The farmer is a Christian.  The seed is the Gospel of Jesus Christ. This is the biblical Gospel: God is loving and just, man is sinful and separated from God, Jesus died to assume the wrath of God for sinful man and was resurrected, man must respond in faith (not intellectual assent but trust, surrender) to what Christ has done.

The soil is the heart of the lost.  When the farmer sows the seed he sleeps.  He can rest in the power of the seed producing life in the soil by itself, and he doesn't fully understand how it happens - it just does.

When the seed has produced full maturity, then the harvest comes.  

Application: We are to share the Gospel with others but we cannot manufacture or manipulate a response of faith in them. We simply plant the seed and trust God for the results.  Because the Gospel is the power of God unto salvation (Rom 1:16) and not our powers of persuasion, we can rest in that.  We don't have to lose sleep over our ability to "sell" the Gospel, we rest in the supernatural power of the Gospel to transform lives.

When a person is "ripe" then that person can be "harvested." If fruit is ripe, you don't have to yank on it. When a person is ripe to respond to the gospel, you don't have to yank on them to make a decision. The power of the seed of the Gospel will spring forth to full maturity.