Luke 8:11, "Now the parable is this: The seed is the word of God." (read the rest of the parable) 

This parable illustrates the importance of the Word of God. God's Word contains power, but it has to be planted in our hearts and allowed to germinate before it releases that power. 

The seed is God's Word, and the types of ground are the four major categories of men's hearts. The Word was the same in each situation, but there were different results in each case because of the condition of men's hearts - not because of the Word. The Word doesn't work for everyone because not everyone will allow the Word to work. The variable in this parable is the condition of these hearts. God's Word is always the same. It has the same potential in every heart. 

Good ground doesn't just happen; it must be cultivated. This is the reason why only one out of four persons in Jesus' parable brought forth fruit. It takes a lot of time, effort, and diligence to be a fruitful Christian. The Christian life is not like a hundred-yard dash but rather a 28 mile marathon. It's quicker and easier to raise weeds than it is to raise tomatoes or corn. 

In this whole parable, it was the Word that produced the fruit. The ground simply gave it a place to grow. If we will simply put God's Word in our hearts, protect it, and give it priority in our lives, the Word will produce fruit of itself. Satan has deceived many people into thinking that they don't have the talents or abilities to be fruitful Christians, but they are not the ones who bring forth fruit; it's God's Word. When we protect the Word sown in our hearts, it will do the rest.