Our Lord is very clear when he teaches this parable, the seeds are the word of God and the earth is us; each one of us can decide to be a good land, holding the word (seed) and doing the work in obedience, denying ourselves and doing the work in the world, bearing good fruit ... we must hear with our hearts what God says in his word. There are people who have the "undercover" understanding and can not "understand", not because the Word is difficult to understand, but because their hearts are thickened, hard, they do not allow the seed of the Word to grow.
Blessings brothers!