Well the farmer doing it himself wouldn't be financially attractive (too big setup costs). But if the farmer is the land owner then he could rent it out to a po

Well the farmer doing it himself wouldn't be financially attractive (too big setup costs). But if the farmer is the land owner then he could rent it out to a power business that are good for the upfront costs. The exact point that would happen is when all the cheaper (non-arable) available land is already covered. That might have already happened in specific locations and countries. Note that at the moment upfront costs (the panels) are so expensive it will be a long time before solar panel 'fields' would become any significant fraction of total arable land. I.E. a simple few acre farm field might be worth a couple hundred thousand euros, but totally filling it with panels might cost billions.