Luke 8:11 explains that when the Scripture talks about seed it is referring to the word of God. Its one of those key verses like Rev. 1:20 which explains an object picture that God uses to describe something spiritual.
The parable of the sower in Matthew 13, Mark 4 and Luke 8 is an important one to understand (Mk 4:13). Seeing our souls as a garden enclosed (Song 4:12) helps. It is a very large garden capable of growing all sorts of trees and plants. Just like any garden, if it is left untended it will grow weeds. The best thing is to clear it, break it up (Hosea 10:12) and sow the word of God.
However there is a more general sense to the parable of the sower. If we are sowing the word by preaching it (Romans 10:14,17) then we can probably expect 4 types of hearts upon which it will land: hard ones, stony ones, congested ones and noble ones. If we look at the parable of the sower in isolation then it would appear that only about a quarter of the people who hear the word produce fruit.
But Jeremiah 23:28-29 gives us some idea of the power of that word and some hope to those who may think their heart is hard, stony or clogged up with the cares of this life:
“Is not My word like a fire?” says the Lord, “and like a hammer that breaks the rock in pieces?”
The fire burns up the thorns and the hammer breaks up the path and stones. A seed can have tremendous power in even the most adverse conditions. God’s word is even more effective in the heart.