Theophylact of Ohrid

By the Kingdom of God He means God's providence concerning us. The "man" is God Himself, Who for our sake became man. He cast "seed into the ground," that is, the evangelical preaching.

Commentary on Mark ↗

Gregory the Dialogist

Or else; Man casts seed into the ground, when he places a good intention in his heart; and he sleeps, when he already rests in the hope which attends on a good work. But he rises night and day, because he advances amidst prosperity and adversity, though he knows it not, for he is as yet unable to measure his increase, and yet virtue, once conceived, goes on increasing. When therefore we conceive good desires, we put seed into the ground; when we begin to work rightly, we are the blade. …

Catena Aurea by Aquinas ↗

Bede

And he said to them: Such is the kingdom of God, as if a man casts seed upon the earth and sleeps; and rises night and day, and the seed sprouts and grows while he does not know it. For the earth produces fruit by itself, first the blade, then the ear, then the full grain in the ear. And when the fruit has brought itself forth, he immediately wields the sickle, because the harvest is at hand. A man casts seed upon the earth, when he instills good intention into his heart. …

On the Gospel of Mark ↗