Err, perhaps too low nitrogen due to binding with the breakdown of the eelgrass? (just a guess)

Here potaotes literally grow themselves...and the best potato vine I ever saw was an accident that occured in the hay/manure mix of a steer's pen.

TGO1013, you do have a point about the huge vines which I read and promptly forgot.....

I assume we can assume full sunlight ?

How could I avoid a similar soil condition this year? I'd read somewhere potatoes were grown in a combination of seaweed and soil and thought to try it; doesn't seem to have worked for me. My soil test (UMASS Amherst) doesn't include nitrogen as part of the test. My soil here (Cape Cod) is generally sandy, and I've been using eelgrass and compost as major soil amendments. Should I be doing something different for the potato area? Yes, the plot has full sunlight.

soil test doesn't show N because it is not stable like P and K. what is you organic Matter%. I would just build the soil up plant , weed and hill up with the soil. hilling up 16" seems real extreme to me. Potatoes love to grow in sand.

3 feet above the raised bed doesn't sound unusual to me, so I don't think the problem is excess nitrogen. I had a similar thing happen to me the first time I grew potatoes several years ago. I prepared a 3.5 diameter 3-foot tall "tower" that was open to the earth on the bottom by filling it with a foot of well cured compost and good garden soil before planting my tubers. I filled it with a loose mix of compost and straw that was at least 18 inches deep after hilling several times. The plants grew vigorously, but when they began to die down and I went to harvest, I found only a handful of tiny tubers. Later that year when I decided to dismantle the tower, I found the base filled with roots from my maple trees. I suspect the tree roots robbed the potatoes of nutrients and adequate moisture. May sound weird, but I have found that tree roots can destroy a raised bed. I now grow all my potatoes in large containers and get good yields (10 to 1 or better).