mouth to ascertain the levels in the soil.
Additionally, the use of copper containing products in organic farming is being monitored closely from what I can gather, so it may face more severe challenges in the future if what I am reading is to be taken as correct - but maybe someone who farms organically may be ale to give a clearer understanding, as I am not a real farmer..