Slowly heat the soy milk to 105 F (40 C), whisking occasionally to keep it from scalding on the bottom of the pot.
Stir the culture into the milk, making sure it is well mixed. (If you are using store bought yogurt you may need to up to 6 tbsp yogurt just in case it is a weak culture.)
Pour the warm soy milk into jars or containers.
Maintain at around 105F (40 C) until it is set (at least 4 hours,or if you like a strong yogurt then you can leave it for up to 24 hours.) The soy yogurt should have thickened slightly. See notes for details on how to strain soy yogurt to thicken it.
It’s important to make sure the soy milk is labelled as Preservative Free. Otherwise, the preservatives might prevent it from culturing. Alternatively, try making your own soy milk.
Soy milk forms a fairly thick yogurt without any added thickeners, however, if you want a really thick Greek-style yogurt, line a colander with cheesecloth and drain the “whey” out of the yogurt for 2-3 hours. The whey can be used as a probiotic culture for fermenting grains and vegetables, or you could just add it to a smoothie.