Potato Fertiliser.

Andreadon- Earthing up prevents your potatoes going green.

Aidy- You want the root system to go down and establish a deep root as this will benefit the plant in dry summers. The potatoes will grow towards the surface with the energy/ feeding from the soil. We place our nitrogen below the potatoes and the potash is broad cast on the soil before hand. but in the past all the fertilizer was placed below the potato.
You can spray seaweed onto tha Haulms to help thing but we found it to be of little benefit. Does that help?

Yep, sounds logical. I must admit to not seeing the roots below the seed spud at harvesting, having said that I probably have never paid much attention. Time for an experiment I feel. Cheers