[jqm]

for a 120 pound adult....

But I weigh over 200 pounds. So 9+ potatoes a day from me to get all the essential amino acids. That's a pretty high starch to protein ratio and would be nearly my whole intake in calories to get a bare minimum of amino acids. Essentials aside, is that enough protein? Also at that point I would have fulfilled my daily requirement of calories but be still be short vitamins A, K, E, calcium, selenium, fatty acids and who knows what else.

That aside, potatoes are one plant. I don't dispute you can eat certain plants or combinations of plants to get all essential amino acids. To restate "the idea you can get sufficient protein by eating just any old plant is... wrong". It is. You'd be in serious nutrition trouble if you tried to live on just broccoli or lettuce. You have to carefully balance your diet as a healthy vegetarian and it is far easier just to add some animal protein.

For the record I still eat a "mostly" vegetarian diet. But not completely. And I feel a lot better and have more energy than when I did ate a full vegetarian diet.