Not sure if this is the best site ... But ... If not please forward ,,,, . Tx

2013 was my second year as a potato farmer.
Year one I put my seed potatoes too close to the clay base on my property and (I believe) that the watering and rain created a pool around the potatoes and essentially drowned them � 4X8X2 feet raised boxes yielded less than two handfuls of harvest.
This year I made two 4x4 boxes and put the seeds higher up on a bed of good earth - Ozette and purple somethings.
Vigorous growth. Added five layers of boards each 6 inches high. Almost three feet of 4x4 growing area.
Curious harvest. The top layer of both boxes had potatoes. The bottom layer had potatoes. The middle three layers � almost two feet � had zero potatoes.
As the plants grew, did I cover them with new soil too soon? Thus not giving the new root systems to form? Each time the plants got seven inches high I added another layer of 6 inch boards and added dirt leaving about an inch or so of the tops showing.
Cannot think of why I would only have potatoes at the top and bottom of my boxes.
Your thoughts and suggestions are most welcome as I plan the same for 2014.
Thank you.
john