Jan, we too dug out potatoes the other day. Yours looking good!
Thod l share something with you guys. On the same topic.
Althugh not completely fair, l like to judge my potato growing sucsess by the seed vs yeald ratio. So one part seed vs what l get out.
1/5 is considered minimum and 1/10 a reasonable yeald here, from what l managed to find online.
This year, we had terible drought. Potatoes were not watered once.
I had 2 beds.
One was early potatoes, a tiny patch, right next to the Kursk. Its where we had a huge bonfire 2 years back, burning the twigs left over from making charcoal. And there was a lot of unburnt wood in the ground also. We harvested this patch early, most potatoes underripe. They are best that way anyways 
Rototilled. Fertilised with aged manure. Hilled once. No mulch.
Yeald ratio 1/17, but counting in that a lot was imature bushes.
Second feald was bigger. For winter potatoes. Planted later too. Fertilised with aged manure, rototilled. Hilled once. No mulch. But much dryer soil, less organic matter.
Yeald ratio 1/5.
Now, just for fun, l perswaded my friend to a test and he had half his potatoes mulched and half not mulched, he forgot to weight the two separately but in the end, his together yeald ratio was 1/13. He did say mulched ones were much better thugh.
Now, today, we got some winter crops in with the kids. My 4 year old daughter planted a single seed potato in her part of the garden (they each have a small, 1m2 plot) and, althugh it was still growing, she wanted to digg it out.
This garden was never tilled. Never fertilised other thain ash and mulch. This is how it looked like prior to the operation.
And hay mulch retracted. Note, the growing potatoes made a hill!
This is the biggest potato of the bush, and the biggest harvested anywhere this year.
Thats over 9 pounds of food, produced out of a 2 oz seed potato. And some old hay. That, and other complete neglect. And the ratio? 1/90!!!
Now, either the kid has a real big green thumb or there is something real valuable to be learned here. Might be a bit of both…