Preheat the oven to 375 degrees F (190 degrees C).
In a medium bowl, cream together the margarine, brown sugar and white sugar until smooth. Mix in the egg and peanut butter. Combine the baking soda, soy flour, tapioca flour and potato flour; stir into the batter to form a dough. Roll teaspoonfuls of dough into balls and place them 2 inches apart onto ungreased baking sheets.
Bake for 8 to 10 minutes in the preheated oven. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.
