Mark 4:26 And he said, So is the kingdom of God, as if a man should cast seed into the ground;
Listen to Jesus speaking: We already know in a parable the seed means God's word and the ground is a heart. So, the kingdom of God is as if a man should speak forth God's word into another's heart. Our responsibility before God is to broadcast his word.
Mark 4:27 And should sleep, and rise night and day, and the seed should spring and grow up, he knoweth not how.
The seed of the word will grow in a heart, depending on the readiness of the heart. It is of God, the work of God, not something we do, so we rest in Jesus.
Mark 4:28 For the earth bringeth forth fruit of herself; first the blade, then the ear, after that the full corn in the ear.
The earth brings forth fruit, it is not the doing of the sower. All he did was plant the seed. Likewise, the word of God once planted, the growing depends on the kind of soul, or the readiness of the heart. God makes the word grow. What the heart does with the word, will be the end result. Satan may plant thorns, the heart may allow this to choke out the word. The thoughts of man or the world and the devil will take root too. What will come forth will depend on what was chosen by the heart and the will of the person.
Mark 4:29 But when the fruit is brought forth, immediately he putteth in the sickle, because the harvest is come.
Will the harvest be good, a crop following the planting of God's word? If so this is the fruit of the kingdom of God. It depends on the heart and mind of the person. A yielded humble heart will increase the things of God. The person will want to grow the kingdom by planting God's word in other hearts. The sickle, the death of a person will mean the harvest is over, the good is gathered into God's house, the bad shall be burned.
Mat 13:30 Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.