The crop that a farmer reaps is usually dependent on what has been sown. A good harvest cannot be expected from bad seed. However, there are no guarantees that good seed will produce a good harvest; sometimes droughts or flooding may intervene, or someone may steal the grain or poison the crop. Paul had a bit to say about sowing and reaping in the sixth chapter of his letter to the Galatians:
You will reap whatever you sow. People who sow to please their sinful nature will reap destruction from their sinful nature. But people who sow to please the Spirit will reap eternal life from the Spirit. We should not get tired of doing good, because we will reap in due course, if we do not give up. We should do good to everyone when we get the opportunity, and especially to members of the family of faith.
Many people might stop at the first sentence, and claim that if you sow some money, you will reap more money, or if you sow good deeds, more good things will happen to you. However, Paul makes it clear that what you get to reap as a reward for doing good is “eternal life from the Spirit”. This is a far greater reward than any temporal reward, but it does mean that your hopes are misplaced if you are hoping to reap earthly things.