This is the 4857th most frequent Polish word.
ostrowskim
"Ostrowskim" is the locative or instrumental form of "Ostrowski," meaning "related to Ostrów" (a place name) or "Ostrowski" (a surname) in Polish.
In this sentence, 'ostrowskim' is used as an adjective describing the district ('powiatem') of Ostrów.
Przejeżdżaliśmy przez ostrowskim powiatem podczas naszej podróży.
We drove through the Ostrów district during our journey.
Here, 'ostrowskim' functions as an adjective describing 'folklorem,' connecting it to Ostrów.
Interesuję się ostrowskim folklorem i tradycjami lokalnymi.
I am interested in the Ostrów folklore and local traditions.
In this context, 'ostrowskim' is an adjective modifying 'krajobrazów,' emphasizing their regional origin from Ostrów.