I have used dried seaweed from the organic catalogue for 2 years and have found it to be a good growth stimulant. I heard that in Jersey they use it on the new pots, so when I planted the potatoes, I dug out a trench and put in wet straw (cos of very dry free-draining soil), and then a bit of soil and a sprinkling of seaweed and then a little sprinkle of soil and then the spud and then the rest of the soil. We have had the best potato year for 3 years...
sb