People complain that potatoes are “only” 2% protein. But …
[P]otatoes are so prolific that you still end up with 500-1000kg of protein per year per hectare of potatoes, versus 164-500kg of protein from soybeans, 98-300kg of protein from wheat, and only 33kg protein from milk produced by cows.
Which is why, where the climate is right, potatoes should be a key component of urban agriculture and home gardens. You do have to eat a lot of them, just be sure not to peel them. From Tom Wagner’s blog.