Soybeans have probably a worse impact on the environment than beef. Most of the deforestation in SA in the past couple of decades was for soybean farms.
According to this source, 77% of soy production is used for animal feed. That only a small percentage of it goes to cattle is irrelevant in terms of the damage caused.
It's the meat industry that is primarily driving deforestation, both directly for pasture, and indirectly for animal feed.