The seed is the Word of God, and its hitting the ground is not the end of the story. A variety of things can happen that will affect the growth of that seed. Some might fall on stone, others might be buried too deep. Rain may come and wash away some of it. Birds may devour others. But because life is in the seed, something will happen.
In the last century, archeologists found wheat and cotton seeds in some of the burial chambers that they excavated. Those seeds—which were probably anywhere from 2,000-4,000 years old—grew when put into the right soil. The spirit of life was still in them, even though they had lain dormant for thousands of years.
This is dramatic confirmation that, if a seed is sown, it will do something when it lands in the right kind of soil. Jesus shows in this parable that the environment affects the seed's growth. When we make the proper application—people are the ground, and our environment and what we do after receiving the seed—the word of truth, containing the doctrines—is what affects its growth. In this analogy, growth represents sanctification, which is the formation of God's image in us by living His way of life empowered by His Spirit. What we do with the seed is "work[ing] out [our] salvation with fear and trembling" (Philippians 2:12). It is the equivalent of rain, sunshine, weeding, fertilizing, so that the potential for fruit is the greatest. Sanctification is worked out through application, by living the doctrines and the truths of God.