Krzysztof:
I beg to differ, I'm from Masovia, between Łódź adn Warsaw, and kartofel is the word used here in everyday speech. Of course people know it's not the "official" word, so in shops or in written texts we use "ziemniak", but in a non formal situations, in spoken language, most people say "kartofel".
I didn`t know about it. It is very interesting, indeed.
It looks Poland is too big and versatile to know correctly every detail. :):)
The area I'm talking about hasn't been influenced by Germans much (it was Russian territory under XIX century partitions).
Yes, it was Russian but there were also many German and Jewish people in the area, e.g., in Łódź.