The Collaborative International Dictionary of English v.0.48:

Soda biscuit \So"da biscuit\, n.
   a bicuit leavened with baking soda and with buttermilk or
   sour milk.
   [PJC]

WordNet (r) 3.0 (2006):

soda biscuit
    n 1: very tender biscuit partially leavened with buttermilk and
         soda [syn: buttermilk biscuit, soda biscuit]