Worker Of The Ground - Genesis 4:2-7

Genesis 4:2-7 ESV 

And again, she bore his brother Abel. Now Abel was a keeper of sheep, and Cain a worker of the ground. In the course of time Cain brought to the Lord an offering of the fruit of the ground, and Abel also brought of the firstborn of his flock and of their fat portions. And the Lord had regard for Abel and his offering, but for Cain and his offering he had no regard. So Cain was very angry, and his face fell. The Lord said to Cain, “Why are you angry, and why has your face fallen? If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it.”

The fruit of the ground was an unacceptable offering to God. The ground had been cursed due to disobedience of Adam. Cain knew this, yet brought that fruit anyway. 

Doing well is following the will of God. Not doing well is doing something else.