Either:
* the ending was taken from clam,
* it is a locative Proto-Indo-European *pl̥h₂emi or *pl̥h₂ēm,
* it is an accusative Proto-Indo-European *pl̥h₂eh₂m
all from Proto-Indo-European *pleh₂- (“flat”).
Cognate with Old Church Slavonic полѥ (polje) (whence Bulgarian and Russian поле (pole, “field”)), Old Armenian հող (hoł, “earth, soil”), German Westfalen.