Soy beans are used in tofu, soy oil (often sold as "vegetable oil"), soy sauce (fermented soy beans and other stuff), soy milk and products made out of it (especially popular with lactose intolerant people) and soy flour (sometimes used to extend meat products).
The is a discussion if soy beans should be considered kitniot. On the one hand, they are called "beans" (this alone is of course a stupid argument) and they are very similar to other beans. Rabbi Blech on kashrut.com writes clearly that "The general custom is to consider soy beans to be Kitniyos" (kashrut.com). On the other hand, there is a tradition that the list of kitniot is fixed and we do not add to that list now. This could be a reason to permit soy beans, as they only came to Europe in the 18th century – far after the minhag developed (jweekly blog, Rabbi Lior at Yeshiva.org).
An additional discussion is about products that are made out of soy but could never be confused with kitniot or chametz, e.g., soy milk. It is not possible to make milk out of chametz. So we have two points where we can be lenient (1 – soy beans were not part of the original minhag, 2 – liquids cannot be confused with chametz), which would be reason enough to permit something (Rabbi Jeff).
Before you go out and buy soy products, be careful to check for actual chametz ingredients. For example soy sauce contains actual chametz – cooked soy beans are mixed with flour (I found a recipe at iloveindia.com).
The OU lists soybeans and edamame (green soybeans). The Star-k list has soy beans, tofu and "Isolated Soy Protein". Kashrut.com has a whole bunch of related items: soy oil, soy lecithin, soy, and tofu.
- Danger of chametz traces: Don’t know.
- Danger of confusion with chametz (raw): No.
- Danger of confusion with chametz (processed): No.
- Botanical categorization: Legume.
- Known in 13th century: No.
- Verdict: Soy beans themselves hould not be kitniot, but are regarded as such by most. Soy products are definitely not kitniot.