Oh dear, you mean my practice of fertilizing my potatoes is not a good thing? Try to tell the taters that!!! It is my opinion that nitrogen and water are the two things most often in short supply in the garden. My whole garden gets a shot of urea, 42% nitrogen, at time of planting and a side dressing later. This is in addition to all the organic matter I can find to go on it too. I keep hearing things like: "Nitrogen makes the potato grow lots of leaves at the expense of roots." Well, I am here to tell you it takes leaves to make the food that is stored in the tubers. More leaves = more food.
I could go on about this, but the only way you can find out for yourself is to make two plots. Plant both with potatoes, everything else being equal, but fertilize one well at planting and side dress it 5 weeks later. Then measure the weight of tubers that you get from each plot at harvest.
Try it with carrots too.