Sterquilinus, Sterculius, or Stercutus (“fertilizer”) was the god of fertilizing the earth. Perhaps he was synonymous with Picumnus. This name was also used for Saturn.
The early Romans were an agrarian civilization and most of their gods (eg Pomona, Ceres, Flora, Dea Diaza) were responsible for rural life. Therefore, the Romans needed a deity who would oversee fertilization. Sterquilinus was just such a god.