Gina, to start with your last question first, I do rotate my potatoes and other stuff around, although pretty haphazardly, I'm afraid... Whether it's absolutely necessary, I can't say; but I figure it can't hurt, and different plants use different nutrients and different zone levels in the soil, as well as rotation can help suppress various disease.
As far as potatoes being a slow grower ... where I am I probably should have planted in mid April to early May, but actually planted June 5th this year; although I'll start harvesting small potatoes sometime in July, most will be dug in September.
Keep in mind that the temperatures here are still barely going above 70F (on a good day), and normally might hit the low 90s less than half a dozen times in the summer; the last two years, though, I don't think we've hit 90 at all, and have had very, very few nights over 70F (and this year seems to be shaping up to be more of the same). So I honestly just don't know how it works where you are -- learning to garden there would definitely be a whole new process for me!!
Also, going back to your original post, I don't know about the calcium, either; I pretty much only fertilize my potatoes with a layer of compost added as the plants start growing.
I hope you have great success with the potatoes, and experimenting with starting some sooner and some later can't hurt, in my opinion!
