After much internet research, I came up with a plan to fertilize the potatoes. I followed the advice of one website and worked 1 Tbsp of 10-10-10 into the soil six inches from the stem of each plant yesterday (7-6-10). Thankfully it also rained profusely last night so I didn't have to worry about watering to distribute the fertilizer. The site I found recommended applying the fertilizer 6 weeks after planting, which was just about right (ok, a little late, but it was a busy week last week!). I went back to the garden to weed today and didn't notice any dead or over-fertilized foliage, so that's a good sign!