The subject of this parable is producing fruits of repentance. The particular application is directed towards the Jewish nation which, although some had repented and come to faith in Christ, the majority were hardened in heart. Jesus was doing a take on Isaiah's prophecy mentioned below. The more general application is toward those who are being led through nominalism to true faith in Christ.
Jesus found that his efforts in converting the Jews as a whole to faith in himself was largely unfruitful. And ultimately he would be rejected and killed by them. He was the caretaker of the vineyard. His father was the owner. Despite the barrenness, he intercedes for the Jews, much as Moses had done when God was going to destroy them.
The fruitfulness he was looking for was fruits of repentance, as verse 5 indicates. If a person says he believes, but has no outworkings to his faith, such faith is a dead faith, having no salvific value and being good for nothing. Jesus fertilizes people's faith in a number of ways. To the proud, he'll fertilize with humiliation. To the humble he may provide miraculous signs of healing, so as to affirm their faith. But he always uses his word to edify.
Why should it use up the soil?
Unfruitful people are a drain on the church. Of course it may take a number of years discipling and training new converts to become fruitful laborers. But many times, such effort seems wasted. They become mere pew-sitters, having little or no application of faith, perhaps feeling religious and feeling morally superior to others, like the Pharisees were, but lacking in the necessary humility to produce fruit.
Don't test God's patience. God won't endure the stiff-necked for long. "God does all these things to a man-- twice, even three times-- to turn back his soul from the pit, that the light of life may shine on him." 约伯记３３：２９，３０
"The vineyard of the LORD Almighty is the house of Israel, and the men of Judah are the garden of his delight. And he looked for justice, but saw bloodshed; for righteousness, but heard cries of distress."
|A man had a fig tree planted in his yard
And could find no fruit though he looked very hard
So he said to the man who took care of the tree
"I've been coming here for how many years? Three!
I've found no fruit inspite of the toil.
Why should it even use up the soil?"
"Sir," he replied, "Leave it one more year,
I'll dig around it and fertilize with care.
If then it bears fruit, that will be good!
If not, we'll make it into firewood."