Planting in the Ger’s field is not considered a Kinyan, since it has no immediate benefit, but plowing is a Kinyan. Why would we differentiate between planting and plowing which are both considered Melachos on Shabbos; and although planting is merely preparation for future growth, so is plowing merely a preparation for future planting (see Rashi Shabbos 73b D”H Guma)?