Irish tenant farmers grew potatoes as a subsistence crop, while the wealthy landlords grew wheat and oats as a cash crop to export. It is in fact a great illustration how growing food by itself will not stave off a famine when the social relation demands that thousands must starve for a few to get even wealthier.

Re: your last point, I have been careful not to blame capitalism specifically in this thread. Wheat was grown as a cash crop in Ancient Rome. The development of the slaver latifundia system leading to the economic marginalisation of the plebeians was a major source of class conflict in their society.