Shinshu University
Soga was born in the city of Niigata, Niigata. He entered Shinshu University, later known as Ōtani University, and graduated in 1901. After graduation from Shinshū, Soga returned to Niigata and became the adopted son-in-law of the priest of Jō'on-ji, a temple in Mitsuke, Niigata.