The critical elements of sourdough
It's interesting - and mildly humorous - to see the many suggested ingredients noted here to create a sourdough starter. If we look at the basic elements needed for sourdough, the only absolutes are flour, water, and bacteria (from the air and/or the surface of natural materials such as fruits, etc.). It's the process that's important in creating sourdough, and as long as you start with the necessary absolutes, and give the process enough time, you'll get a healthy sourdough starter.
I say the process is the most important component because most folks attempting to make sourdough are too impatient to wait long enough for the process to be complete. When you first mix your flour and water together, and introduce bacteria from the air and/or other sources, you have created an amazing "zoo" environment where many, many different kinds of bacteria, good and bad, begin to do their magic.
Some are faster than others, and some are more aggressive than others - and in fact, one of the less useful bacteria is very aggressive in the early, less acidic, environment of your flour/water mix - and this is what you see on day two and three of your process, when you notice bubbles forming.
But this early, active bacteria is about to be killed off by the increasingly acidic condition created by the natural process of the starches changing into sugars which are then consumed by the many bacteria in your mix. On day four, the bubbles will be gone, and many folks will consider their effort failed, and pitch the mix!
This is too bad, because, had they been more patient, they would have allowed their mix the time it needed to create a nice acidic condition, which is absolutely necessary for the good, sourdough bacteria to begin growing and multiplying. This will not happen before day 6 or 7 - maybe even longer.
I'm not suggesting that a basic flour/water/ base is the only way to go in creating a sourdough - my own best sourdough creation was made by using back-yard apples and grapes along with the flour and water - but I am suggesting that most other "additions" such as noted above are unnecessary - How would you ever know if any of those additions did anything positive? Most likely, as long as you included flour, water, and enough time for the process to work, you'd have a successful sourdough anyway!