He was not strong enough. He created the Sudra colour (caste), as Pushan (as nourisher). This earth verily is Pushan (the nourisher); for the earth nourishes all this whatsoever.

In plain words: Brahman still did not feel complete. So it made the Sudra class, the workers, in the form of Pushan, the god who feeds and nourishes. The earth itself is Pushan, because the earth feeds every living thing.