Why does the Torah add the words "asher Yizare'a"?
Rashbam: To teach us that whatever is attached to the ground is Tahor. 1
Ba'al ha'Turim: It teaches us that Tamei seeds that are planted become Tahor. 2
What is the Pasuk coming to teach us?
Why do we need two Pesukim (here and Pasuk 34) to teach us the Hechsher of b'Chi Yutan)?
Ramban, Moshav Zekenim (from Toras Kohanim): The earlier Pasuk is referring to food receiving Tum'ah from an earthenware vessel, whereas here it is referring to receiving Tum'ah from the Sheretz itself.
Ramban #2, Moshav Zekenim #2: Whereas the earlier Pasuk is referring to meat and other food becoming b'Chi Yutan, here it is referring to [edible] seeds that one intends to plant.