As you may already know, SCD is a diet free of many things. One of the items not allowed is soy. Elaine Gottschall explains in the knowledge base.
But others have discovered the drawbacks to soy along the way. Here's a great link from Free the Animal, which explains why soy should be regarded as a highly processed junk food.
The only exception for SCD is the vitamin E found in certain supplements, which can be derived from soy (usually referred to as soy lecithin). Soybean oil used to be considered legal, but changes in processing forced a revision.
Here's a good post from Cheeseslave, who lets us know that the inmates at Illinois State Prison are overdosing on soy.