The first of the first fruits of your ground you shall bring to the house of the LORD your God (vs. 26a; see 23: 19a). (religion-online.org)
Maybe I am reading too much into it, but fruit of the ground indicates to me Cain gathered fruit that had fallen from the tree onto the ground. (redeeminggod.com)
3 In the course of time Cain brought to the Lord an offering of the fruit of the ground, 4 and Abel brought of the firstlings of his flock and of their fat portions. (religion.blogs.cnn.com)