I have always heard this talked about as if the type of soil cannot be changed. However, this commentary gave me much food for thought just by looking at Palestinian farming customs.
In first-century Palestine, it was common for farmers to sow their seed first, and then go back and plow the soil. In this way, the seed could be mixed in with different types of soil, and some hard or rocky patches of soil could be broken up and softened, helping the seed to bear greater fruit. While some of the soil may not be the most fertile at the beginning of the process, by the end, it has a far greater chance of supporting the life and fruitfulness of the seed it has received.
In a similar way, none of us should think that because we see hardness or difficulties in our lives now, that we are beyond hope of change, or that it's too late for us. God can "plow" us up at any time, making us more receptive to the work he has sown in us and more able to bear the abundant fruit that his seed is capable of producing. We should always keep our eyes and ears open, looking for ways that God may be trying to work a greater softening in our hearts, a greater receptivity to his word.