קצר qtsr "reap, harvest" a rural word, only found most often in Micah, Hosea and Ruth. In Amos the participle, as a noun, is used at 9:13, with the participle קצר "plowman".
קָצִיר qatsir   "harvest" not a rare word, but only found in Amos at 4:7, like the English word "harvest" means either the produce harvested or the time/season when this is brought in.